Andrea S Grunst
email: andrea.grunst@univ-lr.fr

Melissa L Grunst

David Grémillet

Akiko Kato

Paco Bustamante

Céline Albert

Émile Brisson-Curadeau

Manon Clairbaux

Marta Cruz-Flores

Sophie Gentès

Samuel Perret

Eric Ste-Marie

Katarzyna Wojczulanis-Jakubas

Jérôme Fort

Mercury Contamination Challenges the Behavioral Response of a Keystone Species to Arctic Climate Change

Keywords: Climate change, behavioral ecotoxicology, toxicant-induced climate change sensitivity, multiple stressors, behavioral plasticity, mercury (Hg), diving behavior

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction: Global climate change is reconfiguring ecosystems, exposing organisms to multiple interacting stressors, including thermal stress, resource redistribution, and altered pollutant exposure [START_REF] Noyes | The toxicology of climate change: Environmental contaminants in a warming world[END_REF][START_REF] Sherwood | An adaptability limit to climate change due to heat stress[END_REF][START_REF] Bartley | Food web rewiring in a changing world[END_REF][START_REF]Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change[END_REF] . A pressing research priority involves understanding how multiple stressor interactions will affect biological systems as climate change progresses [START_REF] Crain | Interactive and cumulative effects of multiple human stressors in marine systems[END_REF][START_REF] Orr | Towards a unified study of multiple stressors: divisions and common goals across research disciplines[END_REF][START_REF] Tekin | Using a newly introduced framework to measure ecological stressor interactions[END_REF] . In particular, mercury (Hg) contamination of marine ecosystems is a persistent problem, which might be aggravated by altered Hg cycling in warming oceans [START_REF] Cossa | Methylmercury manufacture[END_REF][START_REF] Eagles-Smith | Modulators to mercury risk to wildlife and humans in the context of rapid global change[END_REF][START_REF] Schartup | Climate change and overfishing increase neurotoxicant in marine predators[END_REF] . Thus, amplified toxicological effects of Hg exposure threaten to impair organismal resiliency to other climate change-associated stressors [START_REF] Hooper | Interactions between chemical and climate stressors: A role for mechanistic toxicology in assessing climate change risks[END_REF][START_REF] Pirotta | Understanding the combined effects of multiple stressors: A new perspective on a longstanding challenge[END_REF][START_REF] Turschwell | Interactive effects of multiple stressors vary with consumer interactions, stressor dynamics and magnitude[END_REF] , especially by affecting patterns of physiological and behavioral plasticity whereby animals adjust to environmental variability [START_REF] Esparza | Mercury, legacy and emerging POPs, and endocrine-behavioural linkages: Implications of Arctic change in a diving seabird[END_REF][START_REF] Grémillet | Little auks buffer the impact of current Arctic climate change[END_REF][START_REF] Atwood | Rapid environmental change drives increased land use by an arctic marine predator[END_REF][START_REF] Hall | Behavioural plasticity modulates temperature-related constraints on foraging time for a montane mammal[END_REF] . Indeed, in its highly toxic methylated form (methylmercury, MeHg), Hg acts as a potent neurotoxin and endocrine disrupter, which can substantially alter physiology, energetics and behavioral capacity [START_REF] Whitney | Impacts of Sublethal Mercury Exposure on Birds: A Detailed Review[END_REF][START_REF] Gerson | Environmentally relevant methylmercury exposure reduces the metabolic scope of a model songbird[END_REF] . Thus, Hg exposure has the potential to exacerbate declines in behavioral performance under challenging environmental conditions (e.g. resource limitation, predation risk), making behavioral reaction norms steeper and generating synergistic effects [START_REF] Esparza | Mercury, legacy and emerging POPs, and endocrine-behavioural linkages: Implications of Arctic change in a diving seabird[END_REF][START_REF] Kobiela | Risk-taking behaviours in zebra finches affected by mercury exposure[END_REF][START_REF] Bustnes | Multiple stressors in a top predator seabird: potential ecological consequences of environmental contaminants, population health and breeding conditions[END_REF] . For example, Hg-exposed zebra finch (Taeniopygia guttata) increased foraging latency under predation risk more than controls, with implications for energy intake [START_REF] Kobiela | Risk-taking behaviours in zebra finches affected by mercury exposure[END_REF] . Concomitantly, Hg contamination might inhibit behavioral adjustments, canalizing reaction norms. For instance, Hg exposure can cause anemia and lower haemoglobin's oxygen carrying capacity, which might constrain diving behavior in aquatic avian and mammalian predators faced with shifting prey distributions [START_REF] Olsen | Effect of methylated mercury on the diving frequency of the common loon[END_REF] .

The Arctic marine biota is vulnerable to joint effects of climate change and environmental Hg contamination, such that the Arctic presents an important arena for studying how Hg exposure might affect animal resiliency to climate change [START_REF] Krabbenhoft | Global Change and Mercury[END_REF][START_REF]AMAP Mercury Assessment. Summary for Policy-makers[END_REF] . The Arctic is warming > 3 times faster than other regions, causing cascading effects on ecosystem structure [START_REF]Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change[END_REF] . For example, warming sea surface temperature (SST) and drastic reductions in summer sea ice coverage (SIC) are reconfiguring foraging landscapes, leading to behavioral and bioenergetic challenges [START_REF] Eamer | Life linked to ice. A guide to sea-ice associated biodiversity in this time of rapid change[END_REF][26][START_REF] Jakubas | Habitat foraging niche of a High Arctic zooplanktivorous seabird in a changing environment[END_REF][START_REF] Blanchet | Space-use strategy affects energy requirements in Barents Sea polar bears[END_REF] . Moreover, effects of climate change on the cryosphere and biochemical cycling are altering contaminant exposure patterns, with rising Hg contamination in some Arctic seabird and marine mammal populations presenting a special concern [START_REF]AMAP Mercury Assessment. Summary for Policy-makers[END_REF][START_REF] Fort | Does temporal variation of mercury levels in Arctic seabirds reflect changes in global environmental contamination, or a modification of Arctic marine food web functioning?[END_REF][START_REF] Morris | Temporal trends of mercury in Arctic biota: 10 more years of progress in Arctic monitoring[END_REF][START_REF] Tartu | A Uturn for mercury concentrations over 20 years: how do environmental conditions affect exposure in Arctic seabirds?[END_REF] .

We used a keystone Arctic seabird, the little auk (dovekie, Alle alle), as a model species to explore whether effects of Hg exposure on behavioral plasticity could underlie interactive effects of Hg contamination and other climate change-associated stressors on Arctic animals. Little auks are the most abundant seabird in the North Atlantic (~30-40 million breeding pairs globally), and act as ecosystem engineers by transferring nutrients between marine and terrestrial ecosystems [START_REF] González-Bergonzoni | Small birds, big effects: the little auk (Alle alle) transforms high Arctic ecosystems[END_REF] . Energy-rich sea iceassociated copepods upon which little auks specialize are declining with climate change, and smaller Atlantic species moving northward, altering the little auk's resource base [START_REF] Jakubas | Habitat foraging niche of a High Arctic zooplanktivorous seabird in a changing environment[END_REF][START_REF] Amélineau | Arctic climate change and pollution impact little auk foraging and fitness across a decade[END_REF][START_REF] Ershova | Sea ice decline drives biogeographical shifts of key Calanus species in the central Arctic Ocean[END_REF] . Little auks exhibit high flight and dive costs and operate close to maximum energy capacity, rendering them vulnerable to energetic stress [START_REF] Gabrielsen | Field and laboratory metabolism and thermoregulation in dovekies (Alle alle)[END_REF][START_REF] Ste-Marie | Accelerating animal energetics: high dive costs in a small seabird disrupt the dynamic body acceleration -energy expenditure relationship[END_REF] . Behavioral plasticity could buffer little auks, and ecosystems they underpin, against climate change [START_REF] Grémillet | Little auks buffer the impact of current Arctic climate change[END_REF] . However, Hg exposure in little auks from East Greenland has risen concurrent with climate change, threatening to impair behavioral plasticity and reproductive performance [START_REF] Amélineau | Arctic climate change and pollution impact little auk foraging and fitness across a decade[END_REF][START_REF] Fort | Spatial Ecotoxicology: Migratory Arctic Seabirds Are Exposed to Mercury Contamination While Overwintering in the Northwest Atlantic[END_REF] .

We leveraged temporal variation in environmental conditions across five years at a little auk colony in East Greenland combined with advanced biologging techniques (triaxial accelerometery) and individuallevel Hg contamination measurements to test two hypotheses: (1) Warm oceanic conditions (high sea surface temperature (SST), low SIC) affect little auk time activity budgets (TABs), increasing energetically expensive flying and diving, and altering behavioral performance traits (flight and dive durations); and (2) Mercury exposure exacerbates or inhibits changes in TABs, and finer scale adjustments of behavioral performance traits, such as adjustments of dive duration to prey distributions [START_REF] Amélineau | Arctic climate change and pollution impact little auk foraging and fitness across a decade[END_REF] .

Effects of Hg exposure on behavioral plasticity could constitute a mechanism driving multiple stressor interactive effects by altering little auks' resiliency to climate change, which given the species' keystone status, could have reverberating effects throughout Arctic ecosystems.

Methods

Field methods:

We studied little auks at a colony at Ukaleqarteq (Kap Høegh; 70°44′N, 21°35′W), East Greenland, during the breeding season (June-August, 2017-2021). The East Greenland Current transports Arctic waters southward past Ukaleqarteq, promoting favourable foraging conditions for little auks, but SST and SIC vary interannually, and climate change is increasingly elevating SST and reducing SIC [START_REF] Fort | Does temporal variation of mercury levels in Arctic seabirds reflect changes in global environmental contamination, or a modification of Arctic marine food web functioning?[END_REF][START_REF] Amélineau | Arctic climate change and pollution impact little auk foraging and fitness across a decade[END_REF] .

Little auks nest under rocks, often in accessible locations. The sexes are of similar size and contribute equally to parental care [START_REF] Jakubas | Predicting the sex of Dovekies by discriminant analysis[END_REF][START_REF] Wojczulanis | The Little Auk Alle alle: an ecological indicator of a changing Arctic and a model organism[END_REF] . Thus we did not distinguish between the sexes.

We equipped little auks with accelerometers (Axy4, Technosmart, 25 x 10 x 5 mm, 3.0 g, including tape used for attachment, < 2% body mass) to derive records of TABs and behavioral performance traits (dive/flight duration). During each season (July 19 to August 5, 2017-2021), we deployed accelerometers during chick rearing, beginning when chicks were ~3-6 days old, as estimated by size [START_REF] Konarzewski | The influence of weather conditions on growth of Little Auk Alle alle chicks[END_REF] or determined via hatching date. Sample sizes per year were 17, 7, 20, 11, and 7 across 2017-2021. We captured birds near nests and secured accelerometers to ventral feathers using tesaâ tape. Handling time was kept to a minimum at the time of accelerometer deployment to limit effects on behavioral patterns. Birds were recaptured within 3-11 days. Occasionally, shorter datasets (0.5-2 days) were obtained due to battery difficulties. Data < 3/4 th day (18 hrs) in duration (one dataset from 2021, 13 hr in duration, containing 132 dives) were not used in analyses of TABs, but were used when analysing other behavioral traits (proportion deep diving, dive and inter-dive durations; see below). Removing this short dataset from the analysis did not substantially alter results involving diving behavior. At recapture, we obtained small blood samples (~0.2 mL) via brachial venepuncture for Hg analysis.

Determining behavioral traits from accelerometer data: Acceleration was logged in three dimensions at 50 Hz, with X corresponding to surge (back to front), Y to sway (side to side), and Z to heave (dorsal to ventral). Temperature was logged at 0.2 Hz. We used Igor Pro 9.0 (WaveMetrics, Inc., Lake Oswego, OR, USA) combined with Ethographer 2.05 [START_REF] Sakamoto | Can Ethograms Be Automatically Generated Using Body Acceleration Data from Free-Ranging Birds[END_REF] to process data and classify behaviors. In brief, acceleration waves (data vectors measured at equal intervals) in all dimensions were smoothed using a rolling algorithm (1 sec sliding window) in Igor, to extract gravitational acceleration. We calculated body pitch using the formula: atan(XW_B / (sqrt(YW_B 2 + ZW_B 2 )) ´ (180/pi), and vectorial dynamic body acceleration (VDBA) using the formula: sqrt((XW_B-XW) 2 +(YW_B-YW) 2 +(ZW_B-ZW) 2 ), where XW_B, YW_B and ZW_B are smoothed acceleration waves [START_REF] Patterson | A comparison of techniques for classifying behavior from accelerometers for two species of seabird[END_REF][START_REF] Wilson | Estimates for energy expenditure in free-living animals using acceleration proxies: A reappraisal[END_REF] . We applied continuous wavelet transformation to Z dimension waves to derive dominant wing beat frequency across time (1 Hz resolution). We applied K-mean cluster analysis to dominant frequency waves, using four clusters, which resulted in clusters corresponding to flying (~10-11 Hz), descending dives (~3.5-4.5 Hz), rest (~0 Hz), and intermediate activity (~0-3.5 Hz). These values are consistent with previous work in little auks [START_REF] Ste-Marie | Accelerating animal energetics: high dive costs in a small seabird disrupt the dynamic body acceleration -energy expenditure relationship[END_REF] .

We wrote custom functions in Igor to classify behavior, using cluster analysis output, pitch, VDBA and temperature. Flying was identified based on wing beat frequency of ~10-11 Hz. Dives started when wing beat frequency was ~3.5-4.5 Hz and pitch was between -10 and -30°, and ended when pitch increased to > 40° and then returned to zero at the water surface. We subdivided dives into deep dives, wherein the descent phase with high wing beat frequency lasted > 5 seconds, and shallower dives, wherein this phase lasted £ 5 seconds. Histograms revealed that these criteria captured a bimodal distribution in dive durations. We calculated proportion of time deep versus shallower diving (hereafter, proportion deep diving) as a metric of variation in foraging behavior. Time resting at the colony was identified via absence of wing beat and elevated temperature (> ~17°C), following a period of flight. Time resting on sea ice was classified based on initial high pitch, indicating standing, absence of wing beat, and elevated temperature (lower than for colony; ~12-16°C; Fig. S1). Birds were classed as resting on water based on low wing beat, low temperature (> ~10°C), and absence of classification into another category. We adjusted temperature and pitch ranges associated with behaviors depending on positioning of accelerometers on individuals, with pitch standardized to zero during flight. We calculated TABs (flying, resting at colony, on ice, diving, on water) across each accelerometer dataset's duration (Fig. S1), rather than daily TABs, as Arctic breeding little auks are not constrained by daily light-dark cycles, and lack circadian rhythms in behavior [START_REF] Wojczulanis-Jakubas | Little auks under the midnight sun: diel activity rhythm of a small diving seabird during the Arctic summer[END_REF] .

We also used accelerometer data to determine durations of foraging trips, dives, inter-dive intervals, and flights, proportion time spent flying during foraging trips versus around the colony (hereafter, proportion foraging flight), and chick provisioning rates. Breeding little auks display a dual foraging strategy, with short trips used for provisioning chicks and longer trips for self-maintenance. We assumed that short and long trips derive from two log-normal distributions [START_REF] Welcker | Flexibility in the bimodal foraging strategy of a high Arctic alcid, the little auk Alle alle[END_REF] . We log-transformed trip duration, and used K-mean clustering in R (R Core 2021) to group foraging trips into two categories. The cutoff value separating short and long trips was 3.9 hrs., which is lower than previously published values (~5.5-6 hrs.) [START_REF] Brown | Divergent diving behaviour during short and long trips of a bimodal forager, the little auk (Alle alle)[END_REF][START_REF] Jakubas | Foraging strategy of the little auk Alle alle throughout breeding season -switch from unimodal to bimodal pattern[END_REF] . However, we preferred to use the value deriving from our dataset. We determined whether time on the water surface was part of a dive bout (inter-dive interval) using the standard classification method in R package diveMove [START_REF] Luque | Diving Behaviour Analysis in R[END_REF] . The bout ending criteria was 308 sec, and we assumed dives were £ 120 sec [START_REF] Ste-Marie | Accelerating animal energetics: high dive costs in a small seabird disrupt the dynamic body acceleration -energy expenditure relationship[END_REF] . Thus, inter-dive intervals were defined as periods between dives £ 308 sec. Dive and inter-dive durations can be interdependent [START_REF] Elliot | Time allocation by a deep-diving bird reflects prey type and energy gain[END_REF] , and might be constrained by contamination-linked physiological changes [START_REF] Olsen | Effect of methylated mercury on the diving frequency of the common loon[END_REF] . Thus, we were interested in whether interrelationships between dive-and inter-dive durations were altered in Hg-contaminated birds. We determined proportion of foraging flight by classifying flights embedded within periods of resting at the colony as colony-based, and other flight as foraging-based, and taking the quotient (foraging / colony-based flight). We estimated chick provisioning trips per day as the number of colony visits ³ 30 secs long (minimum time to feed chicks) that proceeded a foraging trip containing ³ 5 dives. Video-recording data from accelerometer-equipped birds showed that colony visits fulfilling these criteria were invariably associated with birds returning with a full gular pouch and feeding the chick.

Mercury contamination: Blood samples were centrifuged for 10 min at 3500 rpm, and red blood cells (RBCs) stored in 70% ethanol pending analysis. RBCs were later freeze-dried for 48 hrs and homogenized, after which total Hg concentrations were measured using an advanced Hg analyser spectrophotometer (Altec AMA 254) at the Littoral Environnement et Sociétés (LIENSs) laboratory, La Rochelle University [START_REF] Bustamante | Total and organic Hg concentrations in cephalopods from the North Eastern Atlantic waters: Influence of geographical origin and feeding ecology[END_REF] . Mercury in avian blood consists primarily of MeHg (> 90%) [START_REF] Evers | Patterns and interpretation of mercury exposure in freshwater avian communities in northeastern north America[END_REF][START_REF] Chastel | Mercury contamination and potential health risks to Arctic seabirds and shorebirds[END_REF] . Thus, total blood Hg provides a good proxy for MeHg exposure. Two or more Hg measurements were taken per bird, using small aliquots of dried RBCs (~1.5-5 ng), until standard deviation was < 10%. We regularly measured a certified reference material (CRM; Lobster Hepatopancreas Tort-3; NRC, Canada; Hg concentration of 0.292 ± 0.022 µg/g dry weight (dw)), and ran blanks before each session. The detection limit was 0.005 µg/g and Tort-3 measurements (mean ± SD) averaged 0.306 ± 0.004 µg/g. Mercury concentrations were not available for 2017, hence analyses involving Hg do not include this year.

Our Hg measurements derived from dried RBCs, rather than whole blood. Thus, for comparison to toxicological thresholds for birds, which have been formulated for measurements taken on whole blood (wet weight (ww)), we used the equation recommended by Ackerman et al. (2016) [START_REF] Ackerman | Avian mercury exposure and toxicological risk across western North America: A synthesis[END_REF] to convert Hg concentrations in dried RBCs to whole blood (ww) equivalents. Namely, we assumed a blood moisture content of 79%, and thus converted from dried RBC concentrations (dw) to whole blood (ww) equivalents by multiplying the concentrations measured in RBCs by 0.21.

Sea surface temperature (SST) and sea ice coverage (SIC):

We focused on relationships between SST and SIC and behavior, as these variables strongly affect foraging conditions for little auks [START_REF] Jakubas | Habitat foraging niche of a High Arctic zooplanktivorous seabird in a changing environment[END_REF][START_REF] Amélineau | Arctic climate change and pollution impact little auk foraging and fitness across a decade[END_REF][START_REF] Karnovsky | Foraging distributions of little auks Alle alle across the Greenland Sea: implications of present and future Arctic climate change[END_REF] . We determined SST within the little auk's primary foraging range at Ukaleqarteq, determined by GPS tracking [START_REF] Amélineau | Where to Forage in the Absence of Sea Ice? Bathymetry as a key factor for an Arctic seabird[END_REF] , using the National Oceanic and Atmospheric Administration's interpolated SST (OISST) high resolution daily dataset ( ´ 200 km area. We extracted daily SST across this area during the chick rearing period, across all days of accelerometer deployment. We averaged daily values across the foraging range and each day of accelerometer deployment for use in analyses. SIC data were downloaded from the University of Bremen's Sea Ice Remote Sensing database (https://seaice.uni-bremen.de/sea-ice-concentration/amsre-amsr2/). Data derived from brightness temperature measurements from the Advanced Microwave Scanning Radiometer 2 (ASMR2) sensor, mounted aboard the JAXA satellite "Shizuku" (GCOM-W1). The ASI algorithm permits derivation of SIC by differences between brightness temperatures at 89 GHz and polarization, with atmospheric corrections [START_REF] Spreen | Sea ice remote sensing using AMSR-E 89-GHz channels[END_REF] . We gridded data on a polar stereographic grid through near neighbor interpolation with 3.125 km resolution. We extracted daily SIC values, and derived averages from data spanning the foraging range and days of accelerometer deployment, as for SST.

Statistical Analysis:

We used R 4.1.2 [START_REF] Core | R: A language and environment for statistical computing[END_REF] to process and analyze data. We performed ANOVAs to compare SST, SIC and Hg concentrations (log transformed) among years, and R package emmeans for post-hoc comparisons (Tukey method) [START_REF] Lenth | Emmeans: Estimated Marginal Means, aka Least-Squares Means[END_REF] . We used linear mixed effect models (LMMs) with Satterthwaite approximations for degrees of freedom and year as a random effect to assess whether blood Hg varied with SST/SIC (R packages lme4, lmerTest) [START_REF] Bates | Fitting Linear Mixed-Effects Models Using lme4[END_REF][START_REF] Kuznetsova | lmerTest Package: Tests in Linear Mixed Effects Models[END_REF] .

We analyzed TABs using Dirichlet (mixed) models fit in JAGS, via R2jags [START_REF] Regular | Why timing is everything: Energetic costs and reproductive consequences of resource mismatch for a chick-rearing seabird[END_REF][START_REF] Su | R2jags: Using R to Run[END_REF] . The Dirichlet distribution is a multivariate generalization of the beta distribution, suitable for multivariate compositional data [START_REF] Douma | Analysing continuous proportions in ecology and evolution: A practical introduction to beta and Dirichlet regression[END_REF] . JAGS implements Bayesian models via Markov Chain Monte Carlo (MCMC) simulation. We ran four chains simultaneously with 50,000 repetitions, burn in of 25,000, thinning of 25, and vague priors. All estimated parameters had an Rhat value of ~1, indicating convergence. We first compared TABs among years (fixed effect), with predicted differences determined as derived quantities.

We then used Dirichlet mixed models, with year as a random effect, to explore relationships between SST, SIC (log transformed), and TABs. We assessed effects of Hg within the year-comparison (fixed effect) model, and included an interaction between Hg and SST/SIC in the mixed model. Data were transformed using the DirichletReg package prior to analysis to avoid zeros [START_REF] Maier | DirichletReg: Dirichlet Regression. R package version 0[END_REF] . We analyzed proportion deep diving and proportion foraging flight using beta regression (mixed) models in R2jags, and the same approach as above. Predicted differences and parameter estimates from models fit in R2jags were considered significant when 95% credible intervals (CIs) did not cross zero. We used graphical inspection and deviance information criterion (DIC) to assess model fit.

We analyzed among year variation in foraging trip and flight durations (per trip) using LMMs and emmeans, with bird identity as a random effect (multiple trips per individual). Long and short trips were analyzed separately. We used LMMs to analyze effects of SST/SIC, Hg and interactions on these variables, with bird identity and year as random effects. The same analyses, but without repeated measures on individuals, were performed for maximum continuous flight duration, deep dive duration, and inter-dive duration (individual-level averages). We included interactions between Hg and inter-dive or dive durations in models involving diving behavior, as dive and inter-dive durations can be interdependent [START_REF] Elliot | Time allocation by a deep-diving bird reflects prey type and energy gain[END_REF] . We also performed a model predicting deep dive duration from the interaction between foraging trip duration and SST/SIC, to assess the hypothesis that birds reach colder, more profitable waters, in which diving behavior could differ, on longer trips. Individual identity was a random effect.

Results

Variation in oceanic conditions and Hg contamination: Across years, SST experienced by little auks

during accelerometer deployment within their primary foraging range averaged (mean ± SE) 2.20 ± 0.224°C (range -0.932, 7.416°C), and SIC averaged 3.33 ± 0.867 % (range: 0.016-14.812 %). SST (F4,57 = 43.66, P < 0.001) and SIC (F4,57 = 36.84, P < 0.001) varied among years, with SST warmer and SIC lower in 2018 and 2021 (Table 1). Blood Hg averaged 1.088 ± 0.039 µg/g dw (range: 0.579-1.745 µg/g dw), or, for comparison to toxicological benchmarks, ~0.228 µg/g ww (range: 0.121-0.366 µg/g ww), assuming blood moisture content of 79% [START_REF] Maier | DirichletReg: Dirichlet Regression. R package version 0[END_REF] . Blood Hg (µg/g) concentrations varied somewhat among years (F3,40 = 7.706, P < 0.001) (Table 1).

Warm oceanic conditions affect TABs and foraging patterns:

Variation in oceanic conditions (SST, SIC) among years correlated with reshaping of TABs. In high SST-low SIC years (2018, 2021), little auks were predicted to spend as much as 11.7% (2.7%, 21.1%) more time flying, 23.2% (12.8%, 32.9%) more time on water, 21.4% (14.8%, 28.1%) less time resting on ice, and 27.6% (21.9%, 32.6%) less time resting at the colony relative to in cold years (Fig. 1a-e., Fig. S2). However, time resting at the colony was also low in the coldest year of 2020 (Fig. 1b). Percent time diving was consistent across years, at 15-19% (Fig. 1d). Dirichlet mixed models substantiated strong effects of SST and SIC on TABs. Beta coefficient estimates for SST (and SIC) had 95% CIs not crossing zero for time resting on ice (-0.679 (-0.998, -0.341)) and at the colony (-0.329 (-0.626, -0.045)) (Table S1). Predicted values for other behavioral categories were also affected by SST and SIC, as predictions for behavioral categories are interdependent (Fig. 2a). Time resting on ice was predicted to drop from ~25% to < 3%, time resting at the colony decline (~22 to 10%), time flying (~14 to 30%) and on water (~24 to 38%) increase, and diving time remain constant (~17-18%), as SST increased from < 0°C to > 7°C (Fig. 2a).

Little auk foraging trip and flight patterns were altered under warm conditions, with long trips primarily affected (Fig. S3a-d; Table S2-S6). Long trip duration (F4,49 = 5.022, P < 0.001), flight time per long trip (F4,56 = 6.801, P < 0.001), and maximum continuous flight duration (F4,56 = 3.321, P = 0.016) were greatest during high SST years (Table S2-S4, Fig. S3a,b). In the warmest year ( 2021 Little auks spent more time flying around the colony in cold years, with predicted differences as high as 11.2% (7.7%, 15.1%), reflecting higher colony attendance (Fig. S3e, Fig. S4a).

However, in mixed models, there was no overall effect of SST (0.022 (-0.332, 0.376)) or SIC (-0.037 (-0.363, 0.276)) on proportion foraging flight.

Diving behavior changed with oceanic conditions. Little auks were predicted to spend as much as 15.7% (4.2, 26.8) more time deep rather than shallow diving in high SST years (Fig. S3f, Fig. S4b). In mixed models, beta coefficient estimates were 0.293 (0.025, 0.555) and -0.143 (-0.246, -0.037) for SST and SIC, respectively, with proportion deep diving predicted to increase from 69.4% (57.3, 80.3) to 88.3% (78.9, 94.5) under the lowest (-0.9°C) versus highest (7.4°C) SST (Fig. 2b). Deep dive (F6 = 15.01, P < 0.001) and inter-dive duration (F6 = 9.305, P < 0.001) were longest in high SST years (Table S7-S8; Fig. S3g,h), although post-hoc comparisons were non-significant. However, the mixed model indicated that little auks made longer dives when SST was high (b ± SE = 2.194 ± 0.797, t41 =3.246, P = 0.002; Fig. 2c). Moreover, foraging trip duration interacted with SST to predict deep dive duration, with dives shorter on longer foraging trips when SST in the primary foraging range was high (b ± SE = -0.014 ± 0.004, t397 = -2.966, P = 0.003).

14

Chick provisioning rates (feeds/day) varied among years (F4 =2.929, P = 0.029), tending to be highest in cold years (e.g. 2019: (2.580 (2.114, 3.040)), 2020: 2.810 (2.116, 3.500)), and lowest in warm years (e.g. 2021: 1.540 (0.688, 2.390); Fig. S3i, Table S9). However, post-hoc comparisons were nonsignificant (P > 0.169), and effects of SST (b ± SE = -0.131 ± 0.172, t5 = -0.762, P = 0.477) and SIC (b ± SE = 0.101 ± 0.081, t15 = 1.239, P = 0.234) on provisioning rates were non-significant in mixed models.

Mercury concentrations unrelated to TABs, but associated with dive bout patterning

TABs did not vary with blood Hg concentrations, irrespective of oceanic conditions. Beta coefficient estimates for Hg had 95% CIs overlapping zero in the year-comparison model (Fig. S5). In the mixed model, there was no statistically significant effect of Hg on TABs, additively or in interaction with SST, with 95% CIs overlapping zero, and DICs higher than in models including SST alone. However, little auks with higher blood Hg concentrations increased inter-dive interval duration with dive duration more than less contaminated individuals, as reflected by a significant interaction between dive duration and Hg in predicting inter-dive interval duration (b ± SE = 6.704 ± 2.991, t40 = 2.241, P = 0.031; Fig. 3). The same pattern held when analyzing observations of every dive (and inter-dive) made by individuals, in sequence, rather than means (interaction: b ± SE = 1.980 ± 0.244, t32350 = 8.120, P < 0.001).

Discussion: Multiple stressor effects involving exposure to contaminants, such as Hg, are attracting growing concern in the context of rapid climate change [START_REF] Noyes | The toxicology of climate change: Environmental contaminants in a warming world[END_REF][START_REF] Crain | Interactive and cumulative effects of multiple human stressors in marine systems[END_REF][START_REF] Hooper | Interactions between chemical and climate stressors: A role for mechanistic toxicology in assessing climate change risks[END_REF] . Yet, understanding of mechanisms underlying such effects remains limited. We addressed this deficit by exploring joint effects of climate change-associated environmental variables and Hg contamination on a keystone Arctic seabird's behavioral ecology. We hypothesized that warm oceanic conditions induce costly behavioral changes, and that Hg exposure might exacerbate or inhibit behavioral responses, generating interactive effects. Below we discuss evidence for these hypotheses, and implications of our findings.

TABs and behavioral performance demands under warm conditions: Climate change is increasingly exposing little auks to warm oceanic conditions, which alter resource distribution [START_REF] Amélineau | Arctic climate change and pollution impact little auk foraging and fitness across a decade[END_REF] . Our data demonstrate that warm conditions reconfigure TABs in these sea-ice adapted seabirds, reducing time resting at the colony and on ice, and increasing time in flight and on water. Accelerometer data revealed that sea ice serves as an important resting substrate for little auks at Ukaleqarteq in years with substantial SIC. Thus, loss of ice will likely reduce rest quality for little auks at sea, with energetic and health implications [START_REF] Lovvorn | Modeling marine protected areas for threatened eiders in a climatically changing Bering Sea[END_REF][START_REF] Siegel | Sleep viewed as a state of adaptive inactivity[END_REF] .

Indeed, thermoregulatory costs are elevated for seabirds resting in water [START_REF] Lovvorn | Modeling marine protected areas for threatened eiders in a climatically changing Bering Sea[END_REF] , although warming SST may reduce these costs 67 . A previous study could not differentiate energetic costs of resting on water versus land or ice for little auks, but did not differentiate land versus ice [START_REF] Ste-Marie | Accelerating animal energetics: high dive costs in a small seabird disrupt the dynamic body acceleration -energy expenditure relationship[END_REF] . Energetic costs of resting on land at the colony might be elevated by anti-predator vigilance [START_REF] Wojczulanis | Changes in the Glaucous Gull predation pressure on Little Auks in southwest Spitsbergen[END_REF] . Little auks also reduced time resting at and flying around the colony during warm periods, further reducing rest, and altering colony-based social dynamics, which are critically important in many seabirds [START_REF] Grissot | Parental Coordination of Chick Provisioning in a Planktivorous Arctic Seabird Under Divergent Conditions on Foraging Grounds[END_REF][START_REF] Courbin | The dance of the Cape gannet may contain social information on foraging behaviour[END_REF] .

As rest and social time decreased, foraging and behavioral performance demands increased. Little auks spent more time flying and made longer continuous flights under warmer conditions, suggesting an increase in foraging range. Indeed, even when there was virtually no sea ice within the primary foraging range around Ukaleqarteq, tagged birds spent some time on ice, with ice reached via prolonged flight (> 2 hrs.). Studies at other colonies suggest that little auks can fly > 100 km to reach profitable foraging areas at marginal sea ice zones when foraging conditions are poor near colonies [START_REF] Jakubas | Foraging by little auks in the distant marginal sea ice zone during the chick-rearing period[END_REF] , and similar increases in foraging range also occurs in other seabird species [START_REF] Fayet | Local prey shortages drive foraging costs and breeding success in a declining seabird, the Atlantic puffin[END_REF] . Increased foraging range can maintain energy intake, but decreases net energy return for central place foragers. Thus, foraging range commonly correlates negatively with breeding performance in central place foragers, including little auks [START_REF] Boersma | Foraging distance affects reproductive success in Magellanic penguins[END_REF][START_REF] Jakubas | Foraging closer to the colony leads to faster growth in little auks[END_REF] .

Accordingly, little auk chick provisioning rates at Ukaleqarteq tended to decline with warm conditions, when flight times were longest. Little auks at Ukaleqarteq also increased proportion deep diving and dive duration with warm SST, likely reflecting altered copepod distribution [START_REF] Amélineau | Arctic climate change and pollution impact little auk foraging and fitness across a decade[END_REF] . Previous analyses suggest that energetic costs do not scale to dive duration in little auks [START_REF] Ste-Marie | Accelerating animal energetics: high dive costs in a small seabird disrupt the dynamic body acceleration -energy expenditure relationship[END_REF] , but utilized small sample sizes and did not consider dive depth, which could make identifying energetic patterns difficult. Longer dives might challenge physiological limitations by approaching aerobic dive capacity [START_REF] Kooyman | The physiological basis of diving to depth: birds and mammals[END_REF][START_REF] Williams | What triggers the aerobic dive limit? Patterns of muscle oxygen depletion during dives of emperor penguins[END_REF] . Thus, results suggest that warm conditions force little auks at Ukaleqarteq to adopt more energetically and physiologically demanding behavior, with rest reduced, activity increased, and behavioral performance demands intensified, at diminishing return.

Behavioral correlates of Hg contamination:

We hypothesized that Hg exposure might reduce the capacity of birds to maintain behavioral performance or upregulate costly behavior given altered foraging landscapes under warm conditions. To the contrary, little auk Hg concentrations were not related to TABs. When compared to toxicological benchmarks for avian blood [START_REF] Chastel | Mercury contamination and potential health risks to Arctic seabirds and shorebirds[END_REF] , little auk blood Hg concentrations at Ukaleqarteq (which fall in the range of 0.2-1.0 µg/g ww) suggest low toxicological risk, despite recent increases in Hg exposure, which could explain this result. However, a recent study on thick-billed murres (Uria lomvia) found interrelationships between thyroid hormone levels, time spent diving and Hg exposure with only slightly higher total blood Hg (mean ± SE = 1.51 ± 0.05 µg/g dw, or ~0.31 µg/g ww, range: 0.81-2.74 µg/g dw, or ~0.17-0.36 µg/g ww). Murres with elevated blood Hg showed altered triiodothyronine (T3) levels when sea ice break up was early, and murres were likely stressed due to poor foraging conditions, but not given more usual ice conditions. In turn, elevated T3 levels were associated with reduced time diving, suggesting that thyroidal dysregulation in Hg exposed birds could affect activity budgets under stressful environmental conditions [START_REF] Esparza | Mercury, legacy and emerging POPs, and endocrine-behavioural linkages: Implications of Arctic change in a diving seabird[END_REF] .

Rather than affecting TABs, Hg exposure could alter behavioral performance traits or temporal patterning of behavior. Little auks with higher Hg concentrations exhibited changes in dive bout patterning, with inter-dive breaks lengthened when individuals were making long dives. Mercury exposure might cause physiological impairments, such as decreased oxygen carrying capacity [START_REF] Olsen | Effect of methylated mercury on the diving frequency of the common loon[END_REF] , forcing dive bout restructuring as birds approach aerobic dive capacity. Mercury can interfere with heme-proteins including haemoglobin and myoglobin, which play critical roles in oxygen carrying capacity in blood and muscles, respectively [START_REF] Piscopo | Novel Insights into Mercury Effects on Hemoglobin and Membrane Proteins in Human Erythrocytes[END_REF][START_REF] Alazoumi | Effect of Antioxidants on Heavy Metals Induced Conformational Alteration of Cytochrome C and Myoglobin[END_REF] . Indeed, Hg contaminated common loons (Gavia immer) showed increased diving frequency, concurrent with reduced dive duration, suggesting reduced aerobic dive capacity [START_REF] Olsen | Effect of methylated mercury on the diving frequency of the common loon[END_REF] . In contrast, little auks with higher Hg maintained dive duration, but lengthened inter-dive breaks. Adjusting inter-dive durations might allow Hg-contaminated birds to perform long dives despite physiological impairments when long dives are essential to prey capture. However, lengthening breaks might eventually reduce time underwater or resting outside of dive bouts, with implications for foraging efficiency, digestion and recovery [START_REF] Elliot | Time allocation by a deep-diving bird reflects prey type and energy gain[END_REF][START_REF] Auer | Aerobic scope explains individual variation in feeding capacity[END_REF][START_REF] Enstipp | The dive performance of immature king penguins following their annual molt suggests physiological constraints[END_REF] .

Effects of Hg contamination on little auk diving behavior are predicted to increase with warming conditions. When SST was high and SIC low, little auks at Ukaleqarteq spent more time deep diving and increased dive duration (Fig. 2b,c). Thus, as Hg-contaminated birds increased inter-dive durations when making longer dives, Hg contamination might increasingly constrain diving patterns and foraging efficiency as SST increases with climate change. Changes in Hg exposure with climate change might also combine with foraging challenges to elevate toxicological effects on little auks and ecosystems they underpin [START_REF]AMAP Mercury Assessment. Summary for Policy-makers[END_REF] .

In conclusion, warm oceanic conditions affect TABs and finer scale behavior in little auks, a sea-ice adapted Arctic seabird. Mercury exposure threatens to alter behavioral responses, generating climate change-by-contamination interactive effects. Mercury contamination was not correlated with TABs.

However, birds with high Hg concentrations took longer inter-dive breaks when making long dives, suggesting physiological constraint. A next step will be exploring the nature of this constraint, for instance by measuring oxygen carrying capacity. As little auks engaged in longer dives given warm SST, subtle behavioral effects of Hg-contamination threaten to increasingly constrain this keystone species' foraging efficiency in a warming Arctic, with ecosystem-wide repercussions.

Supporting Information: Additional statistical results, which were too extensive to include in the main text.

Figure 1 .

 1 Figure 1. Little auk TABs across years at Ukaleqarteq, East Greenland. Relatively cold (blue; average

  ), long trips lasted 27.5 hrs. (20.4, 37.1), little auks flew 7.76 hrs. (5.43, 11.09) during long trips, and maximum continuous flight duration was 2.23 hrs. (1.58, 2.88), compared to 14.4 hrs. (12.3, 16.9), 3.12 hrs. (2.56, 3.80), and 1.04 hrs. (0.563, 1.51) in 2019. In mixed models, flight time per long trip (b ± SE = 0.307 ± 0.068, t10 = 4.511, P < 0.001) and maximum continuous flight duration (b ± SE = 0.255 ± 0.106, t10 = 2.395, P = 0.039) increased with SST, and long trip duration tended to increase (b ± SE = 0.122 ± 0.066, t11 = 1.857, P = 0.090).

Figure 2 .

 2 Figure 2. Little auk a) TABs (points staggered to facilitate viewing), b) proportion time deep versus

Figure 3 .

 3 Figure 3. Interaction between dive duration and Hg concentrations in predicting inter-dive durations

  1/4° global grid; https://psl.noaa.gov/data/gridded/data.noaa.oisst.v2.highres.html). The foraging range was encompassed by -21.57° longitude west, -17.47° longitude east, 69.8° latitude south, and 71.65° latitude north, and comprised an ~160

Table 1 .

 1 Variation in a) sea surface temperature (SST; °C) and b) sea ice coverage (SIC; %), and c) blood 254 mercury (Hg) concentrations (µg/g dw), experienced by little auks at Ukaleqarteq, East Greenland, 255 summarized across study years. We quantified values for environmental variables (SST, SIC) on a per-

	256
	257	individual basis, depending on the period of accelerometer deployment.
		a) SST
		Year	Mean ± SE	95% CI	Range	N
		2017	2.472 ± 0.245 1.981, 2.964 3.312, 2.585	17
		2018	3.615 ± 0.382 2.850, 4.381 3.544, 4.038	7
		2019	1.415 ± 0.226 0.963, 1.868 0.318, 3.353	20
		2020	-0.055 ± 0.305 -0.666, 0.555 -0.932, 0.675 11
		2021	5.886 ± 0.382 5.120, 6.651 4.738, 7.416	7
		Overall	2.20 ± 0.224	1.75, 2.64	-0.932, 7.416 62
		b) SIC
		2017	1.820 ± 0.497 1.052, 3.146 1.589, 2.027 17
		2018	0.081 ± 0.034 0.034, 0.190 0.057, 0.263 7
		2019	1.192 ± 0.300 0.719, 1.974 0.096, 4.736 20
		2020	10.058 ± 3.417 5.093, 19.861 5.165, 14.812 11
		2021	0.035 ± 0.015 0.015, 0.083 0.016, 0.078 7
		Overall	3.33 ± 0.867	2.48, 41.8	0.016, 14.812 62
		c) Blood Hg
		2018	1.210 ± 0.096 1.031, 1.420 0.940-1.488	7
		2019	0.916 ± 0.044 0.831, 1.010 0.579-1.627	19
		2020	1.291 ± 0.082 1.136, 1.470 0.993-1.745	11
		2021	1.193 ± 0.094 1.016, 1.400 1.102-1.272	7
		Overall	1.088 ± 0.039 1.013, 1.169 0.579-1.745	44
	258

Acknowledgements: We acknowledge long-term support from the French Polar Institute (IPEV) to the ADACLIM program (388) administered by JF and DG. The authors are grateful to Maud Brault-Favrou of the platform Analyses ELémentaires of LIENSs laboratory for assistance with Hg analyses, and Clément Bertin for help with SIC data. Thanks also to the CPER (Contrat de Projet Etat-Région) and FEDER (Fonds Européen de Développement Régional) for funding the AMA of LIENSs laboratory, and the Institut Universitaire de France (IUF) for supporting PB as a Senior Member. ASG, MLG, and MCF are supported by the European Union's Horizon 2020 programme (Marie Skłodowska-Curie grants 101025549, 896866, 101024166), and CA was supported by a PhD fellowship from the French Ministry of Higher Education and Research. We thank Nanu travel for facilitating field seasons, and the Northern Scientific Training Program of Canada for funding travel and providing logistical support. This work contributes to research projects ARCTIC-STRESSORS and ILETOP funded by the French National Research Agency (ANR-20-CE34-0006, ANR-16-CE34-0005), the international ARCTOX initiative (arctox.cnrs.fr), the Excellence Chair ECOMM funded by the Region Nouvelle Aquitaine, and the OPUS13 funded by the Polish National Science Center (2017/25/B/NZ8/01417).

Data accessibility statement: Data is available via the Zenodo community of European Commission

Funded Research (OpenAIRE), doi:10.5281/zenodo.7185806, and via Dryad, doi:10.5061/dryad.dbrv15f4t.


Luis Miguel

López Santiago

Miguel Lopez Luis

Santiago López

Santiago

Innovating with limited resources: exploring the development process of a frugal water innovation

Keywords: Frugal Innovation, Design Process, Innovation Process, Base of the Pyramid, Sustainability, Water

In environments where resources are scarce, frugal innovation has positioned itself as an approach with the potential to respond to social and environmental crises. Innovators in BoP countries are making interesting contributions to the development of ingenious and affordable frugal solutions focused on improving the quality of life of BoP communities. Research on frugal innovation has provided insight into its origins, lists of design criteria and attributes. However, there is still a lack of understanding about this approach to innovation seen as a process and the most important strategies at each stage. Applying the case study methodology, we conducted an exploratory analysis of the development of a water access solution in Mexico. The water crisis is a global and urgent problem, especially in marginalized areas. Through semi-structured in-depth semi-structured interviews and an analysis and mapping of a frugal innovation, process was carried out. This study offers a descriptive model of a frugal innovation process developed. A list of frugal innovation strategies implemented is also presented. The exploratory nature of a single case limits generalization. However, the results offer valuable findings for future research. The model presented can serve as support for projects that aim to innovate in a frugal way.

I. INTRODUCTION

There is no doubt that the development of technological innovations has improved the quality of life of society. However, 43% of the world's population still lives in poverty [START_REF]global poverty update from the World Bank[END_REF]. This segment of the world's population has been identified by Prahalad [START_REF] Prahalad | The Fortune at the Bottom of the Pyramid: Eradicating Poverty Through Profits[END_REF] as the Base of the Pyramid (BoP). Since then, various projects, policies and innovation approaches have been developed with the aim of reducing poverty and improving the conditions of these people [START_REF] Kolk | Reviewing a Decade of Research on the 'Base/Bottom of the Pyramid' (BOP) Concept[END_REF]. In recent years, there has been concern about how climate change has led to increasing demand and scarcity of natural resources, such as food, water, land and oil [START_REF] Evans | Resource Scarcity, Climate Change and the Risk of Violent Conflict[END_REF]. An example of this was also the pandemic caused by COVID-19. In this period, all social structures in the world were put to the test. During the health crisis, adaptation of traditional innovation processes was necessary to overcome lack of resources, trade barriers and other contextual constraints to save lives and protect humanity.

In the BoP countries, even before the emergence of COVID-19, there have been major crises and problems related to the environment, justice and social equity. However, entrepreneurs, activists, and ordinary people have developed a significant number of creative, appropriate, and affordable solutions to address these issues [START_REF] Pansera | Crafting Sustainable Development Solutions: Frugal Innovations of Grassroots Entrepreneurs[END_REF]. Although these contexts are affected by the lack of and difficult accessibility to resources, local innovators have set some interesting precedents in the field of innovation. Their actions, strategies and ingenuity have generated ingenious low environmental impact responses focused on promoting social inclusion and fighting poverty.

In the academic and industrial community, this approach and its solutions have been identified as Frugal Innovation (FI). This concept encompasses a philosophy whose main idea is to do more with less for a greater number of people [START_REF] Radjou | Frugal Innovation, How to do more with less[END_REF]. FI highlights the capacity for creativity, sobriety and ingenuity to face all kinds of constraints and limitations [START_REF] Bhatti | Emerging Concepts in Innovation[END_REF], [START_REF] Vesci | How to save the world during a pandemic event. A case study of frugal innovation[END_REF]. It is because of the social and environmental benefits, that frugal innovations have found their place within companies, policy makers and academics.

Frugal Innovations have their merit and relevance, since in various scenarios of social and environmental crises they have been able to provide solutions and promote entrepreneurship and sustainability. So far, the literature has emphasized that in order to innovate frugally, a series of criteria and attributes focused on functionality, cost reduction and resource optimization must be taken into account [START_REF] Weyrauch | What is frugal innovation? Three defining criteria[END_REF]. However, although these findings have allowed a better understanding of FI, the literature also mentions that studies are needed to provide models or tables describing a FI as a process [START_REF] Hossain | Grassroots innovation: The state of the art and future perspectives[END_REF], [START_REF] Knizkov | Frugal Processes: An Empirical Investigation Into the Operations of Resource-Constrained Firms[END_REF].

In this paper we present an exploratory analysis of the mapping of a FI process at the BoP. Our study aims to identify the main stages and activities in the development of a technical solution under the FI approach. In addition to this, we present a p. 2 Colloque InterUT 2023 Systèmes sûrs et durables number of particular strategies implemented during the process to cope with the lack of resources and improve the solution.

For our analysis, we have studied the case of the development of a ceramic drinking water filter. This device is a response to the depletion and contamination of water that has affected more than a thousand communities and about 680,000 inhabitants in the central region of Mexico [START_REF] De | How We Work -Caminos de Agua[END_REF]. Applying a case study methodology [START_REF] Yin | Case Study Research, Design and Methods[END_REF], [START_REF] Eisenhardt | Building Theories from Case Study Research[END_REF], in-depth interviews were conducted with the founding director of the NGO Caminos de Agua. This included the elaboration of block diagrams to retrospectively represent their design process, as well as technical documents of the project [START_REF] Langley | Strategies for theorizing from process data[END_REF].

This article is structured as follows. First, we present a theoretical framework, then we describe the research methodology. Then we describe the FI case, its development process and the strategies implemented. We then discuss the implications and limitations of our study.

The article concludes with the final conclusion and outlook.

II. PRIOR RESEARCH

Frugal Innovation Approach

According to authors such as Khan [START_REF] Khan | How frugal innovation promotes social sustainability[END_REF] and Rosca, Reedy, & Bendul [START_REF] Rosca | Does Frugal Innovation Enable Sustainable Development? A Systematic Literature Review[END_REF], the implementation of FI can help to meet the United Nations sustainable development goals. FI have been able to provide solutions to problems in different sectors, such as transportation, health, access to water and energy, etc. [START_REF] Hossain | Pursuing Frugal Innovation for Sustainability at the Grassroots Level[END_REF], [START_REF] Hossain | Mapping the frugal innovation phenomenon[END_REF]. There is still an ongoing debate in the literature on how to define FI. The conceptual boundaries remain unclear, particularly with other innovation approaches that have originated in parallel to the development of a theory for innovation at the Base of Pyramid. Soni and Krishnan [START_REF] Soni | Frugal innovation: aligning theory, practice, and public policy[END_REF] mention that the originality of this approach to innovation is that it is not a single monolithic entity, but can be seen as a process, an outcome (product or service), a way of thinking, or a way of life. For Hossain [START_REF] Hossain | Mapping the frugal innovation phenomenon[END_REF], Frugal Innovation [...] is as a product, service or a solution that emerges despite financial, human, technological and other resource constraints, and where the final outcome is less pricey than competitive offerings (if available) and which meets the needs of those customers who otherwise remain un-served" (p. 199).

Advances in FI research have also made it possible to identify a series of criteria and attributes that must be present in a solution. These attributes focus on functionality, resource optimization, cost reduction and cultural acceptance of the product or service [START_REF] Pisoni | Frugal approach to innovation: State of the art and future perspectives[END_REF], [START_REF] Jagtap | Frugal-IDeM : An Integrated Methodology for Designing Frugal Innovations in Low-Resource Settings[END_REF][9], [START_REF] Santiago | Exploratory Study of the Integration of Frugal Innovation in the Design of Products for the BoP[END_REF]. Contextual factors such as socio-cultural and geographical elements are determinant in defining some specific attributes present in frugal innovations (robustness, ease of use, materials, etc.). Tiwari et al. [START_REF] Tiwari | Frugal Innovations in the mirror of scholarly discourse: Tracing theoretical basis and antecedents[END_REF] mention that engineering processes can benefit from including a frugality approach. Although research on FI has not reached a state of theoretical maturity, there is a consensus that its implementation has enabled has enabled the generation of innovative and affordable solutions to meet essential needs. Products and services obtained through an IF process have been key to address local problems and open new markets for both local and multinational companies [START_REF] Pansera | Crafting Sustainable Development Solutions: Frugal Innovations of Grassroots Entrepreneurs[END_REF], [START_REF] Rosca | Business models for sustainable innovation -an empirical analysis of frugal products and services[END_REF].

Water crisis at the BoP

The preservation and conservation of drinking water is part of the objectives of sustainable development [START_REF] Wehn | Exploring the dynamics of water innovation: Foundations for water innovation studies[END_REF]. However, global warming and overexploitation of this resource due to intensive agriculture and industrial activities have led to increasing scarcity, particularly in rural and urban areas of poor and developing countries. Developed countries have also recently been affected by more severe and constant periods of drought. Such was the case in France, during the year 2022, temperatures were the highest ever recorded in the country, which led to a period of significant drought [START_REF]Ministère de la Transition écologique et de la Cohésion des Territoires[END_REF]. If we take again the recent COVID19 pandemic, one of the main strategies to protect against Coronavirus was hygiene and hand washing. However, this measure was not possible for people who simply do not have access to water. According to the United Nations Human Rights Council [START_REF] Unhrc | COVID-19 will not be stopped without providing safe water to people living in vulnerability[END_REF], "The global struggle against the pandemic has little chance to succeed if personal hygiene, the main measure to prevent contagion, is unavailable to the 2.2 billion persons who have no access to safe water services". In this sense, providing safe, sustainable and accessible water for all requires immediate actions and commitments and solid planning [START_REF] Frank | Can COVID-19 accelerate urban water security?[END_REF].

Regarding water access solutions for marginalized areas, the industry has developed novel products. We can mention Tata Chemicals which is an affordable solution [START_REF] Agnihotri | Low-cost innovation in emerging markets[END_REF], LifeStraw from the Swiss company Vestergaard Frandsen, which can instantly purify water of bacteria by 99.9% [START_REF] Hoffman | LifeStraw Saves Those Without Access to Clean Drinking Water[END_REF]. Although these industrial products are affordable, it is necessary to consider that their development was made possible by high investments in R&D activities. In this sense, we can think that not all disruptive innovations involve a FI process [START_REF] Soni | Frugal innovation: aligning theory, practice, and public policy[END_REF]. Authors such as Hossain [START_REF] Hossain | Mapping the frugal innovation phenomenon[END_REF] and Frank [START_REF] Frank | Can COVID-19 accelerate urban water security?[END_REF] have highlighted that frugal innovations generated by local initiatives and actors are relevant for better water access management. Taking this into account, we consider that a study of the innovation processes and strategies of innovators in the BoP could enrich traditional approaches to solution development in this and other sectors. In other words, an analysis and study of these frugal innovation processes could give new perspectives to design and production practices given the current environmental and social challenges.

III. RESEARCH METHOD

As mentioned in the introductory part, the research and literature on FI has not yet reached a stage of maturity and research is still ongoing. Our research is exploratory in nature and the case studies are positioned as appropriate, as they allow us to have an in-depth description of the phenomenon to be studied [START_REF] Eisenhardt | Building Theories from Case Study Research[END_REF], [START_REF] Yin | Case Study Research, Design and Methods[END_REF]. This methodology has been previously applied to analyze examples of frugal innovations, both in emerging and developed economies [START_REF] Bendul | Sustainable supply chain models for base of the pyramid[END_REF], [START_REF] Molina-Maturano | Understanding and evaluating the sustainability of frugal water innovations in México: An exploratory case study[END_REF].

Case selection and data collection

Gibbert et al. [START_REF] Gibbert | What passes as a rigorous case study?[END_REF] and Eisenhardt [START_REF] Eisenhardt | Building Theories from Case Study Research[END_REF] suggest that the case should be selected for its theoretical relevance, i.e. that it provides detailed information on the research phenomenon. This led us to identify a case that corresponded to the development of a FI at the BoP in the water sector. Our exploratory study contemplates only one case, since our objective is not to generalize but to map a FI process. We analyze the case of Caminos de Agua, a Civil Association located in the State of Guanajuato Mexico. Caminos de Agua has developed a ceramic filter that has allowed access to drinking water to more than 30 marginalized communities in the region. Data were obtained through in-depth interviews with the director and founder of Caminos de Agua [START_REF] Rubin | Qualitative interviewing: The art of hearing data[END_REF]. The interviews were conducted in Spanish in two stages and then transcribed. We resorted to the use of manuals and reports of the Association to triangulate the data obtained [START_REF] De | Annual Report 2021, Meeting the Challenge[END_REF]- [START_REF] De | Filtro Cerámico de Agua Información Técnica[END_REF]. During the interview we asked the participant to make a retrospective mapping of the filter development process [START_REF] Langley | Strategies for theorizing from process data[END_REF], [START_REF] Blessing | DRM, a Design Research Methodology[END_REF], in order to identify the most important stages, key events and strategies of the process.

Data analysis

The data were analyzed inductively to identify main themes [START_REF] Gioia | Seeking Qualitative Rigor in Inductive Research: Notes on the Gioia Methodology[END_REF]. We used the mapping done by the interviewee as a primary source for structuring the innovation process. Subsequently, we began inductively coding the interview transcript using as first codes the concepts and themes expressed by the interviewee. Iteratively, we conducted a literature review to refine the themes expressed by the innovator that are directly related to frugal innovation research. To verify the reliability of our process [START_REF] O'connor | Intercoder Reliability in Qualitative Research: Debates and Practical Guidelines[END_REF], we included as coders two local engineers for their extensive knowledge of the Mexican context and their experience in social innovation. Finally, we obtained a chart that brings together the stages of the frugal innovation process for the development of the ceramic filter, as well as the main activities and strategies for each stage. This diagram was validated by the interviewee.

IV. FINDINGS

The ceramic filter of CAMINOS DE AGUA

The water crisis in central Mexico was the trigger for Caminos de Agua to begin experimenting with appropriate technologies focused on water filtration and purification. In collaboration with universities and local communities, Caminos de Agua has developed. The filter developed by Caminos de Agua is a clear example of frugal innovation at the BoP (Fig. 1). Although this filter may seem like a very simple device, it is actually a complete filtration system that complies with official Mexican water treatment standards. The filter developed by Caminos de Agua is certified by local authorities such as COFEPRIS (Federal Commission for the Protection against Sanitary Risks), is inexpensive, very effective in eliminating biological pathogens and can be adapted to any container.

The inexpensive ceramic filter has been constructed with local materials and is effective in eliminating biological pathogens, adapts to a wide range of containers and can generate more than 30,000 liters of potable water in its 5 year life span. The ceramic filter has had significant success and has been used by Mexican NGOs and humanitarian organizations throughout Latin America.

The Frugal Innovation Process of Caminos de Agua

The development of the ceramic filter is part of a project that began in 2010. While the ceramic filter is one of the most representative frugal solutions of Caminos de Agua, other community projects have been developed in parallel with the same objective. The first successful prototype filters were obtained around 2013. The highest performance filter that has been implemented in the communities was obtained between 2014 and 2015, when the certification process by the Mexican p. 4

Colloque InterUT 2023 Systèmes sûrs et durables health authorities took place. As can be seen, Caminos de Agua's frugal innovation process for the development of the filter required a few years, from its early stages to its maturation.

The frugal innovation process of Caminos de Agua for the development of the filter is presented in the Fig. 2. According to the Caminos de Agua experience, for an innovation to have the desired social and environmental impact, the 6 stages are essential. In the first stage, the motivation and commitment to provide a solution to an environmental or social justice problem is the triggering factor. According to the founder of Caminos de Agua, many of the BoP communities have essential needs that have not been addressed, even by local public policies. In the particular case of Mexico, it is intensive agriculture that has caused an overexploitation of deep reserves, leaving many rural communities without water. Therefore, knowing that we are working on a social welfare project is an essential driving force in the project. The second stage involves direct communication with the community, i.e., the priorities for action and the important problems to be solved must be respected and, above all, defined with them. The director of the Association mentions that the success of a FI project is based on the trust generated among all the stakeholders. Failure to create these bonds of trust will result in early abandonment of the project.

The third stage consists of conducting a series of diagnostics directly in the field, as these provide valuable information on the definition of the problem and an outline of possible actions to be implemented that will have the greatest social and environmental impact. Specific activities within this stage are the clear identification of needs and initial technical challenges. Once the essential need and initial technical requirements have been identified, the fourth stage comes into play. In this stage, the construction of the first prototypes that fulfill the main functions required begins. This stage has a series of iterative activities based on trial and error. The application of traditional knowledge (crafts, arts and empirical experience) and strategies such as DIY are common. The resources used do not have to be as sophisticated and many can be improvised. The ultimate goal is to obtain from this stage is to obtain a prototype that performs the desired functions satisfactorily. The notion of territorial resources emerges at this stage. In the specific case of the ceramic filter, it should be considered that the region in question is known for the quality of its ceramicbased products and crafts. Therefore, there is a latent knowledge and resource in this territory. According to the interviewee, at this stage, the inclusion of socio-cultural values is crucial. Sociocultural values strongly influence the acceptability of the technical solution by the community. Fig. 1 shows the textile element that has the function of covering and decorating the water container. Similarly, within the Mexican context, the shape of the plastic container refers to commercial drinking water (sold in this type of container) and is a symbol of clean water. Stage five is somehow the moment where a definitive technical solution has been reached. At this stage a transition is made from prototypes to final products. The identity of the innovation has been realized and the use of this technology has been generalized within the community. At this stage some process of certification, standardization or adherence to norms may take place. The director of Caminos de Agua emphasizes that the creation of alliances with other actors who share the same values and motivation will allow the dissemination and scalability of the technical solution and of the project in general. Standardization and certification processes can be long and costly, and in this sense, the economic sustainability of the project in general must be considered in parallel. Finally, stage six consists of the implementation of strategies to disseminate the project to other markets or communities with similar needs. Within this stage there may be redesign activities in order to adapt the components of the solution to the context where it will be implemented. If there was a certification in the previous stage, in this phase there is sufficient documentation and research on the project and the frugal innovation. Such documentation will allow thinking about the next improvements that can be made to the final p. 5

Colloque InterUT 2023 Systèmes sûrs et durables product. Constant monitoring and visits to the users of the innovation allows to evaluate the performance of the solution. Throughout the entire innovation process, there is one activity that remains constant, and that is monitoring, documentation and research. This activity makes it possible to iteratively evaluate the impact of the project and identify potential points for improvement. However, the social links that have been created among the stakeholders (including the user community) are key to obtaining relevant information. These stages were identified in the innovation process at Caminos de Agua for the development of the filter. However, the strategies at each stage were crucial to the development of the solution. Below, we present the innovation strategies implemented by the Caminos de Agua team.

Frugal innovations strategies

The literature on FI mentions a series of criteria and attributes that a high-impact frugal innovation should have. However, the presence of these attributes is a consequence of the implementation of some strategy. Through a content analysis we were able to identify a list of the strategies implemented in the innovation process of Caminos de Agua. Figure 2 shows in which stages these were most relevant. In stages one, two and three, which are oriented more towards approaching the community and understanding local needs and problems, communication and dialogues are indispensable. It is at this stage that the human social factor of communication is most relevant to create the trust necessary for collaboration. According to the interviewee, the designer or the work team that is formed must take ownership of the problem in order to understand it in depth and evaluate the technical complexity of a possible solution. For stages three, four and five, a series of strategies focused on do-it-yourself and appropriate use of resources were necessary. In the early stages of the project, there was no infrastructure, workshop documentation or financial resources. Since the construction of a prototype is an uncertain stage, the use of local materials and readily available resources is the best way to overcome the lack of resources.

Participatory design is an important strategy to involve users in the development of the solution. According to the literature, participatory design is a way to bridge cultural and social gaps between designers and users [START_REF] Sanders | Co-creation and the new landscapes of design[END_REF]. This is consistent with an experimentation phase that requires the inclusion of the empirical knowledge of community members [START_REF] Ssozi-Mugarura | Codesigning with communities to support rural water management in Uganda[END_REF]. In communities there are usually people who possess a large amount of knowledge accumulated from experience. This confirms what Gupta [START_REF] Gupta | Innovations for the poor by the poor[END_REF] argues about the combination of formal and informal sciences for the development of frugal innovation. When technical problems require deeper analysis, one should not hesitate to resort to modern engineering science to solve them. In any case, networking with research centers, universities, volunteers and other stakeholders is essential to overcome the problems.

In stages five and six, leveraging the networks created earlier becomes the main strategy. Scaling up the project in other contexts with similar problems is achieved by making a wide dissemination in the network of partners created. The director of Caminos de Agua states that it was thanks to the networks created that the project was able to continue, as there are people who have the necessary skills to support and achieve the objectives. The strategies identified in the innovation process of Caminos de Agua are based on the experience of the Association over the years. These strategies were also declined as specific activities as needed.

V. IMPLICATIONS AND LIMITATIONS

The exploratory nature of our study has limitations. In this research we have focused on frugal innovation in the BoP, particularly in the water sector due to the current challenges of access to this resource. Regarding the methodology employed, we have used a single case study whose main limitation is representativeness and generalization [START_REF] Yin | Case Study Research, Design and Methods[END_REF], [START_REF] Gibbert | What passes as a rigorous case study?[END_REF]. A single case study does not allow a quantitative analysis. However, the detailed description of a case using primary and secondary data sources (company documents, annual reports, etc.) allows to broaden the understanding of the study phenomenon within its context [START_REF] Dooley | Case Study and Theory Building[END_REF], [START_REF] Ridder | The theory contribution of case study research designs[END_REF]. Our article was limited to retrospectively mapping the frugal innovation process of the Asociación Civil Caminos de Agua. The identification of the strategies implemented was also retrospective. Although documentation of the project is available, the long duration of the project means that it is not possible to represent the entire experience of the innovative team. Our study does not present in depth how these strategies are integrated into each activity of the innovation process. As far as the FI is concerned, the research is still ongoing, therefore, it is possible that the concepts used in our article could be updated. The frugal innovation process obtained from the experience of Caminos de Agua could have similarities with other processes of product design and innovation already established in engineering sciences, management and creativity. However, it has the particularity of having been built from the experience of a Civil Association in the BoP. Include strategies that allow the proper development of each stage. These findings could enrich traditional design and innovation practices. Collaborative networks are strong when there is a shared vision of objectives and sensitivity to social and environmental issues. Resource sharing is an essential strategy of innovators at the base of the pyramid. We believe that our study requires further analysis to investigate the nature of these resources and their impact at each stage of the frugal innovation process. The case analyzed in this article corresponds to the reality and needs of p. 6

Colloque InterUT 2023 Systèmes sûrs et durables the Mexican context and the water sector specifically. Although the current water crisis is a global challenge, it is possible that some solutions may only work within the context in which they were developed. However, they can serve as inspiration and feedback for projects that address similar issues.

VI. CONCLUSION AND PERSPECTIVES

In this paper we study a case of frugal innovation in the Latin American region. According to Hossain [START_REF] Tiwari | Frugal Innovations in the mirror of scholarly discourse: Tracing theoretical basis and antecedents[END_REF] the BoP of this region represents a opportunity for further research on FI. The Caminos de Agua case allowed us to map the frugal innovation process of a successful technical solution and some of the most relevant strategies for its implementation. Our study can be enriched by comparing it with other frugal innovation processes in other sectors such as energy access. Similarly, analyzing other cases within the same sector and region would allow contrasting or confirming the results of our research. This exploratory study contributes to a better understanding of frugal innovation, particularly in its process facet. Finally, we believe that innovators, designers, academics and industrialists in other contexts at the base of the pyramid or in developed countries could be inspired by this case to implement similar processes in the development of projects that address current and future social and environmental challenges.

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Figure 1 .

 1 Figure 1. The Ceramic Filter (Source: © Caminos de Agua)

Figure 2 .

 2 Figure 2. The Frugal Innovation Process of Caminos de Agua (Author)

ACKNOWLEDGMENT

This research was carried out thanks to the support from the InSyTE Laboratory of the UTT. We thank the innovators of CAMINOS DE AGUA for their participation and willingness to share their story and experience (https://www.caminosdeagua.org/).