Sara T Daigle

John W Fleeger

James H Cowan

Pierre-Yves Pascal

What is the relative importance of phytoplankton and attached macroalgae and epiphytes to food webs on offshore oil platforms?

Keywords: Artificial reefs, Gulf of Mexico, macroalgae, epiphytes, oil platform consumers

Offshore oil platforms function as artificial reefs but little is known about the food webs that fuel platform-dwelling biota. Carbon and nitrogen isotopes in tissues of biota from two offshore oil platforms were analyzed to determine the potential importance of phytoplankton and platform-associated macroalgae and epiphytic algae to the diets of platform-dwelling consumers. Phytoplankton accounted for an estimated 13-90% and red algae accounted for 5-50% of the basal resource contribution to mobile primary consumers including crabs (Pseudomedaeus agassizii, Teleophrys pococki, Pachygrapsus transverses, Micropanope nuttingi), stenothoid and melitid amphipods and harpacticoid copepods. Sessile suspensionfeeding barnacles and oysters consumed phytoplankton and epiphytic algae, and likely redirected large amounts of pelagic production to small, platform-dwelling consumers. Melitid amphipods and secondary consumers including syllid polychaetes and the blennies Scartella cristata and Hypsoblennius invemar exhibited a distinct contribution from epiphytic algae although all potential basal resources had large, poorly resolved ranges in contributions.

Elemental and isotope analysis of the gut contents of individuals indicated that the nektonic grey triggerfish, Balistes capriscus, was omnivorous, directly consuming reef-derived red algae and epiphytes as well as animal tissue. Although reef-derived resources enter benthic and nektonic food webs surrounding platforms, phytoplankton is the dominant basal resource in the platform-dwelling communities we examined. Platform-derived benthic algae may therefore not be integral to food-web function on artificial reefs, and reef function should be similar in areas both favorable to and unfavorable to reef-associated algal growth.

Introduction

The term artificial reef refers to structures constructed or placed in waters for the purpose of enhancing fishery resources and commercial and recreational fishing opportunities. Some artificial reefs are built exclusively for this purpose. Other artificial reefs are formed by objects constructed for other purposes. In the Gulf of Mexico (Gulf) for example, over 4000 standing oil and gas platforms function as artificial reefs. Like other artificial reefs, platforms support abundant fouling communities of reef-dependent biota. However, platforms differ from most artificial reefs in that they rise to and above the sea surface allowing access to the photic zone for benthic primary producers that inhabit platform surfaces. In the Gulf, reefassociated primary producer communities are dominated by red (e.g., Polysiphonia sp.) and green (e.g., Derbesia sp.) macroalgae and epiphytic microalgae including diatoms and cyanobacteria [START_REF] Carney | Characterization of algal-invertebrate mats at offshore platforms and the assessment of methods for artificial substrate studies: final report[END_REF][START_REF] Gallaway | The ecology of petroleum platforms in the northwestern Gulf of Mexico: A community profile[END_REF][START_REF] Lewbel | Zonation of dominant fouling organisms on northern Gulf of Mexico petroleum platforms[END_REF]. However, the biomass of epibenthic communities on platforms is typically dominated by large, long-lived, sessile consumers including barnacles (e.g., Balanus tintinnabulum) and bivalves (e.g., Chama macerophylla and Ostrea spp.) that act as foundation species supporting a high abundance and diversity of small mobile reef-dependent organisms including amphipods, tanaids, harpacticoid copepods, polychaetes, crabs, and small reef-dwelling fishes [START_REF] Gallaway | The ecology of petroleum platforms in the northwestern Gulf of Mexico: A community profile[END_REF][START_REF] Lewbel | Zonation of dominant fouling organisms on northern Gulf of Mexico petroleum platforms[END_REF]. Furthermore, reef-associated nekton is routinely found in much higher abundances within ~ 60 m of offshore oil platforms than in open water (Stanley and Wilson, 1996;[START_REF] Boswell | Biomass, density, and size distributions of fishes associated with a large-scale artificial reef complex in the Gulf of Mexico[END_REF].

The role of benthic primary producers in food webs on and near artificial reefs is, in general however, poorly understood. Benthic macroalgae will be incorporated into the food web as a basal resource if grazed and assimilated by reef-dwelling consumers, including by suspension feeders that filter macroalgal detritus [START_REF] Kang | Food web structure of a restored macroalgal bed in the eastern Korean peninsula determined by C and N stable isotope analyses[END_REF]Schaal et al., 2010).

Although green algae generally lack chemical or physical defenses and are, therefore, susceptible to grazing [START_REF] Mcconnell | Effects of secondary metabolites from marine-algae on feeding by the sea-urchin, Lytechinus variegatus[END_REF], some species of red algae are unpalatable [START_REF] Kain | Marine ecology[END_REF] and may not contribute to the food web. Benthic microalgae are generally considered highly nutritious for consumers [START_REF] Miller | Microphytobenthos: the ecological role of the "Secret Garden" of unvegetated, shallow water marine habitats. II. Role in sediment stability and shallow water food webs[END_REF], and epiphytic microalgae could be directly grazed by mobile consumers from surfaces associated with reefs, or consumed by suspension feeders after entrainment into the water column. Alternatively, phytoplankton may be consumed by reef-dwelling suspension feeders (e.g., barnacles and bivalves; [START_REF] Gallaway | The ecology of petroleum platforms in the northwestern Gulf of Mexico: A community profile[END_REF], and then redirected to platforms in the form of feces or pseudofeces [START_REF] Beaver | Fishery productivity and trophodynamics of platform artificial reefs in the northwestern Gulf of Mexico[END_REF]. Small, mobile grazers may also consume phytoplankton directly after it adheres to surfaces or is trapped in crevices, or as a by-product of suspension feeding. Taxonomically similar grazing consumers studied in other environments have been found to be selective in their choice of resources [START_REF] Pascal | Seasonal variation in consumption of benthic bacteria by meio-and macrofauna in an intertidal mudflat[END_REF], but the basal resource of choice may change depending on habitat, environmental factors and resource availability [START_REF] Galván | Stable isotope addition reveals dietary importance of phytoplankton and benthic microalgae to saltmarsh infauna[END_REF].

Studies of food webs in hard-bottom ecosystems have described several possible and variable trophic pathways for phytoplankton and reef-associated algae. For example, algae growing on hard bottoms have been found to contribute to the diet of nearby consumers [START_REF] Kang | Trophic importance of benthic microalgae to macrozoobenthos in coastal bay systems in Korea: Dual stable C and N isotope analyses[END_REF][START_REF] Behringer | Stable isotope analysis of production and trophic relationships in a tropical marine hard-bottom community[END_REF][START_REF] Doi | Attached microalgae contribute to planktonic food webs in bays with fish and pearl oyster farms[END_REF][START_REF] Fukumori | Food sources of the pearl oyster in coastal ecosystems of Japan: Evidence from diet and stable isotope analysis[END_REF], and phytoplankton or epiphytes may be important basal resources to consumers inside dense macroalgal beds [START_REF] Kang | Food web structure of a restored macroalgal bed in the eastern Korean peninsula determined by C and N stable isotope analyses[END_REF][START_REF] Gollety | Complexity of the food web structure of the Ascophyllum nodosum zone evidenced by a delta(13)C and delta(15)N study[END_REF]Schaal et al., 2010). However, the relative importance of phytoplankton and endemic micro-and macroalgae to platform-based food webs has not been investigated.

Resolution of the role played by benthic algae on platforms may provide insight into the contribution that these structures make in the trophic dynamics in the Gulf and elsewhere.

High nekton abundance associated with platforms may be due to increased reef-based food resources that enhance biomass production (i.e., the production hypothesis for artificial-reef effects). Alternatively, nekton may aggregate to artificial reefs from surrounding areas to seek refuge from predators (i.e., the attraction hypothesis; [START_REF] Bohnsack | Are high-densities of fishes at artificial reefs the result of habitat limitation or behavioral preference[END_REF][START_REF] Polovina | Artificial reefs: Nothing more than benthic fish aggregators[END_REF]. These hypotheses are not mutually exclusive, and both processes may contribute to the enhancement of secondary production [START_REF] Pickering | Artificial reefs and fisheries exploitation: A review of the 'attraction versus production' debate, the influence of design and its significance for policy[END_REF][START_REF] Wilson | Artificial reefs, the attraction-production issue, and density dependence in marine ornamental fishes[END_REF]. Organic matter produced by benthic micro-and macroalgae will contribute to and enhance a productionbased, artificial-reef effect if benthic algae are incorporated as a basal resource into the diets of organisms living on and around platforms. If so, artificial-reef function may be linked to its endemic algae. However, if benthic algae are unpalatable or are consumed in relatively low amounts, then phytoplankton will serve as the dominant basal resource, and reef function would be less dependent upon reef-associated algae and the factors that control its productivity, composition and biomass. The conversion of platforms to artificial reefs has been proposed as a way to enhance conservation in the deep sea [START_REF] Macreadie | Rigs-to-reefs: will the deep sea benefit from artificial habitat?[END_REF], and shading in shallow water has been shown to affect the community structure of epibiota by reducing macroalgae and altering epifaunal communities (Glasby, 1994); yet current understanding makes it difficult to predict reef function under these conditions.

Basal resource determination and trophic interactions can be quantified using natural abundance isotope analysis. The ratios of heavy to light stable isotopes "track" food sources through a food web and determine trophic relationships among members of food webs [START_REF] Peterson | Stable isotopes as tracers of organic matter input and transfer in benthic food webs: A review[END_REF][START_REF] Fry | Stable isotope ecology[END_REF]. Metabolic differences in primary producers, such as phytoplankton and benthic microalgae, contribute to variation in the composition of naturally occurring isotopes. The low velocity of water in the benthic boundary layer partially equalizes uptake of heavy and light isotopes by benthic microalgae, while phytoplankton obtain nutrients from the water column opportunistically [START_REF] France | C-13 enrichment in benthic compared to planktonic algae -foodweb implications[END_REF]. As a result, epiphytic microalgae are generally enriched in the heavy isotopes ( 13 C and 15 N) while phytoplankton are relatively depleted.

Moreover, many species of red algae (Rhodophyta) are depleted in carbon relative to phytoplankton and epiphytic microalgae due to differences in inorganic carbon sources [START_REF] Raven | Implications of C-13 naturalabundance measurements for photosynthetic performance by marine macrophytes in their natural-environment. Marine Ecology-Progress Series 123:193-205 kelp (Laminaria digitata) forest, as investigated through stable isotopes and chemical assays[END_REF]. When basal resources are sufficiently different from one another, isotope analysis can be useful in determining the contribution of each resource to the diet of consumers.

This study was designed to assess the role of platform-based benthic algae in the food webs of communities residing directly on platforms in the Gulf. Two standing, operational oil platforms were examined. Both support fouling communities that span the entire water column. Dual C and N stable isotope analysis was performed on potential basal resources and consumers to test the null hypothesis that endemic primary production does not contribute to the diet of reef-dwelling consumers on platforms.

Methods

The study site was located approximately 130 km off the coast of Louisiana in the Eugene Island (EI) oil field near 28º 14'10" N, 91º 24'70" W in the northern Gulf (Figure 1). Two standing operational oil platforms located on EI blocks 325 and 346 were chosen for study.

These platforms began production in 1989 and 1998, respectively, and they are located near the continental shelf edge in water depths of 76 m (EI 325) and 95 m (EI 346).

Sampling Procedure

Primary producers and epibenthic consumers residing on platforms were collected in September 2009 and April 2010. These reef-dependent communities were sampled by SCUBA divers on two legs of each platform in September at 0-m, 6-m, 12-m, 18-m, and 30-m depths and in April at 1-m and 5-m depths. At each depth, divers removed barnacles and oysters (and incidentally resident organisms) from the surfaces of the platform legs by hand, placing contents into 250-µm mesh bags. Contents were frozen immediately and stored at -20 ºC. In the laboratory, samples were rinsed 2-3 times over 20-, 63-, and 250-µm mesh stacked sieves to remove and retain organisms that were attached to barnacles and oysters. Epiphytic microalgae was extracted from the 20-µm size fraction using a swirl and decant method following [START_REF] Galván | Natural abundance stable isotopes and dual isotope tracer additions help to resolve resources supporting a saltmarsh food web[END_REF] in which the contents of the sample that settle quickly after disturbance (i.e., heavier sediment and shell fragments) remain in one dish, while the more buoyant contents (i.e., epiphytes) are poured off into another. Epiphytes in the solution were collected on a Whatman glass filter and dried at 75 ºC for at least 48 h. Samples were fumed with HCl to dissolve inorganic carbonates. Animals from appropriate sieve fractions were extracted and identified to the lowest practical taxon. Amphipods in two families (Stenothoidae and Melitidae), 4 species of crab (Teleophrys pococki, Pseudomedaeus agassizii, Pachygrapsus transverses, and Micropanope nuttingi), polychaetes in one family (Syllidae), harpacticoid copepods as a composite taxon, and 2 blenny species (Scartella cristata and Hypsoblennius invemar) were selected for stable isotope analysis. Entire invertebrates and muscle tissue from fishes were used in the analysis. Individuals of the same taxon collected from a particular platform, leg, and depth were pooled into one sample. Not all taxa were available on both collection dates. Sample sizes ranged from 2 to 20 (Tables 1 and3) for platform-dwelling primary producers and mobile fauna on each collection date, and the number of individuals pooled in each sample ranged from 2 to 40 for amphipods, 1 to 4 for all crabs, 20 to 100 for syllid polychaetes, > 400 for harpacticoid copepods and was always 1 for blennies. Numbers of individuals depended both on the availability of organisms in a sample and the mass of individuals. Crabs were acidified using HCl to remove carbonates. The depressor muscle of the tergum of barnacles and the central adductor muscle of oysters were pooled from up to 5 individual barnacles and oysters from each depth for analysis. A total of 10 barnacle-(Balanus tintinnabulum) and 10 oyster-(Chama cf. macerophylla and Ostrea sp.) isotope samples were obtained in this fashion in September, representing over 60 individuals; sample sizes from April were smaller for barnacles (n = 4) and oysters (n = 2). The remaining shells of barnacles and oysters were scraped with forceps to remove attached macroalgae.

Using a dissection microscope, samples were cleaned of shell and debris and visually separated into categories of red or green macroalgae.

Water column particulate organic matter (WPOM), including phytoplankton, was sampled during quarterly cruises from 2008-10 around one or both platforms by towing a 30μm mesh plankton net for 2 to 3 5-min intervals at discrete depths ~250 m from platforms.

Samples around both platforms were collected in September, 2009; however, collections in April 2010 were subsequently misplaced and were unavailable for analysis. WPOM samples were immediately frozen and stored at -20 °C. WPOM was filtered to < 125 µm size fraction using a Whatman glass filter. Samples were fumed with HCl to dissolve inorganic carbonates before isotope analysis. A total of 56 WPOM samples for isotope analysis were obtained.

Four specimens of Balistes capriscus (grey triggerfish, a reef-dependent omnivore) were also collected near platforms by spear-fishing and hook and line in September and December, 2009 as a representative of the surrounding reef-dependent nekton community. Muscle tissue was extracted from the dorsal posterior region of each fish and stored at -20 ºC. The stomachs were removed immediately upon capture and stored in liquid nitrogen. In the laboratory, stomach contents of each fish were examined and descriptive records of the contents were taken. All contents of an individual stomach were pooled together into one sample for isotopic analysis.

Data Analysis

Stable isotope analysis results were determined by the following equation:

δ 13 C or δ 15 N = [(R sample /R standard )-1] x 1000,
where R sample is the ratio of the heavy to light isotopes in the sample (i.e. 13 C/ 12 C and 15 N/ 14 N) and R standard is the ratio of the heavy to light isotopes in PeeDee limestone for carbon values and nitrogen gas in the atmosphere for nitrogen values. Legs from the same platform were considered as subsamples of a platform and were pooled for isotope analysis.

The isotopic composition of carbon and nitrogen of primary producers were compared to each other in separate one-way ANOVAs. To determine if, and how pervasively, primary producers present on platforms were incorporated into the food web, isotopic compositions were compared among primary producers and consumers both graphically in unadjusted δ 13 C and δ 15 N bi-plots as well as quantitatively using a Bayesian isotopic mixing model. SIAR (Stable Isotope Analysis in R; [START_REF] Parnell | Source partitioning using stable isotopes: coping with too much variation[END_REF] incorporates the variability of sources, consumers, and trophic enrichment factors (TEFs) to produce a 95% confidence interval of the percent contribution of each source to a consumer. δ 13 C and δ 15 N values (mean ± 1SD) of three sources (red algae, phytoplankton, and epiphytic microalgae) were included in the model for September 2009. Green algae were included as a fourth source in April 2010 but were not included in the model for September due to its rarity (present in 10% of samples). The isotopic composition of WPOM (a surrogate for phytoplankton) in the Gulf (apart from spring blooms which occur in the immediate coastal zone, [START_REF] Grippo | Spatial variation in basal resources supporting benthic food webs revealed on the inner continental shelf[END_REF], typically shows relatively low levels of variability (Daigle, unpublished), and the average δ 13 C and δ 15 N values of WPOM collected in quarterly cruises were used as a surrogate for phytoplankton in April 2010 SIAR models. Observed δ 15 N values for September 2009 were, however, significantly different from other cruises (see Table 1), and September data were not used to estimate mean WPOM isotope values. Observed values for September were used in SIAR models for September. September and April δ 13 C and δ 15 N values of producers and platform consumers were compared with t-tests, or Mann-Whitney tests for data that failed normality, in order to determine if temporal isotopic shifts in producers contributed to shifts in consumer isotopic compositions. TEFs were obtained both from the literature [START_REF] Vander Zanden | Variation in delta N-15 and delta C-13 trophic fractionation: Implications for aquatic food web studies[END_REF] and from empirical comparisons of stomach and muscle isotopic compositions from nekton [START_REF] Daigle | What is the importance of oil and gas platforms in the community structure and diet of benthic and demersal communities in the Gulf of Mexico?[END_REF]. TEFs used for all invertebrates were 0.47 (± 1.23) for carbon and 2.92 (± 1.78) for nitrogen for all primary consumers. TEFs used for all fish were 1.52 (± 1.34) for carbon and 3.36 (± 1.95) for nitrogen.

Molar carbon:nitrogen ratios were used to support SIAR estimates of the stomach contents of Balistes capriscus.

Results

The most abundant basal resources accessible to consumers residing directly on platforms were phytoplankton, red algae, green algae, and epiphytic microalgae. Although we did not quantitatively sample macroalgae, relative abundances differed between the two sampling times. In September, green algae were abundant enough for isotope analysis in 2 of 20 samples, suggesting a very low relative abundance. Sufficient red algae was available for analysis in each sample (n = 20) in September, and green and red algae also were available in each sample (n = 4) in April. Based on a qualitative assessment, epiphytic microalgae consisted mostly of pennate diatoms. WPOM was used as a surrogate for phytoplankton. Tests for differences in isotopic composition (based on t-tests or Mann-Whitney tests for data that failed normality) were not significant between platforms (EI 325 and EI 346) for most producers and consumers. Therefore, the two platforms were considered replicates and data were pooled in all analyses.

In September, mean phytoplankton isotope composition was -19.71 and 2.42‰ for δ 13 C and δ 15 N, respectively. Averaged across quarterly collections from 2008-10 (excluding September 2009), phytoplankton δ 13 C and δ 15 N values were -20.78 and 6.33‰ respectively (Table 1). These long-term mean isotope values were used for phytoplankton in April 2010 for all calculations.

Isotope composition varied among primary producer and consumer taxa and between the two sampling times. In September, δ 13 C ranged from -28.17 to -15.76 and in April, from -25.86 to -18.73 among primary producers. δ 15 N ranged from 2.42 to 4.67‰ in September and from 2.30 to 6.34‰ in April among primary producers. In both September and April, red algae were significantly depleted in δ 13 C relative to phytoplankton (Table 2). Epiphytes were significantly enriched in δ 13 C and δ 15 N relative to phytoplankton in September, but not in April.

Green algae in April were significantly depleted in δ 15 N relative to phytoplankton, but not in September. δ 13 C of phytoplankton and epiphytes was lower in April (by 1.07‰ and 2.97, respectively) than September, but this difference was significant only in phytoplankton (Table 1). Mean δ 15 N of both red and green algae was higher in September than in April (by 1.14 and 0.49 ‰, respectively), but this difference was not statistically significant (Table 1).

Phytoplankton and epiphytes were lower in September (by 3.91 and 1.67‰, respectively) than April; this difference was significant for phytoplankton (p < 0.001), but not for epiphytes (p = 0.096). No differences in δ 13 C were found between September and April for any consumer (Table 3). However, all mobile consumers had significantly lower δ 15 N values in September than in April (Table 3). Crabs had the largest change in δ 15 N, about 3‰ for each species; other taxa were lower in September by 1.25 to 1.88‰. Large, long-lived suspension feeding oysters and barnacles differed less between the two sampling times than small, mobile taxa; the difference in δ 15 N between September and April was 0.28 and -0.67‰ in oysters and barnacles respectively.

To determine which basal resources were incorporated by the fouling community, isotope composition of primary producers was compared to that of reef-dwelling consumers (Figures 2 and3). The unadjusted δ 13 C values of consumers ranged from -22 to -17.6 ‰ in September (Table 3, Figure 2) and from -20.1 to -17.8 ‰ in April (Table 3, Figure 3); both ranges were similar to phytoplankton. The δ 15 N values of consumers ranged from 3.7 to 12.1‰ in September (Table 3, Figure 2) and from 5.8 to 9.3 ‰ in April (Table 3, Figure 3).

In September, 95% confidence intervals obtained from the SIAR basal resource model (using red algae, epiphytes and phytoplankton as sources) were generally broad but indicates that most platform consumers utilized more than one basal resource (Table 4). The highest maximum possible contribution for most taxa was from phytoplankton. For the crabs Pseudomedaeus agassizii, Pachygrapsus transverses,Teleophrys pococki and Micropanope nuttingi, harpacticoid copepods and stenthoid and melitid amphipods (all of which appear to feed as primary consumers based on their δ 15 N values, Table 4), the largest fraction of the diet was estimated to be derived from phytoplankton (confidence intervals ranged from ~13-90%), with a smaller but important fraction of the diet coming from red algae (CI = ~10-50%).

Epiphyte minimums were near 0 for these taxa (except for melitid amphipods) although possible epiphyte maxima ranged widely (from ~20-43%, Table 4). Estimated diets of melitid amphipods, syllid polychaetes and fishes (S. cristata, H. invermar and B. capriscus) had higher minimum contributions from epiphytes (at least 10%, and for syllid polychaetes, 30%), and maximum contributions from all sources were potentially large (35-80%), indicating either poor resolution or the use of multiple basal resources. Of these, syllid polychaetes and fishes were feeding at the level of secondary consumer or higher based on 15 N values (Table 4). Long-lived suspension feeders, i.e., barnacles and oysters, indicated high potential contributions from epiphytes and phytoplankton while minimums from red algae were near zero (Table 4).

Minimums for epiphytes were 42 and 14% for barnacles and oysters respectively. Barnacles were enriched in 15 N, suggesting they feed as primary and secondary consumers.

In April, 95% confidence intervals around the percent contribution from the SIAR model of the 4 potential basal resources (red algae, green algae, epiphytes and phytoplankton) for consumers were broad and overlapping, indicating a poor ability to resolve the food web. Most primary producers had lower confidence limits < 5% and upper limits ranged from 23-77% (Table 5). Again, highest possible maximums contribution for most consumers came from phytoplankton. Confidence intervals for the contribution of phytoplankton to the crab P.

transverses and for barnacles were best defined and ranged from 14-75%, and from 12-77% respectively (Table 5). 4).

However, specimens 1 and 2 had confidence intervals around stomach content similar to the muscle mean value, and had molar C:N ratios of 5.31 and 6.26. Specimen 3 suggested significant contributions from red algae (95% confidence intervals for red algal contribution was 36-89%) while confidence intervals for phytoplankton and epiphytes were broad and included 0 (Table 4). The molar C:N ratio of the gut contents of specimen 3 was 10.81. Specimen 4 had a δ 13 C value very similar to epiphytic diatoms. The 95% confidence interval around the percent contribution of epiphytic microalgal to specimen 4 was 19-84% (Table 4). The molar C:N ratio of this specimen was 8.56.

Discussion

Conditions in September 2009 allowed for the delineation of basal resources for several of the platform-dwelling consumers studied. For example, phytoplankton and red algae were separated by large differences in δ 13 C (-19.1 for phytoplankton and -28.2‰ for red algae), and phytoplankton and epiphytes differed in both δ 15 N and δ 13 C. The potential to differentiate basal resources in the diet of consumers in April 2010 was lower, however, because primary producer isotope composition was similar. Significantly, a large temporal isotopic shift occurred between September 2009 and April 2010 and differed in direction among primary producers (i.e., δ 15 N increased in epiphytes and phytoplankton across this time but decreased in red and green algae). Such temporal shifts facilitate isotope tracking and can be powerful Our dietary estimates for primary consumers on the platforms are especially well supported. Two lines of evidence suggest that phytoplankton was the most important food resource for a diverse group of mobile consumers that included the crabs Pseudomedaeus agassizii, Pachygrapsus transverses, and Teleophrys pococki, harpacticoid copepods, and stenthoid and melitid amphipods. Firstly, the SIAR model indicated that confidence intervals around the estimated contribution of phytoplankton to the diet among these taxa ranged from 34-90%. In addition, the crab Micropanope nuttingi was estimated to have a large contribution of phytoplankton in its diet, with confidence intervals ranging from 13-71%. Secondly, the δ 15 N for these taxa increased significantly with the increase in phytoplankton δ 15 N from September to April, further supporting the importance of phytoplankton as a basal resource for these small, relatively short-lived primary consumers. For the crabs in particular, the temporal shift in δ 15 N was large and quantitatively similar to the shift in phytoplankton (both were ~3‰, Table 3), and suggests the consumption of macroalgae was of little importance even though reef crabs are often considered algal grazers (e.g., [START_REF] Barry | Diet, food preference, and algal availability for fishes and crabs on intertidal reef communities in Southern California[END_REF]. Similarly, [START_REF] Kang | Food web structure of a restored macroalgal bed in the eastern Korean peninsula determined by C and N stable isotope analyses[END_REF] reported that the decorator crab Hyastenus elongatius living in a macroalgal bed primarily consumes phytoplankton. Consumers may select for phytoplankton resources over red algae due to increased palatability [START_REF] Kain | Marine ecology[END_REF] because secondary metabolites in red and brown algae can reduce palatability and deter grazers [START_REF] Vallim | Feeding-deterrent properties of diterpenes of Dictyota mertensii (Phaeophyceae, Dictyotales)[END_REF]. In addition, the biomass and production of platform-associated algae may be overshadowed by the quantity of phytoplankton made available to consumers in seawater flowing over artificial structures. [START_REF] Beaver | Fishery productivity and trophodynamics of platform artificial reefs in the northwestern Gulf of Mexico[END_REF] estimated that carbon fixation of benthic algae on platforms accounts for 1% of the gross primary production and that the majority of the carbon on platforms is derived from pelagic phytoplankton.

The SIAR model estimated that red algae were the second most important basal resource for taxa in this group (except for melitid amphipods), with confidence intervals of contributions ranging from 5-43%. The crab M. nuttingi also was found to consume red algae, with confidence intervals that ranged from 20-50%. Epiphytes were of relatively minor importance with confidence interval minimums close to or at 0 for these consumers (again except for melitid amphipods in which epiphytes were the second highest contributor). Reef crabs are likely capable of directly ingesting macroalgae after tearing it, but little is known about assimilation efficiencies of red algae, although some invertebrates are able to assimilate it with high efficiency (e.g., [START_REF] Cox | Feeding preferences and the relationships between food choice and assimilation efficiency in the herbivorous marine snail Lithopoma undosum (Turbinidae)[END_REF]. Smaller reef-dependent taxa with mouthparts that are unable to tear macroalgae, i.e., amphipods and harpacticoid copepods, also consumed red algae (perhaps decomposing red algae). Although red algae were very abundant and readily available on the platform at the time of our study, they were not the dominant basal resource for any consumer, including crabs (that might be expected to consume large quantities of macroalgae) and suspension feeders that may benefit from drift algae.

Similar findings that the dominant macroalga is not the most common prey item have been reported for grazers on rocky shores and some artificial reefs [START_REF] Kang | Food web structure of a restored macroalgal bed in the eastern Korean peninsula determined by C and N stable isotope analyses[END_REF][START_REF] Gollety | Complexity of the food web structure of the Ascophyllum nodosum zone evidenced by a delta(13)C and delta(15)N study[END_REF]Shaal et al., 2010).

Large, sessile, suspension-feeding epibiota comprise most of the biomass of platformdwelling communities [START_REF] Gallaway | The ecology of petroleum platforms in the northwestern Gulf of Mexico: A community profile[END_REF]. The long-lived barnacles and oysters dominant on the platforms showed a distinct dietary importance of epiphytes (confidence intervals generated by the SIAR model for the contribution from epiphytes ranged from 42-87% for barnacles and 14-57% for oysters) in September, although the contribution from phytoplankton could be substantial for both taxa (as high as 49% of the diet in barnacles and 80% in oysters). A significant contribution from macroalgae was not indicated, however, even though studies in macroalgal beds have shown that a large contribution of macroalgae to the diet of suspension feeders may occur from consumption of drift or detrital algae [START_REF] Kang | Food web structure of a restored macroalgal bed in the eastern Korean peninsula determined by C and N stable isotope analyses[END_REF]Schaal et al., 2010). The SIAR model may be less accurate for slow-growing suspension feeders because tissue isotope composition represents long-term means for ingested C and N, and tissue composition was therefore likely not in equilibrium with primary producers at the time of collection. Furthermore, the observed shifts in phytoplankton and epiphyte isotope values between September and April were not reflected in barnacle or oyster tissue. It is therefore possible that phytoplankton is more important to the diet of suspension feeders than indicated by the model. Barnacles were more enriched in 15 N than oysters suggesting they feed on a mix of primary producers and primary consumers, as do some other suspension feeders [START_REF] Dupuy | Feeding rate of the oyster Crassostrea gigas in a natural planktonic community of the Mediterranean Thau Lagoon[END_REF][START_REF] Lonsdale | Influence of suspension-feeding bivalves on the pelagic food webs of shallow, coastal embayments[END_REF][START_REF] Gollety | Complexity of the food web structure of the Ascophyllum nodosum zone evidenced by a delta(13)C and delta(15)N study[END_REF]. Barnacles may consume entrained single-celled consumers and microzooplankton from the platforms that ingest epiphytes. [START_REF] Beaver | Fishery productivity and trophodynamics of platform artificial reefs in the northwestern Gulf of Mexico[END_REF] measured energy flow and production rates on two platforms in the northern Gulf and suggested that suspension feeders assimilate carbon (and therefore obtain energy) from the water-column and redirect it to reef-dependent and reef-associated consumers. Their resulting secondary production, feces and/or pseudofeces may be consumed by reef-dependent and reef-associated organisms. Our data also suggest that suspension feeders may make phytoplankton and perhaps epiphytes available to small mobile consumers on platforms.

A group of platform-dwelling, secondary consumers enriched in 15 N (including sylllid polychaetes and the blennies S. cristata and H. invemar) was notable because of the higher diversity of basal contributions and contributions from epiphytic algae could be high.

Confidence intervals of the percent contribution from epiphytes ranged from 10-70 for these taxa, and epiphytes had the highest minimum dietary contribution in syllids. Syllid polychaetes were present in both collections, and the increase in epiphyte δ 15 N (1.67‰) was similar to the increase in δ 15 N (1.10‰) in syllid tissue from September to April. Other possible basal resources had large ranges among these consumers suggesting either a diverse range of resources were used or that the SIAR model was poorly resolved. [START_REF] Fauchald | The diet of worms: A study of polychaete feeding guilds[END_REF] reported that syllid polychaetes sometimes feed on smaller encrusting invertebrates such as hydroids and bryozoans that feed on epiphytes (Shaal et al., 2010). The dietary importance of epiphytic algae to shallow-water consumers may be great (e.g., [START_REF] Galván | Stable isotope addition reveals dietary importance of phytoplankton and benthic microalgae to saltmarsh infauna[END_REF] The fishes we studied are known to be omnivorous on platforms. [START_REF] Mobley | Diet of Scartella cristata: An artificial habitat-associated blenny (Pisces: Blenniidae)[END_REF] and [START_REF] Mendes | Diet and trophic plasticity of an herbivorous blenny Scartella cristata of subtropical rocky shores[END_REF] found that S. cristata consumes algae and small invertebrates such as amphipods on rocky reefs, and the grey triggerfish, B. capriscus, can feed on algae, and soft-bodied and protected (i.e., barnacles and oysters) prey (Vose and Nelson, 1994). Isotope composition of all three fishes was very similar, suggesting the use of similar food resources. A diverse use of food-web resources by these consumers may also explain the isotope composition for these secondary consumers. In a recent review, [START_REF] Mccann | The more food webs change, the more they stay the same[END_REF] concluded that most secondary consumers feed on herbivores utilizing all extreme primary producer end members of food webs, increasing the diversity of basal food sources.

Although our study was focused on the platform-dwelling community, we sought evidence for a platform-based isotopic signal in triggerfish as a representative of the surrounding nektonic food web. We examined both the muscle and gut content isotope composition of triggerfish in an effort to better understand how omnivory affects isotope composition. Individual B. capriscus stomach contents were highly variable. One of the four fish examined had a high estimated contribution from red algae (with confidence intervals ranging from 36-86%) and one had a high estimated contribution from epiphytes (19-84%).

Molar C:N ratios suggest that both individuals consumed plant tissue directly [animal tissue typically has a molar C:N ratio of approximately 3-5 (McConnaughey and McRoy, 1979), while benthic plant tissue typically has a ratio > 10, [START_REF] Atkinson | C:N:P ratios of benthic marine plants[END_REF]]. Two other individuals had last consumed a meal of animal tissue, based on gut content C:N ratio. The omnivory observed in the isotopic analysis of B. capriscus stomach contents was poorly reflected in the SIAR analysis of muscle tissue. It is possible that triggerfish assimilate red algae with low efficiency. Because muscle tissue isotopic composition represents an average of the diet over several months, evidence of omnivory of B. capriscus may be obscured when analyzing only muscle tissue. It is possible that reef-dependent resources are channeled through omnivorous fishes to other nekton in the reef-based food web but this transfer may be difficult to detect with isotopes. We also sampled selected species of non-reef-dwelling, benthic-feeding fish and bottom-dwelling scallops in the vicinity of these platforms (data reported in [START_REF] Daigle | What is the importance of oil and gas platforms in the community structure and diet of benthic and demersal communities in the Gulf of Mexico?[END_REF].

Isotope composition was nearly identical in all samples of the same species including those 250 m, 1.5 km and > 6 km distant from platforms, indicating that platform-derived resources contribute little to food webs surrounding platforms. More food-web research is needed to determine the importance of reef-dependent primary producers in the larger platformassociated food web.

The general assumption of the production hypothesis to explain the artificial reef effect is that platforms stimulate novel production by platform-dwelling primary and secondary producers, which fuels higher growth rates of resident and transient nekton [START_REF] Bohnsack | Are high-densities of fishes at artificial reefs the result of habitat limitation or behavioral preference[END_REF].

Increased food resources are often cited as the main contributor to higher densities of commercially important fish as well as other nekton around platforms [START_REF] Bull | An indication of the process -offshore platforms as artificial reefs in the Gulf of Mexico[END_REF][START_REF] Gallaway | The ecology of petroleum platforms in the northwestern Gulf of Mexico: A community profile[END_REF]Stanley and Wilson, 1996;[START_REF] Stone | Comparison of fish populations on an artificial and natural reef in the Florida Keys[END_REF]. Our work suggests that phytoplankton and platform-based micro-and macroalgae all contribute to the diets of platform-dwelling animals. However, dietary contributions from red algae and epiphytes were generally smaller than contributions from phytoplankton and were significant in specific taxa, not dominant throughout the platform-dwelling assemblage. Some primary consumers utilized smaller amounts of red algae, while others, especially secondary consumers, had a broader mix of basal resources including epiphytes. Given the high flow rates and large volumes of seawater passing through and over platforms [START_REF] Keenan | Zooplanktivory by Blue Runner Caranz ctyos: a potential energetic subsidy to Gulf of Mexico fish populations at petroleum platforms[END_REF] and the potential ability of suspension feeders to capture and redirect phytoplankton to platforms, it seems important to ask if platforms serve primarily as sinks, concentrating pelagic primary production, rather than significant producers of organic matter that fuel nektonic food webs. There are several ways to further test the sink vs. source conjecture. The use of highly enriched isotopes to label the benthic algae accessible to platform consumers may yield more definitive conclusions on the contribution of epiphytes and macroalgae to consumers including suspension feeders [START_REF] Drake | Salt marsh ecosystem biogeochemical responses to nutrient enrichment: A paired n-15 tracer study[END_REF][START_REF] Galván | Stable isotope addition reveals dietary importance of phytoplankton and benthic microalgae to saltmarsh infauna[END_REF][START_REF] Galván | Natural abundance stable isotopes and dual isotope tracer additions help to resolve resources supporting a saltmarsh food web[END_REF]. Fatty acid composition can also be used to determine the contribution of food resources to nektonic consumers [START_REF] Kharlamenko | Determination of food sources of marine invertebrates from a subtidal sand community using analyses of fatty acids and stable isotopes[END_REF][START_REF] Carney | Characterization of algal-invertebrate mats at offshore platforms and the assessment of methods for artificial substrate studies: final report[END_REF]. Energy flow estimations can be used to compare the rate of macroalgae and epiphyte production with the grazing rate or capacity of suspension feeders as well as other consumers (see [START_REF] Beaver | Fishery productivity and trophodynamics of platform artificial reefs in the northwestern Gulf of Mexico[END_REF].

Decommissioned platforms are on occasion converted into permanent artificial reefs as part of rigs-to-reefs programs to preserve ecosystem function that develops on the structures during production [START_REF] Dauterive | Rigs-to-reefs policy, progress, and perspective[END_REF][START_REF] Kaiser | The Louisiana artificial reef program[END_REF][START_REF] Macreadie | Rigs-to-reefs: will the deep sea benefit from artificial habitat?[END_REF]. Decommissioned platforms are either toppled or partially removed (the top portion is removed and placed adjacent to the base), reducing the height of the structures in order to prevent interference with vessels [START_REF] Macreadie | Rigs-to-reefs: will the deep sea benefit from artificial habitat?[END_REF]. In deep or highly turbid shallow water, the rigs-to-reef conversion reduces surface area of or eliminates hard substrates that reach into the photic zone. If communities are highly dependent on platform-based primary production, then this policy would lead to a decrease in artificial reef function. However, in this study we did not find that consumers were highly dependent on reef-dwelling benthic primary production. Instead, we estimated that phytoplankton is the most important basal resource to these communities.

Phytoplankton may be grazed by the high biomass of suspension-feeders attached to platforms or may be accessible to mobile consumers as it comes in contact with and adheres to the complex structures of platforms and epibiota. Reef-dependent suspension feeders have high rates of secondary production and egestion that may be passed on to reef-dependent consumers [START_REF] Beaver | Fishery productivity and trophodynamics of platform artificial reefs in the northwestern Gulf of Mexico[END_REF][START_REF] Carney | Characterization of algal-invertebrate mats at offshore platforms and the assessment of methods for artificial substrate studies: final report[END_REF]. Thus, reef function may not be strongly affected by factors (such as light) that influence platform-associated primary production, and the functional significance of artificial reefs in the aphotic zone might be expected to be equivalent to those in the photic zone. If so, rigs-to-reefs programs may be successful in the deep sea and artificial reefs in shaded areas or the aphotic zone should function as expected.

Fisheries-resource policies often manage resources at platforms by assuming they increase fish production. Our research does not invalidate this premise, but suggests a re-evaluation of the trophic pathways that lead to increased fish production is desirable. 3. 3. OYSTERS-Ostrea sp.

POLYCHAETA-Syllidae

  Because δ 13 C values of the stomach contents of B. capriscus were highly variable and sometimes different than muscle mean values, individual values were used in the SIAR model for September. Mean muscle values of the 4 fish combined indicate broad 95% confidence intervals around contributions of red algae, phytoplankton, and epiphytic microalgae (Table

  tools in determining the relative importance of basal resources (McCutchan and Lewis 2002; Perga and Gerdeaux 2005; Grippo et al. 2011).

  but little comparative research has been conducted on the functional role of epiphytes on open water reefs.

Figure 1 .

 1 Figure 1. Location of study area and oil platforms sampled (insert).

Figure 2 .

 2 Figure 2. δ 13 C vs. δ 15 N bi-plot of consumers residing on platform legs obtained during the September 2009 cruise along with primary producers. Primary producers are expressed as means with error bars (±1 SD). Though error bars are not included on consumers, these data points represent means of 3-20 samples of pooled individuals; standard deviations are reported in Table3.

Figure 3 .

 3 Figure 3. δ 13 C vs. δ 15 N bi-plot of consumers residing on platform legs obtained during the April 2010 cruise along with primary producers. Primary producers are expressed as means with error bars (±1 SD). Though error bars are not included on consumers, these data points represent means of 1-4 samples of pooled individuals; standard deviations are reported inTable 3.

  Figure 2.

Table 1 .

 1 Mean (standard deviation) δ 13 C and δ 15 N values of primary producers near or on platforms used in SIAR mixing models.Note that 15 N values for phytoplankton and epiphytes increased from September 2009 to April, 2010 while 15 N values for red and green algae decreased. t-test (for normal data) or a Mann-Whitney test (for data that failed normality) results comparing Sept and April for δ 13 C and δ 15 N are reported as p values (bold p-values are significant).

			Sept.09			April 10		p, δ 13 C p, δ 15 N
	Source	δ 13 C	δ 15 N	Sample	δ 13 C	δ 15 N	Sample
				size			size
	Phytoplankton -19.71 (1.19) 2.42 (0.94)	12	-20.78 (1.31)* 6.33 (0.96)*	44	0.0303 <0.001
	Red Algae	-28.17 (3.48) 3.44 (0.65)	12	-25.86 (3.65)	2.30 (2.14)	4	0.2493 0.3665
	Epiphytes	-15.76 (3.25) 4.67 (1.41)	20	-18.73 (1.36)	6.34 (1.75)	5	0.0599 0.0960
	Green Algae	-19.68 (0.11) 4.15 (0.08)	2	-22.91 (7.42)	3.66 (1.25)	3	0.7728 0.7728

*Phytoplankton was not collected in April 2010 and values shown are means of a series of quarterly collections (phytoplankton values from September 2009 were not included).

Table 2 .

 2 Pair-wise comparisons of δ 13 C and δ 15 N values of potential sources of primary producers on and near platforms collected in September 2009 and April 2010. Tukey test results are reported as p-values (significant p-values are in bold).

		September 2009	April 2010
	Producer Comparisons	δ 13 C	δ 15 N	δ 13 C	δ 15 N
	Phytoplankton vs. Red Algae	<0.0001 0.1264	0.0002 <0.0001
	Phytoplankton vs. Epiphytes	0.0029 <0.0001 0.1827	1.000
	Phytoplankton vs. Green Algae	1.000	0.194	0.3359	0.0016
	Red Algae vs. Epiphytes	<.0001	0.0223 <0.0001 <0.0001
	Red Algae vs. Green Algae	0.0022	0.8421	0.272	0.4136
	Epiphytes vs. Green Algae	0.2724	0.9219	0.0435	0.0124

Table 3 .

 3 Mean δ 13 C and δ 15 N (standard deviation) of consumers from platform communities sampled in September 2009 and April 700 2010. p-values compare September and April isotopic composition within each taxon. Significant p-values (p < 0.05) are in bold.

	701
	702
			Sample Size		δ 13 C			δ 15 N
	Fouling Community
			Sept.	Apr.	Sept.	Apr.	p	Sept.	Apr.	p
	Crabs				Mean (SD)	Mean (SD)		Mean (SD)	Mean (SD)
		Pseudomedaeus agassizii	11	3	-21.04 (0.73)	-20.10 (1.07)	0.0919	5.14 (1.13)	8.16 (0.60)	0.0009
		Pachygrapsus transversus	9	3	-19.91 (0.92)	-19.53 (0.14)	0.2609	5.11 (0.96)	8.12 (0.98)	0.0009
		Teleophrys pococki	11	1	-21.49 (1.07)	-20.6	0.4408	3.67 (1.01)	6.63	0.0186
		Micropanope nuttingi	5	--	-21.27 (1.08)	--	--	5.63 (1.03)	--	--
	Copepoda
		Harpacticoida	20	4	-20.63 (0.86)	-20.42 (1.00)	0.6758	3.91 (0.72)	5.79 (0.59)	0.0001
	Amphipoda
		Stenothoidae	11	4	-21.20 (0.85)	-20.97 (0.59)	0.6380	4.58 (0.79)	5.83 (0.27)	0.0099
		Melitidae	13	4	-19.80 (0.69)	-19.39 (0.53)	0.3375	5.68 (0.69)	7.13 (0.42)	0.0013
	Polychaeta
		Syllidae	20	4	-18.74 (0.48)	-18.62 (0.42)	0.6528	8.34 (0.60)	9.44 (0.81)	0.0046
	Suspension-
	Feeders	Balanus sp.	10	4	-17.55 (1.91)	-17.99 (0.25)	0.5356	8.65 (1.31)	9.32 (0.75)	0.3613
		Ostrea sp.	10	2	-19.02 (0.49)	-18.31 (0.07)	0.0789	6.93 (1.85)	6.65 (0.24)	0.3544
	Blennidae
		Scartella cristata	11	--	-18.89 (0.27)	--	--	10.94 (0.94)	--	--
		Hypsoblennius invemar	10	--	-18.50 (0.24)	--	--	12.08 (0.69)	--	--
	Nekton
		Balistes capriscus muscle	4	--	-17.83 (0.55)	--	--	10.84 (0.74)	--	--

Acknowledgments

We thank M. Grippo, K. Simonsen, C. Saari, J. Saari, D. Kulaw, D. Johnson, and K. Limon for their help in field sampling and sample preparation. Thanks also to B. Fry and K. Harms for helpful comments, D. Felder for assistance in crab identifications, and the University of California at Davis for isotope analysis. This research was funded through the Louisiana Department of Wildlife and Fisheries and the Louisiana Artificial Reef Program.