Anne Lorrain

Differential δ13C and δ15N signatures among scallop tissues: implications for ecology and physiology

There have been several studies where the isotopic composition of organisms has been determined seasonally, but fewer have examined separate organs. In this context, separate organs (e.g. gonad, digestive gland and muscle) of a suspension-feeder, the scallop Pecten maximus, were used to assess seasonal changes of both stable isotopes and biochemical components. Our study used multiple indicators [stable carbon and nitrogen isotope ratios, biochemical components and seston chlorophyll-a (chl a)] to track nutritive activity and energy allocation in P. maximus from the Bay of Brest (France). In addition to seasonal variation in the isotopic composition of P. maximus tissues, we found strong differences in the mean isotopic signatures of different organs. This has serious implications for interpretation of animal diets and potential use in animal physiology. Furthermore, we present evidence that seasonal variations of metabolism will cause changes in the isotopic composition not related to changes in the diet. Interpretation of isotopic data may require consideration of values from several separate organs. Finally, y 15 N appears powerful to track metabolite fates in the scallop P. maximus. D 2002 Elsevier Science B.V. All rights reserved.

Strong kinetic effects on Sr/Ca ratios in the calcitic bivalve Pecten maximus

Although Sr/Ca ratios in abiogenic calcite are strongly controlled by precipitation rates, such a kinetic effect has never been demonstrated in calcitic bivalve shells. Therefore, we report Sr/Ca ratios together with daily growth rates in the calcitic shells of four individuals of the bivalve Pecten maximus (age class I). Ratios of Sr/Ca were found to be variable among individuals that grew at the same location, illustrating that vital effects dominate over environmental controls. Although daily growth rate was correlated with shell Sr/Ca ratios, it explained only half of the Sr/Ca variations. However, daily shell surface area increment, an estimation of the total quantity of carbonate precipitated for a given time, explained 74% of the Sr/Ca variability in the shells of P. maximus. This proves, for the first time in a calcitic bivalve, that shell Sr/Ca partitioning is mainly controlled by kinetic effects. The Sr/Ca ratio should therefore be tested as a potential proxy of calcification rate in modern or fossil calcitic biocarbonates.

Growth anomalies in Pecten maximus from coastal waters (Bay of Brest, France): relationship with diatom blooms

The aim of this study was to characterize the daily shell growth of Pecten maximus from early 1998 to late spring 1999 in the Bay of Brest with a careful qualitative and quantitative description of the pelagic primary production. Our results, in accordance with previous studies, demonstrate that there are episodic declines in the growth rate. Concurrent events of low growth rate and large bottom-concentrations of algae following diatom blooms (Cerataulina pelagica or Rhizosolenia delicatula), suggests that this high concentration of phytoplankton on the bottom layer may affect food intake or respiratory activity of the scallops by gill clogging or oxygen depletion. In this study, silicic acid or phosphorus are inferred to be limiting factors responsible for the collapse of the spring bloom. Further, we suggest that, in an N-enriched ecosystem, Si or P-limitation and the changing sinking velocities of phytoplankton, could affect the biology of benthic suspension-feeders.

Sequential Isotopic Signature Along Gladius Highlights Contrasted Individual Foraging Strategies of Jumbo Squid (Dosidicus gigas)

Background Cephalopods play a major role in marine ecosystems, but knowledge of their feeding ecology is limited. In particular, intra- and inter-individual variations in their use of resources has not been adequatly explored, although there is growing evidence that individual organisms can vary considerably in the way they use their habitats and resources. Methodology/Principal Findings Using δ13C and δ15N values of serially sampled gladius (an archival tissue), we examined high resolution variations in the trophic niche of five large (>60 cm mantle length) jumbo squids (Dosidicus gigas) that were collected off the coast of Peru. We report the first evidence of large inter-individual differences in jumbo squid foraging strategies with no systematic increase of trophic level with size. Overall, gladius δ13C values indicated one or several migrations through the squids lifetime (∼8–9 months), during which δ15N values also fluctuated (range: 1 to 5‰). One individual showed an unexpected terminal 4.6‰ δ15N decrease (more than one trophic level), thus indicating a shift from higher- to lower-trophic level prey at that time. The data illustrate the high diversity of prey types and foraging histories of this species at the individual level. Conclusions/Significance The isotopic signature of gladii proved to be a powerful tool to depict high resolution and ontogenic variations in individual foraging strategies of squids, thus complementing traditional information offered by stomach content analysis and stable isotopes on metabolically active tissues. The observed differences in life history strategies highlight the high degree of plasticity of the jumbo squid and its high potential to adapt to environmental changes.

Role of pelagic crustaceans in the diet of the longnose lancetfish Alepisaurus ferox in the Seychelles waters

The role of pelagic crustaceans in the diet of the longnose lancetfish Alepisaurus ferox was investigated from stomach content analysis of fish collected in the waters surrounding the Seychelles Archipelago in the western Indian Ocean. Crustaceans accounted for 88.4% by prey number and 73.7% by reconstituted mass during the South-West monsoon season. During the North-East monsoon season, crustaceans remained the main prey group and accounted for 63.7% by prey number and 46% by reconstituted mass. There was a clear seasonal pattern with the portunid crab Charybdis smithii predominating during the South-West monsoon season and the pelagic stomatopod Natosquilla investigatoris during the North-East monsoon season. This pattern likely reflects variations in prey availability linked to current movements around the Seychelles Archipelago. Most of the prey consisted of slow-swimming and small individuals, which occur in dense swarms during the pelagic phase of their life history. The dietary trends of lancetfish are consistent with opportunistic feeding on the most abundant prey. They exploit short food chains based on carnivorous crustaceans, and play an important role in the pelagic trophic functioning of the western Indian Ocean.

Setting the stage for a global-scale trophic analysis of marine top predators: a multi-workshop review

Global-scale studies of marine food webs are rare, despite their necessity for examining and understanding ecosystem level effects of climate variability. Here we review the progress of an international collaboration that compiled regional diet datasets of multiple top predator fishes from the Indian, Pacific and Atlantic Oceans and developed new statistical methods that can be used to obtain a comprehensive ocean-scale understanding of food webs and climate impacts on marine top predators. We loosely define top predators not as species at the apex of the food web, but rather a guild of large predators near the top of the food web. Specifically, we present a framework for world-wide compilation and analysis of global stomach-contents and stable-isotope data of tunas and other large pelagic predatory fishes. To illustrate the utility of the statistical methods, we show an example using yellowfin tuna in a “test” area in the Pacific Ocean. Stomach-contents data were analyzed using a modified (bagged) classification tree approach, which is being prepared as an R statistical software package. Bulk δ15N values of yellowfin tuna muscle tissue were examined using a Generalized Additive Model, after adjusting for spatial differences in the δ15N values of the baseline primary producers predicted by a global coupled ocean circulation-biogeochemical-isotope model. Both techniques in tandem demonstrated the capacity of this approach to elucidate spatial patterns of variations in both forage species and predator trophic positions and have the potential to predict responses to climate change. We believe this methodology could be extended to all marine top predators. Our results emphasize the necessity for quantitative investigations of global-scale datasets when evaluating changes to the food webs underpinning top ocean predators under long-term climatic variability.

Diazotrophs: a non-negligible source of nitrogen for the tropical coral Stylophora pistillata.

ABSTRACT Corals are mixotrophs: they are able to fix inorganic carbon through the activity of their symbiotic dinoflagellates and to gain nitrogen from predation on plankton and uptake of dissolved organic and inorganic nutrients. They also live in close association with diverse diazotrophic communities, inhabiting their skeleton, tissue and mucus layer, which are able to fix dinitrogen (N2). The quantity of fixed N2 transferred to the corals and its distribution within coral compartments as well as the quantity of nitrogen assimilated through the ingestion of planktonic diazotrophs are still unknown. Here, we quantified nitrogen assimilation via (i) N2 fixation by symbiont diazotrophs, (ii) ingestion of cultured unicellular diazotrophs and (iii) ingestion of natural planktonic diazotrophs. We estimate that the ingestion of diazotrophs provides 0.76±0.15 µg N cm−2 h−1, suggesting that diazotrophs represent a non-negligible source of nitrogen for scleractinian corals. Summary: Nitrogen limits coral productivity in tropical waters; diazotrophs represent an important source of nitrogen for scleractinian corals.

Seabirds supply nitrogen to reef-building corals on remote Pacific islets

Seabirds concentrate nutrients from large marine areas on their nesting islands playing an important ecological role in nutrient transfer between marine and terrestrial ecosystems. Here we investigate the role of guano on corals reefs across scales by analyzing the stable nitrogen isotopic (δ15N) values of the scleractinian coral Pocillopora damicornis on fringing reefs around two Pacific remote islets with large seabird colonies. Marine stations closest to the seabird colonies had higher nitrate + nitrite concentrations compared to more distant stations. Coral and zooxanthellae δ15N values were also higher at these sites, suggesting that guano-derived nitrogen is assimilated into corals and contributes to their nitrogen requirements. The spatial extent of guano influence was however restricted to a local scale. Our results demonstrate that seabird-derived nutrients not only spread across the terrestrial ecosystem, but also affect components of the adjacent marine ecosystem. Further studies are now needed to assess if this nutrient input has a positive or negative effect for corals. Such studies on remote islets also open fresh perspectives to understand how nutrients affect coral reefs isolated from other anthropogenic stressors.

Responses of Two Scleractinian Corals to Cobalt Pollution and Ocean Acidification

The effects of ocean acidification alone or in combination with warming on coral metabolism have been extensively investigated, whereas none of these studies consider that most coral reefs near shore are already impacted by other natural anthropogenic inputs such as metal pollution. It is likely that projected ocean acidification levels will aggravate coral reef health. We first investigated how ocean acidification interacts with one near shore locally abundant metal on the physiology of two major reef-building corals: Stylophora pistillata and Acropora muricata. Two pH levels (pHT 8.02; pCO2 366 μatm and pHT 7.75; pCO2 1140 μatm) and two cobalt concentrations (natural, 0.03 μg L-1 and polluted, 0.2 μg L-1) were tested during five weeks in aquaria. We found that, for both species, cobalt input decreased significantly their growth rates by 28% while it stimulated their photosystem II, with higher values of rETRmax (relative Electron Transport Rate). Elevated pCO2 levels acted differently on the coral rETRmax values and did not affect their growth rates. No consistent interaction was found between pCO2 levels and cobalt concentrations. We also measured in situ the effect of higher cobalt concentrations (1.06 ± 0.16 μg L-1) on A. muricata using benthic chamber experiments. At this elevated concentration, cobalt decreased simultaneously coral growth and photosynthetic rates, indicating that the toxic threshold for this pollutant has been reached for both host cells and zooxanthellae. Our results from both aquaria and in situ experiments, suggest that these coral species are not particularly sensitive to high pCO2 conditions but they are to ecologically relevant cobalt concentrations. Our study reveals that some reefs may be yet subjected to deleterious pollution levels, and even if no interaction between pCO2 levels and cobalt concentration has been found, it is likely that coral metabolism will be weakened if they are subjected to additional threats such as temperature increase, other heavy metals, and eutrophication.

A global perspective on the trophic geography of sharks

Sharks are a diverse group of mobile predators that forage across varied spatial scales and have the potential to influence food web dynamics. The ecological consequences of recent declines in shark biomass may extend across broader geographic ranges if shark taxa display common behavioural traits. By tracking the original site of photosynthetic fixation of carbon atoms that were ultimately assimilated into muscle tissues of 5,394 sharks from 114 species, we identify globally consistent biogeographic traits in trophic interactions between sharks found in different habitats. We show that populations of shelf-dwelling sharks derive a substantial proportion of their carbon from regional pelagic sources, but contain individuals that forage within additional isotopically diverse local food webs, such as those supported by terrestrial plant sources, benthic production and macrophytes. In contrast, oceanic sharks seem to use carbon derived from between 30° and 50° of latitude. Global-scale compilations of stable isotope data combined with biogeochemical modelling generate hypotheses regarding animal behaviours that can be tested with other methodological approaches.Carbon isotopic analysis reveals global biogeographic traits in shark trophic interactions, and sheds light on the diverse foraging behaviour of sharks.