Carbon Transfer Processes of Food Web and Trophic Pathways in a Tropical Eutrophic Seagrass Meadow

Seagrass meadows provide important habitats and rich organic carbon sources for consumers at different trophic levels but are threatened by accelerating eutrophication in coastal waters. Nevertheless, at present, carbon transfer processes throughout the food web and trophic pathways in eutrophic seagrass meadows are still poorly known. To resolve this issue, carbon sources of different trophic communities in a eutrophic tropical seagrass meadow [Xincun (XC) bay, South China Sea] under eutrophication were examined in summer and winter using dual stable isotopes. The δ13C value of omnivores and carnivores overlapped more with that of herbivores and planktivores/filter feeders, which mainly overlapped with that of epiphytes in summer and macroalgae in winter. Meanwhile, epiphytes and macroalgae exhibited high biomass and corresponding highest contribution to herbivores, omnivores, and carnivores in summer and winter, respectively. These results suggest that the grazing food chain was the main trophic pathway in this eutrophic seagrass meadow, and that the transfer of carbon flow in the grazing food chain was mainly dominated by the proliferating epiphytes or macroalgae carbon. In contrast, the contribution of seagrass to detritivores in both seasons was higher than that of other food sources. Our findings suggest that in eutrophic tropical seagrass meadows, the proliferation of epiphytes or macroalgae induced by high nutrient loading, as well as their seasonal changes, has a greater impact on the transfer of carbon in the grazing food chain than that in the detritus food chain, and the seagrass fueled the food web mainly through the detritus food chain.

Congruent trophic pathways underpin global coral reef food webs

Ecological interactions uphold ecosystem structure and functioning. However, as species richness increases, the number of possible interactions rises exponentially. More than 6,000 species of coral reef fishes exist across the world’s tropical oceans, resulting in an almost innumerable array of possible trophic interactions. Distilling general patterns in these interactions across different bioregions stands to improve our understanding of the processes that govern coral reef functioning. Here, we show that across bioregions, tropical coral reef food webs exhibit a remarkable congruence in their trophic interactions. Specifically, by compiling and investigating the structure of six coral reef food webs across distinct bioregions, we show that when accounting for consumer size and resource availability, these food webs share more trophic interactions than expected by chance. In addition, coral reef food webs are dominated by dietary specialists, which makes trophic pathways vulnerable to biodiversity loss. Prey partitioning among these specialists is geographically consistent, and this pattern intensifies when weak interactions are disregarded. Our results suggest that energy flows through coral reef communities along broadly comparable trophic pathways. Yet, these critical pathways are maintained by species with narrow, specialized diets, which threatens the existence of coral reef functioning in the face of biodiversity loss.

The Canadian Beaufort Shelf trophic structure: evaluating an ecosystem modelling approach by comparison with observed stable isotopic structure

Climate-driven impacts on marine trophic pathways worldwide are compounded by sea-ice loss at northern latitudes. For the Arctic, current information describing food web linkages is fragmented, and there is a need for tools that can describe overarching trophic structure despite limited species-specific data. Here, we tested the ability of a mass-balanced ecosystem model (Ecopath with Ecosim, EwE) to reconstruct the trophic hierarchy of 31 groups, from primary producers to polar bears, in the Canadian Beaufort Sea continental shelf. Trophic level (TL) estimates from EwE were compared with those derived from two nitrogen stable isotope (SI) modelling approaches (SI linear and scaled) to assess EwE accuracy, using a dataset of 642 δ15N observations across 282 taxa. TLs from EwE were strongly, positively related to those from both SI models (R2 > 0.80). EwE performed well (within 0.2 TL) for groups with relatively well-known diets or for taxa characterized by fewer trophic connections (e.g., primary consumers). Performance was worse (> 0.5 TL) for species groups aggregated at coarse taxonomic levels, those with poorly documented diets, and for anadromous fishes. Comparisons with SI models suggested that the scaled approach can overestimate the TL of top predators if ecosystem-specific information is not considered.

Disturbance-Mediated Changes in Coral Reef Habitat Provoke a Positive Feeding Response in a Major Coral Reef Detritivore, Ctenochaetus striatus

Coral reefs are undergoing global phase shifts from coral-dominated to algae-dominated stages. The negative effects of this substratum shift on the diversity and abundance of fish have been well documented, but the influence on fish feeding is less studied, which may limit a deeper understanding of trophic pathways in such a disturbed system. In this study, we investigated the feeding response of a numerically dominant fish species Ctenochaetus striatus to different substrate types, including hard coral, short algal turfs (SATs, <5 mm), and long algal turfs (LATs, >5 mm), on reefs in the South China Sea. The biomass of C. striatus showed an inverted U-shaped relationship with coral coverage and a significant positive correlation with SAT coverage (p < 0.05), indicating that rising SAT coverage associated with moderate coral loss provoked a feeding response in C. striatus. Stomach contents of C. striatus, analyzed using high-throughput sequencing (HTS), were dominated by algal sequences (relative read abundance, RRA > 80.0%), including macroalgae, filamentous algae, and microalgae (e.g., Symbiodinium and Prorocentrum). The sequence number and diversity of microalgae (mainly dinoflagellates) tended to be abundant (RRA 13.5–36.5%) with increased SAT cover, but brown algae sequences (RRA 17.2–57.8%) or green algae sequences (RRA > 50.7% except one site) dominated the stomach content DNA in reefs with high coral cover and high LAT or macroalgal cover, respectively. Considering the limited ability of C. striatus to remove mature algae, macroalgal DNA might be from algal debris. Our results indicate that C. striatus populations respond positively to conditions of moderate coral loss through increases in body condition identified as increased biomass. These responses are correlated to the expansion of SAT’s as coral cover declined, however, this relationship reverses if coral loss is high due to the succession of LAT’s over SAT’s and a corresponding decrease in the quality of food available. Our use of HTS has nevertheless identified the importance of detritivory in the flow of energy through reefs in the Anthropocene which are increasingly becoming depauperate in hard coral.

Linking Microbial Functioning and Trophic Pathways to Ecological Status in a Coastal Mediterranean Ecosystem

Coastal marine ecosystems host complex microbial communities whose composition and metabolism are influenced by continental inputs and mesoscale properties of seawater masses. The identifying traits of the phytoplankton and bacteria such as biomass, size, shape and their metabolism related to organic matter production and degradation, recognized as indicators of the functioning of an ecosystem, were observed in the Gulf of Manfredonia (South Adriatic Sea, Italy) in late spring. This Gulf area is characterized by terrestrial inputs and mesoscale circulation influence such as coastal waters flowing southward from the North Adriatic and offshore waters interested by the Ionian Sea. Water samples were grouped in clusters (Coastal, Intermediate, Offshore and Deep Systems) according to the water column properties. Phytoplankton community biomass and composition, autotrophic and total prokaryotic abundances and microbial metabolism such as enzyme activity rates and prokaryotic heterotrophic production were analyzed to elucidate the trophic pathways with the objective to infer on the ecosystem status. As expected, size-fractionated phytoplankton biomass and production showed greater concentration in coastal waters with prevalence of the largest fractions (micro- and nano-) supported by the diatoms. Conversely, lower biomass and production were measured in all off-shore waters, mainly sustained by smallest fractions (nano-sized phytoflagellates and picophytoplankton). Total and autotrophic prokaryotic abundance decreased from coastal to offshore stations, inversely with respect to cell volume. Prokaryotic heterotrophic production was just below 50% compared to that of phytoplankton in all waters, evidencing an active biomass synthesis. High alkaline phosphatase and leucine aminopeptidase in coastal and offshore waters suggested the quick regeneration of Phosphorus and protein decomposition, respectively. Different levels of phytoplankton-bacteria association might provide a tool to define the ecological status of the studied system in the observed period; an approach to ecosystem assessment exportable to other coastal systems is proposed.

Trophic structure of a nektobenthic community exploited by a multispecific bottom trawling fishery in Northeastern Brazil

We used complementary stable isotope (SIA) and stomach content (SCA) analyses to investigate feeding relationships among species of the nektobenthic communities and the potential ecological effects of the bottom trawling of a coastal ecosystem in northeastern Brazil. Carbon (δ13C) and nitrogen (δ15N) compositions were determined for five basal sources and 28 consumers, from zooplankton to shrimp and fish species. Fishes and basal sources showed a broad range of δ15N (fishes: 6.49–14.94‰; sources: 2.58–6.79‰) and δ13C values (fishes: -23.86 to -13.71‰; sources: -24.32 to -13.53‰), while shrimps and crabs exhibited similar nitrogen and carbon ratios. Six trophic consumer groups were determined among zooplankton, crustaceans and fishes by SIA, with trophic pathways associated mostly with benthic sources. SCA results indicated a preference for benthic invertebrates, mainly worms, crabs and shrimps, as prey for the fish fauna, highlighting their importance in the food web. In overall, differences between SCA and the SIA approaches were observed, except for groups composed mainly for shrimps and some species of high δ15N values, mostly piscivorous and zoobenthivores. Given the absence of regulation for bottom trawling activities in the area, the cumulative effects of trawling on population parameters, species composition, potentially decreasing the abundance of benthic preys (e.g., shrimps, worms and crabs) may lead to changes in the trophic structure potentially affect the food web and the sustainability of the fishery.

Food Web Structure and Trophic Interactions Revealed by Stable Isotope Analysis in the Midstream of the Chishui River, a Tributary of the Yangtze River, China

Understanding energy flow and nutrient pathways is crucial to reveal the dynamics and functions of riverine ecosystems and develop appropriate conservation strategies. In this study, we utilized stable isotopes of δ13C and δ15N to examine the fundamental characteristics of trophic position, trophic niche, and carbon source for the food web in the midstream of the Chishui River, a tributary to the Yangtze River. Our results showed that stable isotope signatures among different sorts of basal resources and consumers were significantly distinguishable and that the food chain consisted of four trophic levels, indicating the multiple trophic pathways and long food chain length here. The trophic guilds of fish were classified into four categories, in which herbivorous and carnivorous fish showed greater trophic diversity and omnivorous fish had higher trophic redundancy, which meant that there was a stable trophic niche structure in the study area. Phytoplankton and periphyton presented the largest contributions to consumers, indicating that autochthonous productivity was the dominant carbon source in the midstream of the Chishui River. Since the Chishui River is still in a natural condition without any dam constructions, the autochthonous productivity, stable trophic niche structure, multiple trophic pathways and long food chain length found here demonstrate its high conservation value. Therefore, the strategy to refrain from damming on this river should persist into the future.