Trophic Environments Research Articles

Clustering and classification of vertical movement profiles for ecological inference of behavior

AbstractVertical movements can expose individuals to rapid changes in physical and trophic environments—for aquatic fauna, dive profiles from biotelemetry data can be used to quantify and categorize vertical movements. Inferences on classes of vertical movement profiles typically rely on subjective summaries of parameters or statistical clustering techniques that utilize Euclidean matching of vertical movement profiles with vertical observation points. These approaches are prone to subjectivity, error, and bias. We used machine learning approaches on a large dataset of vertical time series (N = 28,217 dives) for 31 post‐nesting leatherback turtles (Dermochelys coriacea). We applied dynamic time warp (DTW) clustering to group vertical movement (dive) time series by their metrics (depth and duration) into an optimal number of clusters. We then identified environmental covariates associated with each cluster using a generalized additive mixed‐effects model (GAMM). A convolutional neural network (CNN) model, trained on standard dive shape types from the literature, was used to classify dives within each DTW cluster by their shape. Two clusters were identified with the DTW approach—these varied in their spatial and temporal distributions, with dependence on environmental covariates, sea surface temperature, bathymetry, sea surface height anomaly, and time‐lagged surface chlorophyll a concentrations. CNN classification accuracy of the five standard dive profiles was 95%. Subsequent analyses revealed that the two clusters differed in their composition of standard dive shapes, with each cluster dominated by shapes indicative of distinct behaviors (pelagic foraging and exploration, respectively). The use of these two machine learning approaches allowed for discrete behaviors to be identified from vertical time series data, first by clustering vertical movements by their movement metrics (DTW) and second by classifying dive profiles within each cluster by their shapes (CNN). Statistical inference for the identified clusters found distinct relationships with environmental covariates, supporting hypotheses of vertical niche switching and vertically structured foraging behavior. This approach could be similarly applied to the time series of other animals utilizing the vertical dimension in their movements, including aerial, arboreal, and other aquatic species, to efficiently identify different movement behaviors and inform habitat models.

Perils of ingesting harmful prey by advanced snakes.

The advanced snakes (Alethinophidia) include the extant snakes with a highly evolved head morphology providing increased gape and jaw flexibility. Along with other physiological and morphological adaptations, this allows them to immobilize, ingest, and transport prey that may be disproportionately large or presents danger to the predator from bites, teeth, horns, or spines. Reported incidents of snakes failing to consume prey and being injured or killed during feeding mostly reflect information in the form of natural-history notes. Here we provide the first extensive review of such incidents, including 101 publications describing at least 143 cases of mortality (including six of 'multiple individuals') caused by ingestion or attempted consumption of injurious prey. We also report on 15 previously unpublished injurious feeding incidents from the USA, Austria, and Bulgaria, including mortality of five juvenile piscivorous dice snakes (Natrix tessellata) from a single location. Occurrences are spread across taxa, with mortality documented for at least 73 species from eight families and 45 genera. Incidents were generally well represented within each of three major categories: oversized prey (40.6%), potentially harmful prey (40.6%), and predator's behavioural/mechanical errors (18.9%). Reptile (33%) and fish (26%) prey caused disproportionately high mortality compared to mammals (16%). Feeding can be dangerous throughout a snake's life, with the later stages of feeding likely being more perilous. The number of reports has increased over time, and the data seem biased towards localities with a higher number of field-working herpetologists. We propose a standardized framework, comprising a set of basic information that should ideally be collected and published, and which could be useful as a template for future data collection, reporting, and analyses. We conclude that incidents of mortality during feeding are likely to be more common than previously assumed, and this hypothesis has implications for the ecology of persistence where populations are impacted by changing trophic environments.

Fine-scale larval fish distributions and predator-prey dynamics in a coastal river-dominated ecosystem

River plumes discharging into continental shelf waters have the potential to influence the distributions, predator-prey relationships, and thus survival of nearshore marine fish larvae, but few studies have been able to characterize the plume environment at sufficiently fine scales to resolve the underlying mechanisms. We used a high-resolution plankton imaging system and a sparse convolutional neural network to automate image classification of larval fishes, their planktonic prey (calanoid copepods), and gelatinous planktonic predators (ctenophores, hydromedusae, and siphonophores) over broad spatial scales (km) and multiple pulses of estuarine water exiting Mobile Bay (Alabama, USA) into the northern Gulf of Mexico from 9-11 April 2016. Fine-scale (1 m) plankton distributions were examined to analyze predator-prey relationships across 3 distinct plume regimes that varied by degree of wind-forcing and mixing rates. In calm wind conditions, the water column was highly stratified, and fish larvae and zooplankton were observed aggregating in a region of river plume-derived hydrodynamic convergence. As winds strengthened, the water column was subjected to downwelling and highly turbulent conditions, and there was decreasing spatial overlap between larval fishes and their zooplankton prey and predators. Our results indicate that high-discharge plume regimes characterized by strong wind-forcing and turbulence can rapidly shift the physical and trophic environments from favorable to unfavorable for fish larvae. Multiple pathways for both nearshore retention and advective dispersal of fish larvae were also identified. Documenting this variability is a first step toward understanding how high discharge events and physical forcing can affect fisheries production in river-dominated coastal ecosystems worldwide.

Consequences of plume encounter on larval fish growth and condition in the Gulf of Mexico

Freshwater input into nearshore continental shelf waters from coastal river-estuarine plumes can greatly alter the physical and trophic environments experienced by fish larvae. However, the biological consequences of plume encounter on larval fish survival remain equivocal, largely due to the extreme variability of these systems but also because traditional sampling techniques alone are too coarse to effectively characterize the dynamic biophysical environment at spatiotemporal scales relevant to individual larvae. Using a multidimensional approach, we simultaneously collectedin situimagery and net samples of larval fishes and zooplankton from the Mobile Bay plume (Alabama, USA) and ambient continental shelf waters during a high discharge event (8-11 April 2016). We measured the effects of plume encounter on growth and condition of larval striped anchovyAnchoa hepsetusand sand seatroutCynoscion arenarius, 2 prominent nearshore species in the northern Gulf of Mexico. Size-frequency distributions of both species indicated that larger individuals were present in shelf waters but absent from plume waters. Otolith microstructure analysis revealed that recent growth of both focal species was significantly lower for plume-collected larvae during the last few days prior to capture. Furthermore, plume larvae were in poorer morphometric condition (skinnier at length) than their shelf counterparts, despite the fact that there were higher concentrations of zooplankton prey in plume water masses. Taken together, these results suggest that elevated prey concentrations do not necessarily translate to higher growth and condition. High turbulence and turbidity within the plume may physically inhibit the prey capture ability and feeding success of fish larvae.

Changing with the tides: fine-scale larval fish prey availability and predation pressure near a tidally modulated river plume

Tidally controlled river plumes form distinct frontal boundaries that can alter the spatial distributions of larval fishes and their planktonic prey and predators. Variable in nature, they may expose larval fishes to different trophic environments over small spatio-temporal scales, with unknown consequences for survival and recruitment. In the northern California Current, the Columbia River Plume is strongly influenced by twice-daily freshwater injections that create a highly dynamic coastal environment. Using the In situ Ichthyoplankton Imaging System, we examined changes in the fine-scale horizontal and vertical distributions of larval fishes, their prey, and their predators over space and time (ebb/flood tide). In total, 6095 fish larvae and ~1.5 million prey/predator zooplankton were imaged and measured. Plume regions provided substantially higher concentrations of prey and enhanced spatial overlap between larval fishes and their prey relative to oceanic waters. The functionality of river plumes as a refuge from predators was less clear. Predator concentrations were also higher in plume regions, but overlap with larval fishes was taxon-specific and varied with the tide. Notably, regions of high zooplankton concentrations did not necessarily confer high spatial overlap on small scales (meters vertical, kms horizontal) relevant to trophic interactions. Surface salinity and chlorophyll a were the most important factors influencing the spatial overlap of zooplankton with larval fishes. In the vicinity of river plumes, larval fishes experience a diversity of unique prey and predator fields over short spatio-temporal scales, which likely contribute to variable growth and mortality patterns at much finer scales than previously thought.

Trophic environments influence size at metamorphosis and recruitment performance of Pacific oysters

Reproduction and recruitment of benthic invertebrates are influenced by the climate and by the ecological structure of marine ecosystems, along with local anthropogenic pressures such as eutrophication or oligotrophication. Using the Pacific oyster Crassostrea gigas as a biological model, we tested the hypothesis that the variability in prodissoconch II (PII) size (i.e. size at metamorphosis) depends on ecological functioning. Settlement and recruitment were assessed at 5 sampling sites on the French Mediterranean shellfish farmed Thau lagoon during the main summer recruitment events in 3 consecutive years (2012-2014). Hydrobiological and planktonic analyses were conducted at 3 sampling sites. Our results showed that recruitment was extremely heterogeneous, ranging from 0 to 260 +/- 27 SE ind. dm(-2) throughout the ecosystem and was linked with variability in PII size, which ranged from 180 to 296 mu m. The annual temporal pattern of PII sizes appeared to be controlled by temperature during the settlement period, whereas the spatial pattern depended on phytoplankton biomass and on the trophic functioning of the ecosystem. Smaller PII sizes were significantly correlated with the highest phytoplankton biomass, while larger PII sizes were positively correlated with mixotrophic cryptophyte abundance. We found an inverse relationship between PII size and survival after metamorphosis, showing that recruitment success was associated with smaller PII sizes. Regional climate conditions and local trophic functioning appear to be key factors in metamorphosis and consequently contribute to recruitment heterogeneity. Further studies should be performed in other ecosystems following an oligotrophication trajectory to generalize this result.

Trophic plasticity of Antarctic echinoids under contrasted environmental conditions

Echinoids are common members of Antarctic zoobenthos, and different groups can show important trophic diversity. As part of the ANT-XXIX/3 cruise of RV Polarstern, trophic plasticity of sea urchins was studied in three neighbouring regions (Drake Passage, Bransfield Strait and Weddell Sea) featuring several depth-related habitats offering different trophic environments to benthic consumers. Three families with contrasting feeding habits (Cidaridae, Echinidae and Schizasteridae) were studied. Gut content examination and stable isotopes ratios of C and N suggest that each of the studied families showed a different response to variation in environmental and food conditions. Schizasteridae trophic plasticity was low, and these sea urchins were bulk sediment feeders relying on sediment-associated organic matter in all regions and/or depth-related habitats. Cidaridae consumed the most animal-derived material. Their diet varied according to the considered area, as sea urchins from Bransfield Strait relied mostly on living and/or dead animal material, while specimens from Weddell Sea fed on a mixture of dead animal material and other detritus. Echinidae also showed important trophic plasticity. They fed on various detrital items in Bransfield Strait, and selectivity of ingested material varied across depth-related habitats. In Weddell Sea, stable isotopes revealed that they mostly relied on highly 13C-enriched food items, presumably microbially reworked benthic detritus. The differences in adaptive strategies could lead to family-specific responses of Antarctic echinoids to environmental and food-related changes.

Dietary tracers in Bathyarca glacialis from contrasting trophic regions in the Canadian Arctic

This study used fatty acid trophic markers (FATMs) to assess carbon sources of the bivalve Bathyarca glacialis and describe the pelagic-benthic coupling in the Canadian Arctic Archipelago. Four regions characterized by contrasting trophic environments were investigated: Southeastern Beaufort Sea, Victoria Strait, Lancaster Sound and Northern Baffin Bay. Our results suggest that B. glacialis is a non-selective filter feeder, feeding on microalgae, zooplankton, and bacteria. Diet was based mainly on microalgae, especially for coastal populations of the Southeast- ern Beaufort Sea. However, zooplankton and bacteria contributed more significantly to the diet of B. glacialis in bathyal populations than the coastal populations. Local and seasonal environmental conditions likely explain these differences in diet between populations. Furthermore, non- methylene- interrupted (NMI) fatty acids were present in the polar lipids of B. glacialis, which could be produced de novo when access to essential fatty acids (EFAs), required for maintaining membrane structure and function, is limited. This physiological response could help the bivalve to modulate its membrane fluidity in the face of constraints of the deep-sea environment such as low temperatures, high pressure, and when EFAs are less available in its diet. This bivalve species thus has certain attributes that could help it to cope with expected strong modifications in primary production dynamics due to climate-induced changes in the Arctic marine system.

Modeling zooplankton growth in Lake Washington: A mechanistic approach to physiology in a eutrophication model

Many efforts have been made to incorporate our improved understanding of zooplankton physiology and behaviour into mathematical models. The increased complexity, however, has been a major impediment in integrating these advances into management-oriented models and thus bridging the gap between theoretical and applied ecology. In this study, we enhance an existing eutrophication model with a zooplankton somatic growth submodel that simulates the interplay among nitrogen, phosphorus, and highly unsaturated fatty acids (HUFAs) through the grazers’ digestive tracks. We calibrate the newly incorporated parameters (and associated processes) against observed data from the mesotrophic Lake Washington. We extrapolate the model to different trophic environments and tease out the underlying drivers of zooplankton growth. Our analysis suggests that both stoichiometric and HUFA based somatic growth limitations can modulate the zooplankton biomass in mesotrophic environments. Food abundance and mineral P limitation are critical factors of zooplankton growth under oligotrophic conditions, while HUFA availability is the main driving force of plankton dynamics in eutrophic states. Our zooplankton submodel downplays pre-gut regulation in favour of post-gut metabolic processing, which appears to shift the bulk of the non-limiting nutrient recycling from particulate to dissolved form. The homeostatic maintenance of somatic quotas and the dynamic nutrient recycling could also be an important mechanism for shedding light on the controversial hypothesis that the enrichment of natural ecosystems is a destabilizing factor of food web dynamics.

Modelling the role of highly unsaturated fatty acids in planktonic food web processes: Sensitivity analysis and examination of contemporary hypotheses

Aquatic food web models typically treat the constituent trophic levels as static elements interacting with one another and the environment. Dynamic biological stoichiometry has relaxed this assumption and considers evolutionary responses in said elements. The incorporation of organismal response in food web models holds promise for a more realistic portrayal of ecosystem dynamics. Recent advances in aquatic ecology pinpoint the importance of highly unsaturated fatty acids (HUFAs) on food web interactions and ecosystem resilience. In this study, we utilized a HUFA explicit submodel in conjunction with a limiting nutrient–phytoplankton–zooplankton–detritus (NPZD) mathematical system to incorporate elements of the physiology of individual animals into the context of plankton dynamics. Our HUFA-augmented plankton model provided a realistic platform to examine functional properties and physiological strategies that modulate resource procurement in different trophic environments and to effectively link variability at the organismal level with ecosystem-scale patterns. First, we were able to illustrate the implications of the filling-cup hypothesis, in which species’ fitness stems from dynamic HUFA turnover rates in response to bottom-up stresses. We then examined an evolutionary hypothesis of consumer fitness dependence on HUFA quota management strategies, whereby adaptive individuals with low HUFA minimum and optimum requirements gain competitive advantage. Several studies have reported higher HUFA concentrations in consumers than producers, and our results suggest that this pattern could be driven by a combination of conservative turnover and elevated bioconversion rates. Oligotrophic settings showed strong reliance upon exogenous phosphorus subsidies and frequently yielded inverted food web biomass distributions. With the prevalence of eutrophic conditions, consumer growth is primarily controlled by HUFA availability, and the associated biochemical limitation can ultimately result in patterns of algal accumulation. Finally, our study discusses directions to improve the representation of the producer–grazer interactions and thus advance our understanding of the factors that determine the flow of nutrients and energy to the higher trophic levels.

Echinoderms Display Morphological and Behavioural Phenotypic Plasticity in Response to Their Trophic Environment

The trophic interactions of sea urchins are known to be the agents of phase shifts in benthic marine habitats such as tropical and temperate reefs. In temperate reefs, the grazing activity of sea urchins has been responsible for the destruction of kelp forests and the formation of ‘urchin barrens’, a rocky habitat dominated by crustose algae and encrusting invertebrates. Once formed, these urchin barrens can persist for decades. Trophic plasticity in the sea urchin may contribute to the stability and resilience of this alternate stable state by increasing diet breadth in sea urchins. This plasticity promotes ecological connectivity and weakens species interactions and so increases ecosystem stability. We test the hypothesis that sea urchins exhibit trophic plasticity using an approach that controls for other typically confounding environmental and genetic factors. To do this, we exposed a genetically homogenous population of sea urchins to two very different trophic environments over a period of two years. The sea urchins exhibited a wide degree of phenotypic trophic plasticity when exposed to contrasting trophic environments. The two populations developed differences in their gross morphology and the test microstructure. In addition, when challenged with unfamiliar prey, the response of each group was different. We show that sea urchins exhibit significant morphological and behavioural phenotypic plasticity independent of their environment or their nutritional status.

Examination of the effects of nutrient regeneration mechanisms on plankton dynamics using aquatic biogeochemical modeling

The prolonged stratification of lakes due to climate warming is expected to increase the dependence of planktonic food webs on internal nutrient regeneration mechanisms (i.e., microbial mineralization, zooplankton excretion). Our current conceptualization of aquatic communities, however, suggests that while the strength of the recycling feedback loop is indeed related to climate forcing, other biotic factors (e.g., zooplankton community composition) along with the system productivity may also be equally important. What do the contemporary operational models predict about the role of recycling rates in different trophic environments? How tight is the relationship between mineralization rates and lake warming? How realistically do modelers describe the mechanisms by which nutrients in non-living organic matter are recycled into inorganic forms? Our study addresses these questions using a complex biogeochemical model that simulates multiple elemental cycles (C, N, P, Si, O), multiple functional phytoplankton (diatoms, green algae and cyanobacteria) and zooplankton (copepods and cladocerans) groups. We relaxed the assumption of strict zooplankton homeostasis by allowing nutrient use efficiency to vary with food quality. Our analysis shows that the nutrient regeneration rates can play a major role in planktonic food webs, but their relative importance is somewhat inconsistent with the existing paradigm. We provide evidence that the recycled material and the associated energy fluxes can be significant drivers in low as well as in high-productivity ecosystems depending on the period of the year examined. Warmer climatic conditions and longer stratification periods will increase the dependence of lakes on nutrient regeneration rates. The lake productivity response, however, is non-linear and non-monotonic and is modulated by the type of nutrient limitation (nitrogen or phosphorus) experienced. Our study concludes by pinpointing some problems of the existing mathematical representation of the recycling rates, and emphasizes the need to improve our understanding of the interplay among microbial metabolism, trophic state, and lake thermal structure.

Related antipodes: a comparative study on digestive endopeptidases from Northern krill and Antarctic krill (Euphausiacea)

The Antarctic krill, Euphausia superba, and the Northern krill, Meganyctiphanes norvegica, are closely related species but occupy significantly different trophic and climatic environments. E. superba holds a key position as a phytoplankton grazer in the Southern Ocean. The omnivorous M. norvegica is an important member of plankton communities in the Northeast Atlantic. Both species expressed high proteolytic activities which were dominated by serine proteinases. In the stomachs of Antarctic krill, activities of total proteinase, trypsin, and chymotrypsin were significantly higher than in Northern krill. In the midgut glands, however, total proteinase and trypsin activities were similar in both species, but chymotrypsin activity was significantly higher in Antarctic krill. Moreover, Antarctic krill expressed four trypsin isoforms while only one isoform appeared in Northern krill. Chymotrypsin was present in either species as one single isoform. Antarctic krill adapted to the low and patchy distribution of food by elevated enzyme activities and the expression of trypsin isoforms with slightly different catalytic properties. Presumably, these enzymes facilitate in concerted action the efficient utilization of proteins from phytoplankton, the major food. Northern krill, in contrast, seems not to be equipped to face food limitation. It expresses a “simple” or “basic” set of digestive enzymes for utilizing abundant and easily digestible prey.

Distribution of lipid biomarkers and carbon isotope fractionation in contrasting trophic environments of the South East Pacific

Abstract. The distribution of lipid biomarkers and their stable carbon isotope composition was investigated on suspended particles from different contrasting trophic environments at six sites in the South East Pacific. High algal biomass with diatom-related lipids (24-methylcholesta-5,24(28)-dien-3β-ol, C25 HBI alkenes, C16:4 FA, C20:5 FA) was characteristic in the upwelling zone, whereas haptophyte lipids (long-chain (C37-C39) unsaturated ketones) were proportionally most abundant in the nutrient-poor settings of the centre of the South Pacific Gyre and on its easter edge. The dinoflagellate–sterol, 4α-23,24-trimethylcholest-22(E)-en-3β-ol, was a minor contributor in all of the studied area and the cyanobacteria-hydrocarbon, C17n-alkane, was at maximum in the high nutrient low chlorophyll regime of the subequatorial waters near the Marquesas archipelago. The taxonomic and spatial variability of the relationships between carbon photosynthetic fractionation and environmental conditions for four specific algal taxa (diatoms, haptophytes, dinoflagellates and cyanobacteria) was also investigated. The carbon isotope fractionation factor (εp) of the 24-methylcholesta-5,24(28)-dien-3β-ol diatom marker, varied over a range of 16% along the different trophic systems. In contrast, εp of dinoflagellate, cyanobacteria and alkenone markers varied only by 7–10‰. The low fractionation factors and small variations between the different phytoplankton markers measured in the upwelling area likely reveals uniformly high specific growth rates within the four phytoplankton taxa, and/or that transport of inorganic carbon into phytoplankton cells may not only occur by diffusion but also by other carbon concentrating mechanisms (CCM). In contrast, in the oligotrophic zone, i.e. gyre and eastgyre, relatively high εp values, especially for the diatom marker, indicate diffusive CO2 uptake by the eukaryotic phytoplankton. At these nutrient-poor sites, the lower εp values for haptophytes, dinoflagellates and cyanobacteria indicate higher growth rates or major differences on the carbon uptake mechanisms compared to diatoms.

Environmental influences on zooplankton and micronekton distribution in the Bransfield Strait and adjacent waters

The spatial distribution, biomass and size structure of key mesozooplankton species and micronekton in the Bransfield Strait (Antarctica) are described in relation to environmental variables during the austral summer 2001. Stratified (BIONESS) biological sampling at five depth-ranges and CTD casts were performed at 40 stations, including a cross-Strait transect. Six families, 11 genera and 16 species composed the total catch of larval and juvenile fish, which were more abundant in the upper 75 m. Trematomus scotti was the most abundant fish and large individuals dominated at depths >75 m. The fish distribution patterns were associated with the main water masses in the area and with bottom depth. Spatial segregation in density and biomass of krill (Euphausia superba) and salps (Salpa thompsoni) was observed at a relatively small scale, in relation to the main water masses. Also, size-related spatial segregation was found in krill. The present study suggests that the spatial distribution of krill, salps and early life stages of fish, are influenced by the main water masses in the Strait, and that species may adapt their reproductive strategies not only to seasonal production peaks, but to transport processes within water bodies that maximise their fitness through optimum temperature and/or trophic environments.

Distribution of lipid biomarkers and carbon isotope fractionation in contrasting trophic environments of the South East Pacific

Distribution of lipid biomarkers and carbon isotope fractionation in contrasting trophic environments of the South East Pacific

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.

Zooplankton nutrition, storage and fecal lipid composition in different water masses associated with the Agulhas and Subtropical Fronts

Fecal pellets of zooplankton can be important carriers of organic matter from surface to deep waters of the oceans. The summer cruise ANTARES 4 gave us the opportunity to collect zooplankton from the frontal subantarctic zone (station 7 or s-ant) and adjacent subtropical waters (station 8 or s-trop). Triglyceride fatty acid profiles were used to assess the trophic position of two copepods ( Metridia lucens and Pleuromamma spp.), three euphausiids ( Euphausia spinifera, Nematoscelis megalops, and Thysanoessa sp) and a decapod Acanthephyra sp. The euphausiid E. spinifera was studied at the two sites and its reaction to contrasting trophic environments and hydrology was highlighted by variations in its lipid composition. Most species showed various degree of omnivory: E. spinifera from subantarctic showed the highest degree of herbivory on phytoplankton (high proportion of 16:4(n-1), 18:5(n-3), 16:1(n-7)), while the same species at the subtropical station, as well as Thysanoessa sp and M. lucens from the subantarctic station displayed more omnivorous feeding characteristics, partly linked to the abundance of microzooplankton cells. Acanthephirasp, N. megalops and Pleuromammasp from station 8 (s-trop) appear to be essentially carnivorous (high 18:1n-9/18:1n-7 ratio, high levels of 20:1(n-9) or 22:1(n-11)). The trophic position influenced the composition of feces, with higher PUFA percentages in fecal pellets from carnivorous species. Hence, the surface faunal assemblage and the season controlled not only the quantity but also the quality of the food supply as PUFA exported towards the benthic domain.

Spatial distribution and trophic ecology of dominant copepods associated with turbidity maximum along the salinity gradient in a highly embayed estuarine system in Ariake Sea, Japan

The present study aimed to investigate into the feeding ecology of the dominant copepods along a salinity gradient in Chikugo estuary. Copepod composition was studied from samples collected from stations positioned along the salinity gradient of the estuary. Copepod gut pigment concentrations were measured by fluorescence technique and hydrographical parameters such as temperature, salinity, transparency, suspended particulate matter (SPM); pigments such as chlorophyll- a (Chl- a), phaeopigment; and particulate nutrients such as particulate organic carbon (POC) and particulate organic nitrogen (PON) were measured. Two distinct zones in terms of nutrient and pigment concentrations as well as copepod distribution and feeding were identified along the estuary. We identified a zone of turbidity maximum (TM) in the low saline upper estuary which was characterized by having higher SPM, higher POC and PON but lower POC:PON ratios, higher pigment concentrations but lower Chl- a/SPM ratios and higher copepod dry biomass. Sinocalanus sinensis was the single dominant copepod in low saline upper estuary where significantly higher concentrations of nutrients and pigments were recorded and a multispecies copepod assemblage dominated by common coastal copepods such as Acartia omorii, Oithona davisae and Paracalanus parvus was observed in the lower estuary where nutrient and pigment concentrations were lower. Copepods in the estuary are predominantly herbivorous, feeding primarily on pigment bearing plants. However, completely contrasting trophic environments were found in the upper and the lower estuary. It was speculated from the Chl- a and phaeopigment values that copepods in the upper estuary receive energy from a detritus-based food web while in the lower estuary an algal-based food web supports copepod growth. Overall, the upper estuary was identified to provide a better trophic environment for copepod and is associated with higher SPM concentrations and elevated turbidity. The study demonstrates the role of estuarine turbidity maximum (ETM) in habitat trophic richness for copepod feeding. The study points out the role of detritus-based food web as energy source for the endemic copepod S. sinensis in the upper estuary, which supports as nursery for many fish species.

Megaherbivores influence trophic guilds structure in African ungulate communities.

We used a data set of ungulate censuses from 31 natural ecosystems from East and Southern Africa to test two hypotheses: (1) megaherbivores should dominate ungulate communities in ecosystems with high rainfall and low soil nutrient status because of their ability to survive on poor quality food resources, and (2) the abundance of megaherbivores affects the abundance of the mesoherbivores, distinguishing the different feeding guilds: mesograzers, mesobrowsers and mesomixed feeders. Two axes of a multivariate analysis (77% of the variance) discriminated the sites well, the first separating sites dominated by megaherbivores from those dominated by mesoherbivores, and the second representing a gradient between mesograzers and mesobrowsers. Our analysis shows (1) that megaherbivores can be considered to be a separate trophic guild and (2) that mesograzers and mesobrowsers respond differently to variation in their trophic environments. The metabolic biomass density of megaherbivores increased with annual rainfall, but was not related to soil nutrient status, and as predicted, megaherbivores comprised a larger proportion of the biomass of ungulate communities in ecosystems with high rainfall and low nutrient soils. The metabolic biomass density of mesoherbivores increased with rainfall and soil nutrient status. Within the mesoherbivores, the metabolic biomass density of mesograzers showed the same trend, and seemed unaffected by megaherbivores. Conversely, mesobrowsers and mesomixed feeders appeared to be unaffected by rainfall or soil nutrient status, but mesomixed feeders declined when megaherbivores were abundant. This suggests that megaherbivores may compete with the mesomixed-feeder species for food or they may alter the vegetation communities unfavourably. A similar analysis using elephants alone instead of megaherbivores as a group showed that both mesobrowsers and mesomixed feeders were affected significantly by elephant, which is consistent with the fact that most of the effect of megaherbivores on browse resources or woodland habitat is due to elephants. This study shows that the different trophic guilds within African ungulate communities react differently to environmental factors (rain and soil), and that megaherbivores, and particularly elephants, appear to compete with mesomixed feeders and mesobrowsers. These results are relevant for the understanding of the functioning of African ungulate communities and call for further testing with longitudinal data.