Microscale variations of food web functioning within a rocky shore invertebrate community

The assessment of relevant spatial scales at which ecological processes occur is of special importance for a thorough understanding of ecosystem functioning. In coastal ecosystems, the variability of trophic interactions has been studied at different spatial scales, but never at scales from centimetres to metres. In the present study, we investigated the link between habitat structure and small-scale variability of food web functioning on intertidal boulder field ecosystems. Two microhabitats, boulder-top and boulder-bottom, were considered, and the trophic ecology of invertebrate consumers was studied using stable isotope tracers. We found for two of the main suspension feeders of northern Atlantic rocky shores (the sponges Halichondria panicea and Hymeniacidon sanguinea) consistent ¹⁵N enrichment for individuals sampled under boulders, suggesting that these consumers relied on different trophic resource according to the microhabitat inhabited, at a centimetre scale. The high δ¹⁵N signatures found underneath boulders suggested higher use of highly decomposed organic matter in this microhabitat. The isotopic difference between the two microhabitats decreased in higher trophic level consumers, which likely foraged at a spatial scale including both microhabitats. Finally, our results reveal that in highly heterogeneous habitats such as boulder fields, trophic interactions are likely to vary strongly in space, which should be considered in future researches. The link between habitat physical structure and food web variability might also contribute to the high biological diversity characterizing heterogeneous ecosystems.     

Food web analysis in intertidal <Emphasis Type="Italic">Zostera marina</Emphasis> and <Emphasis Type="Italic">Zostera noltii</Emphasis> communities in winter and summer

The food web of two intertidal seagrass (Zostera marina and Zostera noltii) beds that may be influenced by the seasonal variation in food source abundance was studied in winter and in summer with δ¹³C and δ¹⁵N analysis. In spite of high relative variation of abundance of main primary producers at the two sites, the food web did not vary between winter and summer. The δ¹³C range of primary producers was wide. Zostera leaves, the most ¹³C-enriched source, were not consumed directly by grazers. Deposit and filter feeders have a similar δ¹³C and could use a mix of suspended and sedimented organic particulate matter, largely composed of detritus from macroalgae to seagrass. This trophic pathway allows the local incorporation of the high biomass produced by seagrasses. The wide δ¹⁵N range of predators was linked either to a large variety from omnivore to carnivore predators or to the also wide ranges of δ¹⁵N of primary consumers.     

Eutrophication and trophic structure in response to the presence of the eelgrass <Emphasis Type="Italic">Zostera noltii</Emphasis>

In estuaries, eelgrass meadows contribute to fundamental ecosystem functions of estuaries, providing food to several predators and buffering the negative effects of eutrophication. We asked whether the presence of the eelgrass Zostera noltii decreased the nitrogen concentration in the overlying water, affected the sources of nitrogen sequestrated by primary producers and changed the benthic and pelagic food web structures. We also studied the importance of these food webs in providing food to fish. We compared bare sediment to sediment covered by a Z. noltii meadow, and examined nutrient concentrations in the water column and δ¹⁵N in primary producers as indicators of anthropogenic inputs of nutrients. We then measured both δ¹³C and δ¹⁵N in the tissues of plants and consumers to establish food web structures. There were no differences in the concentrations and sources of nitrogen between sites. Rather, δ¹⁵N values indicated anthropogenic inputs of N (e.g. sewage discharges, agriculture) in both sites. There were no major differences in the structure of the planktonic food web, which was in part sustained by particulate organic matter and supported most predator fish, and in the structure of the benthic food web. Nonetheless, there were differences in the sources of food for omnivore consumers and for the detritivore Scrobicularia plana. Overall, the benthic food web did not use food derived from the eelgrass or macroalgae deposited on the substratum. Suspension feeders used particulate and sediment organic matter, whereas the δ¹³C and δ¹⁵N values of the other consumers indicated a likely contribution of benthic microalgae. Furthermore, in both habitats we found large variability in the isotope signatures of benthic macrofauna consumers, which did not allow distinguishing clearly different trophic groups and indicated a high level of omnivory and a mixed diet opportunistically making use of the availability of food in the surroundings.     

Food web structure of a restored macroalgal bed in the eastern Korean peninsula determined by C and N stable isotope analyses

Loss of macroalgae habitats has been widespread on rocky marine coastlines of the eastern Korean peninsula, and efforts for restoration and creation of macroalgal beds have increasingly been made to mitigate these habitat losses. Deploying artificial reefs of concrete pyramids with kelps attached has been commonly used and applied in this study. As a part of an effort to evaluate structural and functional recovery of created and restored habitat, the macroalgal community and food web structure were studied about a year after the establishment of the artificial macroalgal bed, making comparisons with nearby natural counterparts and barren ground communities. Dominant species, total abundance, and community structure of macroalgal assemblage at the restored macroalgal bed recovered to the neighboring natural bed levels during the study period. The main primary producers (phytoplankton and macroalgae) were isotopically well separated. δ¹³C and δ¹⁵N values of consumers were very similar between restored and natural beds but varied greatly among functional feeding groups. The range of consumer δ¹³C was as wide as that of primary producers, indicating the trophic importance of both producers. There was a stepwise trophic enrichment in δ¹⁵N with increasing trophic level. A comparison of isotope signatures between primary producers and consumers showed that, while suspension feeders are highly dependent on pelagic sources, invertebrates of other feeding guilds and fishes mainly use macroalgae-derived organic matter as their ultimate nutritional sources in both macroalgal beds, emphasizing the high equivalency of trophic structure between both beds. Isotopic signatures of a few mollusks and sea urchins showed that they use different dietary items in macroalgal-barren grounds compared with macroalgal beds, probably reflecting their feeding plasticity according to the low macroalgal biomass. However, isotopic signatures of most of the consumers at the barren ground were consistent with those at the macroalgal beds, supporting the important trophic role of drifting algae. Our results revealed the recoveries of the macroalgal community and trophic structure at the restored habitat. Further studies on colonization of early settlers and the following succession progress are needed to better understand the process and recovery rate in the developing benthic community.     

Heterogeneous energetic pathways and carbon sources on deep eastern Mediterranean cold seep communities

Cold seep communities in the Mediterranean Sea have only been discovered two decades ago, and their trophic ecology has been the subject of very few studies. We investigated the benthic food web of two deep chemosynthesis-based ecosystems on the Napoli and Amsterdam mud volcanoes (MVs) in the eastern Mediterranean Sea (~2,000 m depth). Seeping methane has been detected at the surface of both MVs during pioneering cruises and has been hypothesised to be assimilated by benthic fauna as observed in other oceans’ margins. Given the extreme oligotrophic character of the eastern Mediterranean Sea, we a priori expected that chemosynthetic food sources, especially methane-derived carbon (MDC), played a major trophic role in these deep seep communities relative to what has been observed in other seep systems worldwide. We aimed at unravelling the trophic relationships on Napoli and Amsterdam MVs through the analysis of carbon, nitrogen and sulphur isotopes both in the dominant benthic invertebrates including the small endofauna (300 μm < size < 1 cm) and in the sedimented organic matter. In particular, we assessed the fraction of MDC in the tissue of several heterotrophic and symbiotic species. Low mean δ³⁴S and δ¹³C values (0.4 ± 4.8‰ and −31.6 ± 5.7‰, respectively) obtained for mega- and macrofauna suggested that the investigated benthic food webs are virtually exclusively fuelled by carbon of chemosynthetic origin. A few grazer invertebrates (δ³⁴S up to 11‰) depart from this trend and could complement their diet with sedimented and decayed phytoplanktonic organic matter. Faunal δ¹³C values indicated that the oxidation of sulphur is likely the predominant energetic pathway for biosynthesis on both MVs. Nevertheless, mytilid bivalves and small capitellid, ampharetid and spionid polychaetes were ¹³C-depleted (δ¹³C < −37‰) in a way indicating they assimilated a significant portion of MDC. For these later heterotrophic species, MDC ranged between 21 and 31% (lower estimates) and 97 and 100% (upper estimates). However, our results highlighted that the origin of assimilated carbon may be complex for some symbiotic species. The vestimentiferan tubeworm Lamellibrachia sp., which exclusively depends on its sulphur-oxidising endosymbionts, showed a ~20‰ inter-individual δ¹³C variability on a very small spatial scale (<1 m) at the summit of Napoli MV. This mostly reflects the variable isotopic composition of pore-water-dissolved inorganic carbon (DIC) and evidenced that tubeworms (and subsequently their endosymbionts) uptake DIC derived from multiple methane oxidation processes in varying proportions. The lower and upper MDC estimates for the vestimentum of Napoli’s individuals were 11–38 and 21–73%, respectively. Finally, data on trophic ecology of Napoli and Amsterdam MVs clearly corroborate previous geophysical results evidencing the spatial heterogeneity of Mediterranean MV environmental conditions.     

Food sources used by sediment meiofauna in an intertidal Zostera noltii seagrass bed: a seasonal stable isotope study

In an intertidal Zostera noltii Hornem seagrass bed, food sources used by sediment meiofauna were determined seasonally by comparing stable isotope signatures (δ¹³C, δ¹⁵N) of sources with those of nematodes and copepods. Proportions of different carbon sources used by consumers were estimated using the SIAR mixing model on δ¹³C values. Contrary to δ¹⁵N values, food source mean δ¹³C values encompassed a large range, from −22.1 ‰ (suspended particulate organic matter) to −10.0 ‰ (Z. noltii roots). δ¹³C values of copepods (from −22.3 to −12.3 ‰) showed that they use many food sources (benthic and phytoplanktonic microalgae, Z. noltii matter). Nematode δ¹³C values ranged from −14.6 to −11.4 ‰, indicating a strong role of microphytobenthos and/or Z. noltii matter as carbon sources. The difference of food source uses between copepods and nematodes is discussed in light of source accessibility and availability.     

Stable isotopes (δ13C, δ15N) and modelling as tools to estimate the trophic ecology of cultivated oysters in two contrasting environments

Food sources for cultivated marine bivalves generally are not well identified, although they are essential for a better understanding of coastal ecosystems and for the sustainability of shellfish farming activities. In addition to phytoplankton, other organic matter sources (OMS), such as microphytobenthos and detritus (of terrestrial or marine origins), can contribute significantly to the growth of marine bivalves. The aim of this study was to identify the potential food sources and to estimate their contributions to the growth of the Pacific oyster (Crassostrea gigas) in two contrasting trophic environments of Normandy (France): the Baie des Veys (BDV) and the Lingreville area (LIN). Two sites were studied in the BDV area (BDV-S and BDV-N) and one in the LIN area. To estimate the contribution of each type of OMS, we used a combination of stable natural isotope composition (δ¹³C, δ¹⁵N) analysis of OMS and oyster tissue together with a modelling exercise. Field sampling was conducted every 2 months over 1 year. The sampled sources were suspended particulate organic matter from marine (PhyOM) and terrestrial (TOM) origins, microphytobenthos (MPB), detrital organic matter from the superficial sediment (SOM), and macroalgae (Ulva sp., ULV). A statistical mixing model coupled to a bioenergetic model was used to calculate the contributions of each different source at different seasons. Results showed that isotopic composition of the animal flesh varied with respect to the potential OMS over the year within each ecosystem. Significant differences were also observed among the three locations. For instance, the δ¹³C and δ¹⁵N values of the oysters ranged from −20.0 to −19.1‰ and from 6.9 to 10.8‰ at BDV-S, from −19.4 to −18.1‰ and from 6.4 to 10.0‰ at BDV-N, and from −21.8 to −19.4‰ and from 6.3 to 8.3‰ at LIN. The contributions of the different sources to oyster growth differed depending on the ecosystem and on the period of the year. Phytoplankton (PhyOM) predominated as the principal food source for oysters (particularly in the LIN location). MPB, TOM, and ULV detritus also possibly contributed to oysters’ diet during summer and autumn at the BDV-S and BDV-N sites. SOM was not considered an OMS because it was already a mix of the other four OMS, but rather a trophic reservoir that potentially mirrored the trophic functioning of marine ecosystems.     

Influence of seasonal food abundance and quality on the feeding habits of an opportunistic feeder,the intertidal crab <Emphasis Type="Italic">Pachygrapsus marmoratus</Emphasis>

The abundance of trophic sources on the intertidal zone is discontinuous and their supply can vary both in a predictable or unpredictable way. The Mediterranean semi-terrestrial crab Pachygrapsus marmoratus, is known, as adult, to entirely rely on the intertidal trophic sources, and, consequently, it faces the fluctuations of nutritional sources and quality. To clarify the relationships between the feeding habits of an Italian population of P. marmoratus and the temporal variation of its food sources, we carried out a 2-year sampling protocol. Data on seasonal variation in composition of intertidal food item assemblages, on the average content in N and C of the commonest algae, on seasonal changes in crabs feeding habits were collected and compared using a suite of multivariate and univariate techniques. Results showed that P. marmoratus takes advantage of the recruitment phase of the most common invertebrates, affecting and controlling the abundance even of those species whose adults are out of its reach. It can act both as a herbivore, and as a carnivore that shifts between bivalves, more abundant in spring/summer, and the periwinkle Melaraphe neritoides, during winter. In conclusion, we provide evidences on the food choice of this common rocky shore species, which is very plastic and capable of relying on many trophic sources, possibly influencing the abundance and/or the population structure of a number of intertidal populations.     

Influence of nutrients in the feeding ecology of seagrass (Posidonia oceanica L.) consumers: a stable isotopes approach

Nitrogen inputs to coastal environments can considerably alter the abundance of primary producers. However, how herbivores modify their trophic signatures and adjust to changes in food resource conditions remains controversial. Here, we assess the effect of nutrient availability on the diet shifts of the two main Mediterranean herbivores, the Sparid fish Sarpa salpa L. and the sea urchin Paracentrotus lividus (Lmk.) that feed mostly on the seagrass Posidonia oceanica L. (Delile), epiphytes and benthic macroalgae. To do this, we (1) investigate the patterns of isotopic composition (δ¹³C and δ¹⁵N signatures) of the two herbivores and their potential food sources in three areas of contrasting nutrient conditions and, (2) we assess the diet shift along this nutrient gradient by estimating the isotopic nutrient enrichment (i.e., the contribution of δ¹³C and δ¹⁵N signatures in consumers’ tissues relative to potential food sources). Food web signatures of δ¹³C were similar among the three study sites, and no patterns of δ¹³C shift were observed in their diets. In contrast, there was a consistent increase in N contents and δ¹⁵N along the nutrient gradient for all primary producers and their consumers. The rate of δ¹⁵N enrichment was also clearly distinctive between the two herbivores: in P. lividus it increased by 61% along the nutrient gradient, while in S. salpa it remained constant. Our results suggest that sea urchins behave as facultative omnivores and feed on vegetable or mixed diets depending on the trophic status of the system. It is unclear, however, if this modification is behavioral or the consequence of mere changes in the availability of food items, as animal epiphytes (e.g., hydrozoans, bryozoans and ascidians) can also became more abundant on seagrass leaves under increased nutrient conditions. In contrast, adult fish appear to feed on vegetal material independent of nutrient availability in the ecosystem.     

The trophic link between squid and the emperor penguin <Emphasis Type="Italic">Aptenodytes forsteri</Emphasis> at Pointe Géologie,Antarctica

Cephalopod beaks retrieved from stomachs of dead emperor penguin chicks at Pointe Géologie, Terre Adélie, provide information on taxonomic and size composition of the penguin’s squid diet, on the trophic range of the squid species preyed upon and on the fractional trophic impact of the penguin on the whole food web. Emperor penguins prey upon four squid species (Psychroteuthis glacialis, Kondakovia longimana, Gonatus antarcticus, Alluroteuthis antarcticus) and do not take squid larger than 480 mm mantle length. Larger squid live either below the penguin’s diving range or are beyond its handling capacity. Nitrogen stable isotope ratios indicate that squids cover a range of about two trophic levels (2.5–8‰ δ¹⁵N). The impact of the emperor penguin, however, concentrates on the upper part of this range, about 68% of its squid prey being >6‰ δ¹⁵N. The principal components of the emperor’s diet, fish, krill and squid, differ distinctly in average trophic level. Consequently the trophic position of the emperor penguin changes accordingly with diet composition and may differ by almost one trophic level between different emperor penguin colonies.     

Isotopic evidence of distinct feeding ecologies and movement patterns in two migratory predators (yellowfin tuna and swordfish) of the western Indian Ocean

Ecologists primarily use δ¹⁵N values to estimate the trophic level of organisms, while δ¹³C, and even recently δ¹⁵N, are utilized to delineate feeding habitats. However, many factors can influence the stable isotopic composition of consumers, e.g. age, starvation or isotopic signature of primary producers. Such sources of variability make the interpretation of stable isotope data rather complex. To examine these potential sources of variability, muscle tissues of yellowfin tuna (Thunnus albacares) and swordfish (Xiphias gladius) of various body lengths were sampled between 2001 and 2004 in the western Indian Ocean during different seasons and along a latitudinal gradient (23°S to 5°N). Body length and latitude effects on δ¹⁵N and δ¹³C were investigated using linear models. Both latitude and body length significantly affect the stable isotope values of the studied species but variations were much more pronounced for δ¹⁵N. We explain the latitudinal effect by differences in nitrogen dynamics existing at the base of the food web and propagating along the food chain up to top predators. This spatial pattern suggests that yellowfin and swordfish populations exhibit a relatively unexpected resident behaviour at the temporal scale of their muscle tissue turnover. The body length effect is significant for both species but this effect is more pronounced in swordfish as a consequence of their different feeding strategies, reflecting specific physiological abilities. Swordfish adults are able to reach very deep water and have access to a larger size range of prey than yellowfin tuna. In contrast, yellowfin juveniles and adults spend most of their time in the surface waters and large yellowfin tuna continue to prey on small organisms. Consequently, nitrogen isotopic signatures of swordfish tissues are higher than those of yellowfin tuna and provide evidence for different trophic levels between these species. Thus, in contrast to δ¹³C, δ¹⁵N analyses of tropical Indian Ocean marine predators allow the investigation of complex vertical and spatial segregation, both within and between species, even in the case of highly opportunistic feeding behaviours. The linear models developed in this study allow us to make predictions of δ¹⁵N values and to correct for any body length or latitude differences in future food web studies.     

Stable C and N isotope analysis elucidated the importance of zooplankton in a tropical seagrass bed of Santiago Island,Northwestern Philippines

The role of zooplankton in a tropical seagrass ecosystem was investigated in milkfish farms pollution-impacted and -unimpacted seagrass beds in Santiago Island coral reefs, Northwestern Philippines. The aim was to compare between the two sites: (1) abiotic factors and zooplankton community parameters, and (2) the trophic structure using C and N stable isotopes. Low water (98–119?mV) and sediment (–121 to ?138?mV) Oxidation Reduction Potential values indicated a reducing environment in the impacted site. Zooplankton in the impacted site showed the typical community response to eutrophication (low diversity, but high total abundance due to the dominance of the cyclopoid copepod Oithona oculata), generally few elevated δ¹⁵N values, but a significant shift towards depleted ¹³C due to the organic enrichment of fish-farm feeds. Apart from suggesting a highly complex food web with POM and zooplankton as main food sources in the unimpacted site, the Bayesian mixing model simulation generated reduced complexity in feeding interactions between basal sources, zooplankton, and fish including adults of a key fish species, Siganus fuscescens, in the impacted sites. In this study, C and N stable isotope analysis has clarified the importance of zooplankton as fish prey in a seagrass bed food web.     

Stable isotope ratios indicate differential omnivory among syntopic rocky shore suspension-feeders

We utilised stable isotope ratios to assess differences in diet among three indigenous and syntopic rocky shore suspension-feeders (mussels Perna perna, barnacles Tetraclita serrata, and polychaetes Gunnarea gaimardi). We also determined the spatial and temporal variability in the suspension-feeder diets by collecting specimens on two occasions from two regions adjacent to hydrologically distinct river mouths (i.e. one with larger annual freshwater throughput than the other). The results showed that the isotopic niches (used as proxies for trophic niches) of the three species did not overlap and that the barnacles occupied a trophic position (3.4) well above those of the mussels (2) and polychaetes (2.6). We ascribed the interspecific differences primarily to the disparate feeding mechanisms used by the species. Large-scale regional (~50 km) differences in suspension-feeder diets were apparent, but not small-scale (up to a few km north and south of each estuary mouth). The regional differences in diet resulted from the increased availability of estuarine-origin suspended particulate matter (SPM) and zooplankton in the region adjacent to the river with relatively larger freshwater output, although overall incorporation of zooplankton versus mixed SPM into consumer diets was relatively consistent between regions and through time. Temporal shifts in suspension-feeder diets were apparent from stable carbon isotope ratios in the consumers. Our results showcase the measurable effects of regional-scale processes that can alter the food sources for dominant primary consumers in the rocky intertidal, thus potentially affecting entire food webs through bottom-up processes. The clear evidence for trophic niche partitioning offers valuable insights into how potentially strong competitors can coexist.     

Distribution of foraging habitats of male loggerhead turtles (Caretta caretta) as revealed by stable isotopes and satellite telemetry

Most studies on the foraging ecology of loggerhead turtles (Caretta caretta) have focused on adult females and juveniles. Little is known about the foraging patterns of adult male loggerheads. We analyzed tissues for carbon and nitrogen stable isotopes (δ¹³C and δ¹⁵N) from 29 adult male loggerheads tracked with satellite transmitters from one breeding area in Florida, USA, to evaluate their foraging habitats in the Northwest Atlantic (NWA). Our study revealed large variations in δ¹³C and δ¹⁵N and a correlation between both δ¹³C and δ¹⁵N and the latitude to which the loggerheads traveled after the mating season, thus reflecting a geographic pattern in the isotopic signatures. Variation in δ¹³C and δ¹⁵N can be explained by differences in food web baseline isotopic signatures rather than differences in loggerhead trophic levels. Stable isotope analysis may help elucidate residency and migration patterns and identify foraging sea turtle subpopulations in the NWA due to the isotopically distinct habitats used by these highly migratory organisms.     

Stable isotopes changes in the adductor muscle of diseased bivalve Ruditapes philippinarum

In this article, we show how a disease could bias stable isotope analyzes of trophic networks and propose a strategy in the choice of tissues to be analyzed. In the past few years, a new pathology (brown muscle disease or BMD) affecting the posterior adductor muscle of Ruditapes philippinarum has emerged in Arcachon Bay. BMD induces a necrosis of muscle tissues which become infused by conchiolin and hence calcified. As muscle of mollusks are often used for trophic food webs studies through stable isotopic analyzes, this work investigated the effect of BMD on carbon and nitrogen isotopic ratios of anterior and posterior adductor muscles of clams collected in February and August 2007. Infected clams displayed a lower condition index and a posterior adductor muscle δ¹³C enrichment of 1.2‰ in February and 0.7‰ in August. δ¹⁵N of posterior muscles was however not affected by the disease. Anterior muscle of diseased clams remained healthy and displayed the same isotopic signature as both posterior and anterior muscular tissues of healthy clam. Acidification significantly depleted δ¹³C in posterior muscles of infected clams, suggesting calcification, contrary to anterior muscles of infected clam and to both muscles of healthy clams, where no effect was observed. An X-ray diffractometry analysis confirmed the presence of CaCO₃ (aragonite). Trophic food web studies relying on stable isotope ratios should utilize only healthy animals or anterior adductor muscles when expertise in mollusk pathology is lacking.     

Effects of detrital non-native and native macroalgae on the nitrogen and carbon cycling in intertidal sediments

The decay of non-native and native seaweed mixing may modify sediment biogeochemistry and organic matter transfers within benthic food webs according to their composition and biomass. The non-native species Sargassum muticum was deliberately added to the sediment of an intertidal sandflat at different biomass and mixed to the native species Ulva sp. and Fucus vesiculosus. The sediment porewater was then ¹³C and ¹⁵N enriched to test whether both detrital diversity and biomass influenced the transfer of porewater carbon and nitrogen to the sediment and to the macrofauna consumers. More ¹⁵N-nitrogen was mobilized to sediments and macrofauna when the 3-species detrital mixing was buried, probably because this mixing provided species-specific compounds such as polyphenols due to the presence of S. muticum and F. vesiculosus, as well as large amounts of nitrogen due to the presence of Ulva. Our study revealed the importance of detrital diversity and non-native seaweeds for the nitrogen cycling in the benthic food web.     

Stable carbon isotope ratios in Red Sea barnacles (Cirripedia) as an indicator of their food source

The stable isotope ratio ¹²C/¹³C was used to investigate the source of carbon in free-living barnacles and in coral-inhabiting barnacles from the Red Sea. The δ¹³C of most of the barnacles collected on the open shore ranges between −17.5 and −19.7‰, indicating relative enrichment of light carbon originating from the open-sea phytoplankton. Those collected in closed habitats showed heavier isotopic composition. The δ¹³C of the coral-inhabiting barnacles ranges from −14.1 to −16.7‰, suggesting that the carbon contribution of open-sea plankton to these barnacles is less important than it is to free-living barnacles. We hypothesize that coral organic matter and zooxanthellae expelled by the host coral contribute carbon to the barnacle, and that a mixture of this relatively heavy carbon with carbon from other sources is responsible for the high values of δ¹³C in coral barnacles. Received: 28 February 1997 / Accepted: 16 September 1997     

Utilization of nitrogen derived from seabird guano by terrestrial and marine plants at St. Paul, Pribilof Islands, Bering Sea, Alaska

Dense, conspicuous colonies of seabirds and pinnipeds breed on ocean islands throughout the world. Such colonies have been shown to have local impacts on prey populations, but whether or not they affect nutrient cycling has been debated. We determined the natural abundance levels of the stable isotopes (C and N) of primary producers, seabirds and other consumers at and near St. Paul Island, Pribilof Islands, Bering Sea, in summer 1993. Marine primary producers (phytoplankton, as particulate organic matter, and kelp) collected near seabird colonies were ca. 6.5‰ enriched in both ¹⁵N and ¹³C relative to those collected further from shore. Terrestrial plants collected near the seabird colonies were enriched in ¹⁵N (δ¹⁵N ca. 22‰) compared with conspecifics collected away from the colonies (δ¹⁵N ca. 11‰). The trend towards higher δ¹⁵N values in marine and terrestrial plants near bird colonies is consistent with their uptake of ornithogenic N. This ¹⁵N-enrichment of plants using ornithogenic N can be attributed to a combination of two processes: trophic enrichment, and volatilization of ammonia produced during degradation of terrestrially deposited guano. Seabird breeding colonies at St. Paul Island appear to be significant sources of recycled nitrogen for terrestrial plants in the vicinity of colonies and for phytoplankton in the nearshore zone. Received: 4 July 1997 / Accepted: 20 November 1997     

Coral mucus stable isotope composition and labeling: experimental evidence for mucus uptake by epizoic acoelomorph worms

Mucus released by scleractinian corals can act as an important energy and nutrient carrier in coral reef ecosystems, and a distinct isotopic signature would allow following the fate of this material. This study investigates the natural C and N stable isotopic signatures of mucus released by four scleractinian coral genera (Acropora, Fungia, Pocillopora and Stylophora) in comparison with those of suspended particulate organic matter (POM) in seawater of a Northern Red Sea fringing coral reef near Aqaba, Jordan. The natural δ¹³C and δ¹⁵N signatures of coral mucus differed significantly from seawater POM for the majority of seasonal comparisons, but were inappropriate for explicit tracing of mucus in the coral reef food web. Thus, a labeling technique using stable isotope tracers (¹³C and ¹⁵N) was developed that produced δ¹³C values of up to 122 ± 5‰ (mean ± SE) and δ¹⁵N of up to 2,100 ± 151‰ in mucus exuded by Fungia corals. ¹³C and ¹⁵N-enriched compounds were rapidly (within 3 h) and light-dependently transferred from the endosymbiotic zooxanthellae to the mucus-producing coral host. The traceability of ¹⁵N-labeled mucus was examined by evaluating its uptake and potential utilization by epizoic acoelomorph Waminoa worms naturally occurring on a range of scleractinian coral taxa. This tracer experiment resulted in uptake of coral mucus by the coral-associated acoelomorphs and further demonstrated the possibility to trace stable isotope-labeled coral mucus by revealing a new trophic pathway in coral reef ecosystems.     

Biodiversity and trophic structure of nematode assemblages in seagrass systems: evidence for a coupling with changes in food availability

Nematode assemblage composition, trophic structure and biodiversity were followed over an annual cycle in a Posidonia oceanica bed of the NW Mediterranean to test the response to temporally changing food availability (sediment organic matter, bacterial and microphytobenthic biomass). The sediment-water interface of the seagrass meadow was characterised by high particulate organic matter concentrations. Also, seagrass sediments had high organic matter content, chloropigment concentrations and bacterial biomass. All organic matter components (i.e. carbohydrates, proteins and lipids) changed temporally, with higher concentrations in winter and spring and lower concentrations in summer; however, overall, large amounts of organic compounds were potentially available for (seagrass) benthic consumers throughout the year. Nematode assemblage in the P. oceanica bed maintained a high genus number (88 genera), and a trophic structure tightly coupled with the composition of the potential food sources. In agreement with the relevance of microphytobenthos, epigrowth-feeder nematodes (2A) represented the dominant trophic guild. The biomass of predator nematodes (2B) was significantly correlated with the biomass of other nematodes. Non-selective deposit feeders (1B) displayed a close relationship with the concentration of proteins, carbohydrates and biopolymeric carbon (i.e. labile organic detritus). Conversely to what was expected, epigrowth feeders were not correlated with chloropigment concentrations, but showed a significant relationship with the number of dividing bacteria, possibly suggesting a trophic plasticity of the 2A feeding guild. The coupling between temporal changes in food sources and temporal variability of the nematode trophic structure was highlighted by a CANOCO analysis, which allowed us to identify and associate, at each sampling time, nematode genera and their feeding habits, with specific environmental variables and food indicators. Furthermore, species diversity (H') and evenness (J) calculated on nematodes identified to the genus level displayed evident temporal changes, also reflected by the index of trophic diversity (ITD). Both structural and functional diversity were coupled with high concentration and highly heterogeneous composition of the food sources (including organic detritus, microphytobenthic algae and bacteria) potentially available to nematodes. These data suggest that temporal changes in quantity and quality of food sources do not only influence nematode dynamics and trophic composition, but also influence nematode structural and functional diversity.