Planktonic Food Web Research Articles

Is in-situ burning an acceptable mitigation option after a major oil spill? Impact on marine plankton

Major oil spills can impose a significant environmental hazard on the marine ecosystem, and a promising mitigation measure is in-situ oil burning (ISB). However, our knowledge of the impact of the burned residues and soot deposition on the marine ecosystem is still limited. We investigated the effects of burned oil residue and soot deposition on the marine plankton communities of the oligotrophic Eastern Mediterranean Sea with a mesocosm experiment. Three triplicated treatments were tested: (1) Iranian crude oil was added and burned (Burned treatment); (2) soot was collected and deposited with artificial rain (Soot); and (3) a non-contaminated Control. Results revealed that Low Nucleic Acid heterotrophic bacteria, Synechococcus spp., and pigmented pico-nano Eukaryotes (pnEuk) were negatively affected in the Burned and Soot treatments. Viruses, heterotrophic pnEuk and ciliates (in Soot) were crucial for controlling the High Nucleic Acid bacteria. Ciliates and most dinoflagellates showed a negative response to the burned residues but were less affected or were even favored when exposed to soot. Our results show that ISB affected the structure and dynamics of the plankton food web through burned residues and soot depositions. However, since the effects appeared at least three days after the ignition, ISB could be combined with subsequent burned residue collection to minimize its impact on the pelagic ecosystem.

Can planktonic food web topology be retraced by biomass measurements without internal and input flows (production and grazing rate) in freshwater marshes?

An accurate way to estimate the planktonic food web topology is to consider biomass and flows. However, measurements of flows, as production and grazing rate, are time-consuming. In this paper, we retrace food web topology based on three different degrees of information: input flows (production), internal flows (grazing rate) and plankton biomass in freshwater marshes. For that, a meta-analysis of datasets from 4 freshwater marshes of the Charente-Maritime (French Atlantic coast) were used, corresponding to 47 stations/dates and thus to different geographical and temporal situations. The main results were that, globally, the biomass can yield good results in terms of characterization of the contrasted topology of food web, as the herbivorous food web, the microbial food web and the multivorous food web. However, in order to properly distinguish weak, ‘normal‘ multivorous and strong multivorous food web, the only measure of biomass did not appear to be sufficient. To better separate these different types of multivorous food webs, measurements of primary productions or heterotrophic prokaryote biomass, which correlated well with primary productions, appear interesting. This approach can be applied in all aquatic ecosystems.

Effects of mixotroph evolution on trophic transfer

Abstract Plankton form the foundation of marine food webs, playing fundamental roles in mediating trophic transfer and the movement of organic matter. Increasing ocean temperatures have been documented to drive evolution of plankton, resulting in changes to metabolic traits that can affect trophic transfer. Despite this, there are few direct tests of the effects of such evolution on predator–prey interactions. Here, we used two thermally adapted strains of the marine mixotroph (organism that combines both heterotrophy and autotrophy to obtain energy) Ochromonas as prey and the generalist dinoflagellate predator Oxyrrhis marina to quantify how evolved traits of mixotrophs to hot and cold temperatures affects trophic transfer. Evolution to hot temperatures reduced the overall ingestion rates of both mixotroph strains, consequently weakening predator–prey interactions. We found variability in prey palatability and predator performance with prey thermal adaptation and between strains. Further, we quantified how ambient temperature affects predator grazing on mixotrophs thermally adapted to the same conditions. Increasing ambient temperatures led to increased ingestion rates but declines in clearance rates. Our results for individual, pairwise trophic interactions show how climate change can alter the dynamics of planktonic food webs with implications for carbon cycling in upper ocean ecosystems.

Direct and indirect effects of copepod grazers on community structure

Abstract Ecological theory and empirical research show that both direct lethal effects and indirect non-lethal effects can structure the composition of communities. While the direct effects of grazers on marine phytoplankton communities are well studied, their indirect effects are still poorly understood. Direct and indirect effects are inherently difficult to disentangle in plankton food webs. In this study we evaluate the indirect effects of copepod grazers on community function and structure using isolated chemical alarm signals, copepodamides. We expose intact summer and spring communities to direct grazing from copepods, or to chemical alarm cues without the presence of grazers in controlled experiments. The effects of direct grazing on ecosystem function were moderate in both experiments as indicated by levels of chlorophyll and primary production. Indirect and direct effects resulted in changes in the composition of both the eukaryote and prokaryote communities as shown by metabarcoding of 18S and 16S rRNA. Size structure analysis suggests that direct grazing and copepodamide exposure both favoured smaller organisms (< 10–15 μm) corroborating the size-structuring effect of copepod grazers. We conclude that the well-established effect of copepods on phytoplankton communities results from a combination of direct and indirect effects. This is a first attempt to isolate indirect effects of copepods on community structure and the results suggest that a full mechanistic understanding of the structuring effect of copepods will require insights to both direct and indirect effects of consumers as demonstrated for other ecosystems components.

Hypervariable DOM properties in coastal NW Mediterranean sea -evidences of strong human influences and potential consequences for the heterotrophic base of planktonic food webs

Marine dissolved organic matter (DOM) plays a key role in the carbon cycle. There is growing interest for its chemical and ecological properties but its variability in the very heterogeneous coastal environments is poorly documented. In this study, we assess the spatial and temporal variations in marine coastal DOM chemical properties and its potential to fuel the growth of the heterotrophs in the planktonic food-web. We sampled two northwestern Mediterranean coastal stations under contrasting terrestrial and human influences. From January to July 2022, dissolved organic carbon (DOC) concentration progressively increased, although highly variable spatially. DOM composition varied temporally rather than spatially. These variations appeared related to a combination of direct freshwater inputs (tracked by salinity variations), direct human contaminations (tracked by concentrations in copper and lead), and primary production (tracked by concentrations in chlorophyll a). An incubation experiment was used at each sampling date to evaluate the growth potential of heterotrophic prokaryotes and compare it to variations in DOM properties. Significantly higher growth was observed with DOM from a site under higher terrestrial and human influences. Water temperature exerted a higher control on growth than DOM properties. Correlation analysis with DOM sources suggested the uncoupling of phytoplanktonic production and growth of heterotrophic prokaryotes, which appeared better supported by human contaminations and, to a lesser extent, freshwater inputs. Sediment resuspension in harbors and antifouling paints could represent two important sources of bioavailable resources, favoring fast heterotrophic growth and higher net production, respectively. This work suggests that human activities and constructions in harbors have the potential to strengthen the heterotrophic basis of the planktonic food web.

Dissolved organic carbon (DOC) and labile organic compounds' spatial and temporal variations in coastal Indian groundwater: their bioavailability and transfer to neighboring coastal waters of India.

Submarine groundwater drainage (SGD) changes the elemental composition of the neighboring coastal ocean and impacts the biogeochemical cycles. To examine the seasonal and spatial variability in dissolved organic carbon (DOC) and labile organic compound biochemical compounds like dissolved carbohydrates (TDCHO), dissolved proteins (TDPRO), and dissolved free amino acid (TDFAA) concentrations during the dry and wet periods, groundwater samples were taken at 90 locations (180 samples) along the Indian coast. The mean DOC contents in Indian coastal groundwaters were more significant than the global mean values. DOC, TDCHO, TDPRO, and TDFAA concentrations are higher during wet than dry periods. The DOC and labile organic compound showed a substantial positive association with soil organic carbon, and respective labile compounds in soil, population, and land usage and poor relation with woodland territories, implying that soil organic compounds leaching is a source of DOC and other labile organic compounds into the groundwater. DOC and other labile compounds concentrations were linearly associated with population density, land usage, and sewage production, demonstrating that anthropogenic activities tightly regulate the formation of DOC in groundwater. During the wet and dry periods, total labile organic compounds (TDCHO, TDFAA, and TDPRO) constituted 21% and 10.5% of DOC, respectively. Compared to the wet time, more aromatic compounds accumulated during the dry season but were less bioavailable. SGD DOC flux contributed 2-7% of riverine DOC flux to the coastal ocean. The SGD flux from the Indian subcontinent to the nearby northern Indian Ocean accounts for approximately 2% of the worldwide SGD flux. The effect of DOC flux via SGD on coastal bacterial activity, the plankton food web, and the oxygen minimum zone must be studied.

From nutrients to fish: Impacts of mesoscale processes in a global CESM-FEISTY eddying ocean model framework

The ocean sustains ecosystems that are essential for human livelihood and habitability of the planet. The ocean holds an enormous amount of carbon, and serves as a critical source of nutrition for human societies worldwide. Climate variability and change impact marine biogeochemistry and ecosystems. Thus, having state-of-the-art simulations of the ocean, which include marine biogeochemistry and ecosystems, is critical for understanding the role of climate variability and change on the ocean biosphere. Here we present a novel global eddy-resolving (0.1° horizontal resolution) simulation of the ocean and sea ice, including ocean biogeochemistry, performed with the Community Earth System Model (CESM). The simulation is forced by the atmospheric dataset based on the Japanese Reanalysis (JRA-55) product over the 1958–2021 period. We present a novel configuration of the CESM marine ecosystem model in this simulation which includes two zooplankton classes: microzooplankton and mesozooplankton. This novel planktonic food web structure facilitates “offline” coupling with the Fisheries Size and Functional Type (FEISTY) model. FEISTY is a size- and trait-based model of fish functional types contributing to fisheries. We present an evaluation of the ocean biogeochemistry, marine ecosystem (including fish types), and sea ice in this high resolution simulation compared to available observations and a corresponding low resolution (nominal 1°) simulation. Our analysis offers insights into environmental controls on trophodynamics within the ocean. We find that this high resolution simulation provides a realistic reconstruction of nutrients, oxygen, sea ice, plankton and fish distributions over the global ocean. On global and large regional scales the high resolution simulation is comparable to the standard 1° simulation, but on smaller scales, explicitly resolving the mesoscale dynamics is shown to be important for accurately capturing trophodynamic structuring, especially in coastal ecosystems. We show that fine-scale ocean features leave imprints on ocean ecosystems, from plankton to fish, from the tropics to polar regions. This simulation also offers insights on ocean acidification over the past 64 years, as well as how large-scale climate variations may impact upper trophic levels. The data generated by the simulations are publicly available and will be a fruitful community resource for a large variety of oceanographic science questions.

Changes in phytoplankton size–structure alter trophic transfer in a temperate, coastal planktonic food web

AbstractMicrozooplankton grazing is an essential parameter to predict the fate of organic matter production in planktonic food webs. To identify predictors of grazing, we leveraged a 6‐yr time series of coastal plankton growth and grazing rates across contrasting environmental conditions. Phytoplankton size–structure and trophic transfer were seasonally consistent with small phytoplankton cell dominance and low trophic transfer in summer, and large cell dominance and higher trophic transfer in winter. Departures from this pattern during two disruptive events revealed a critical link between phytoplankton size–structure and trophic transfer. An unusual summer bloom of large phytoplankton cells yielded high trophic transfer, and an atypical winter dominance of small phytoplankton resulted in seasonally atypical low trophic transfer. Environmental conditions during these events were neither seasonally atypical nor unique. Thus, phytoplankton size–structure rather than environmental conditions held a key‐role driving trophic transfer. Phytoplankton size–structure is easily measurable and could impart predictive power of food‐web structure and the fate of primary production in coastal ecosystems.

Isotopic evidence for the dietary difference between Rhizostomeae Nemopilema nomurai and Semaeostomeae Cyanea nozakii

Blooms of the Rhizostomeae Nemopilema nomurai and the Semaeostomeae Cyanea nozakii have become more prominent in the coastal waters of China since the end of the 20th century. However, the trophic ecology of these jellyfish species remain incompletely understood. In this study, the trophic characterizations of N. nomurai and C. nozakii populations were assessed using stable isotope analysis (SIA), with a focus on the important bloom area offshore of the Yangtze Estuary. Our results indicated obvious trophic differences between two scyphomedusae. The higher trophic position of the C. nozakii population in the coastal planktonic food web was reflected by its relatively large δ15N value compared to that of N. nomurai. The MixSIAR model indicated that small copepods (<1000 μm) and seston were important food sources for N. nomurai, and showed a stable dietary, irrespective of N. nomurai size. Conversely, C. nozakii exhibited a more diverse diet composition, and gelatinous organisms also were an important part of the diet of C. nozakii. Moreover, a pronounced ontogenetic shift in the diet of C. nozakii was observed, consisting of an increase in the proportion of zooplanktonic prey (excluding seston) in the C. nozakii diet with diameter. This study provides isotopic evidence of the substantial difference in trophic ecology between N. nomurai and C. nozakii, which resulted from the variations in SI values and diet compositions. Inconsistent size-based variation patterns were observed in trophic ontogenetic shifts within the N. nomurai and C. nozakii groups, highlighting a need for further investigation. These results will give insights into the characteristics of trophic ecology and functional roles of Rhizostomeae and Semaeostomeae, and indicate the need for a more careful consideration of the representations of Rhizostomeae and Semaeostomeae in coastal ecosystems, so as not to underestimate the knowledge of taxon-specific ecological effects on energy flow.

Compound-specific stable isotope analyses of fatty acids indicate feeding zones of zooplankton across the water column of a subalpine lake

Spatial and temporal zooplankton feeding dynamics across the water column of lakes are key for understanding site-specific acquisition of diet sources. During this 6-week lake study, we examined stable carbon (δ13C) and nitrogen (δ15N) isotopes and conducted compound-specific fatty acid (FA) stable isotope analysis (CSIA) of edible seston in the epi-, meta-, and hypolimnion, and zooplankton of Lake Lunz, Austria. We predicted that CSIA of essential FA can discern the foraging grounds of zooplankton more accurately than the commonly used bulk stable isotopes. The δ13C and δ15N values of seston from different lake strata were similar, whereas a dual CSIA approach using stable carbon and hydrogen isotopes of FA (δ13CFA and δ2HFA) provided sufficient isotopic difference in essential FA to discern different lake strata-specific diet sources throughout the study period. We present a CSIA model that suggests strata-specific foraging grounds for different zooplankton groups, indicating higher preference of cladocerans for feeding on epilimnetic diet sources, while calanoid copepods retained more hypolimnetic resources. The CSIA approach thus yields strata-specific information on foraging strategies of different zooplankton taxa and provides more details on the spatial and temporal trophodynamics of planktonic food webs than commonly used bulk stable isotopes.

Killing the predator: impacts of highest-predator mortality on the global-ocean ecosystem structure

Abstract. Recent meta-analyses suggest that microzooplankton biomass density scales linearly with phytoplankton biomass density, suggesting a simple, general rule may underpin trophic structure in the global ocean. Here, we use a set of highly simplified food web models, solved within a global general circulation model, to examine the core drivers of linear predator–prey scaling. We examine a parallel food chain model which assumes microzooplankton grazers feed on distinct size classes of phytoplankton and contrast this with a diamond food web model allowing shared microzooplankton predation on a range of phytoplankton size classes. Within these two contrasting model structures, we also evaluate the impact of fixed vs. density-dependent microzooplankton mortality. We find that the observed relationship between microzooplankton predators and prey can be reproduced with density-dependent mortality on the highest predator, regardless of choices made about plankton food web structure. Our findings point to the importance of parameterizing mortality of the highest predator for simple food web models to recapitulate trophic structure in the global ocean.

Brownification in the Eastern Mediterranean Sea: effect of simulated terrestrial input on the planktonic microbial food web in an oligotrophic sea

Terrestrial input to marine and freshwater ecosystems colors the water yellow-brown, causing a phenomenon called “brownification”. The effect of brownification on the marine pelagic microbial food web was studied in the oligotrophic eastern Mediterranean in June 2021 by adding HuminFeed in a 15-day mesocosm experiment with 2 treatments: Control (C, no addition) and HuminFeed (HF, single dose of HuminFeed, 2 mg L-1); and 3 replicates per treatment. HuminFeed caused shading, leading to a decrease in the abundance of photo-autotrophic organisms (cyanobacteria Synechococcus and diatoms). Bacteria were positively affected by the HF addition (mainly in terms of production rather than abundance), benefiting either directly from the dissolved organic carbon (DOC) contained in HuminFeed or indirectly from the trophic cascade through the food web. Despite the decrease in HF bacterial abundance during the experiment, an increase in both the high nucleic acid containing bacteria% and heterotrophic bacterial production were observed, suggesting higher activity at the single cell level. In the HF treatment, the increased abundance of dinoflagellates observed could be due to either a dominance of mixotrophic species or a release from predation by copepods. Both ciliates and copepods were severely impacted by HuminFeed, showing lower abundance and distorted forms (ciliates) and reduced reproductive potential (copepods). In conclusion, in the ultraoligotrophic eastern Mediterranean, the simulated brownification negatively affected autotrophs and top predators while benefiting bacteria, thus indicating a shift in the structure of the plankton food web.

A diatom extension to the cGEnIE Earth system model – EcoGEnIE 1.1

Abstract. We extend the ecological component (ECOGEM) of the carbon-centric Grid-Enabled Integrated Earth system model (cGEnIE) to include a diatom functional group. ECOGEM represents plankton community dynamics via a spectrum of ecophysiological traits originally based on size and plankton food web (phyto- and zooplankton; EcoGEnIE 1.0), which we developed here to account for a diatom functional group (EcoGEnIE 1.1). We tuned EcoGEnIE 1.1, exploring a range of ecophysiological parameter values specific to phytoplankton, including diatom growth and survival (18 parameters over 550 runs) to achieve best fits to observations of diatom biogeography and size class distribution as well as to global ocean nutrient and dissolved oxygen distributions. This, in conjunction with a previously developed representation of opal dissolution and an updated representation of the ocean iron cycle in the water column, resulted in an improved distribution of dissolved oxygen in the water column relative to the previous EcoGEnIE 1.0, with global export production (7.4 Gt C yr−1) now closer to previous estimates. Simulated diatom biogeography is characterised by larger size classes dominating at high latitudes, notably in the Southern Ocean, and smaller size classes dominating at lower latitudes. Overall, diatom biological productivity accounts for ∼20 % of global carbon biomass in the model, with diatoms outcompeting other phytoplankton functional groups when dissolved silica is available due to their faster maximum photosynthetic rates and reduced palatability to grazers. Adding a diatom functional group provides the cGEnIE Earth system model with an extended capability to explore ecological dynamics and their influence on ocean biogeochemistry.

Global change alters coastal plankton food webs by promoting the microbial loop: An inverse modelling and network analysis approach on a mesocosm experiment

Marine organisms are currently, and will continue to be, exposed to the simultaneous effects of multiple environmental changes. Plankton organisms form the base of pelagic marine food webs and are particularly sensitive to ecosystem changes. Thus, warming, acidification, and changes in dissolved nutrient concentrations have the potential to alter these assemblages, with consequences for the entire ecosystem. Despite the growing number of studies addressing the potential influence of multiple drivers on plankton, global change may also cause less obvious alterations to the networks of interactions among species. Using inverse analyses applied to data collected during a mesocosm experiment, we aimed to compare the ecological functioning of coastal plankton assemblages and the interactions within their food web under different global change scenarios. The experimental treatments were based on the RCP 6.0 and 8.5 scenarios developed by the IPCC, which were extended (ERCP) to integrate the future predicted changes in coastal water nutrient concentrations. Overall, we identified that the functioning of the plankton food web was rather similar in the Ambient and ERCP 6.0 scenarios, but substantially altered in the ERCP 8.5 scenario. Using food web modelling and ecological network analysis, we identified that global change strengthens the microbial loop, with a decrease of energy transfer efficiency to higher trophic levels. Microzooplankton responded as well by an increased degree of herbivory in their diet and represented, compared to mesozooplankton, by far the main top-down pressure on primary producers. We also observed that the organisation of the food web and its capacity to recycle carbon was higher under the ERCP 8.5 scenario, but flow diversity and carbon path length were significantly reduced, illustrating an increased food web stability at the expense of diversity. Here, we provide evidence that if global change goes beyond the ERCP 6.0 scenario, coastal ecosystem functioning will be subjected to dramatic changes.

Diatom-mediated food web functioning under ocean artificial upwelling

Enhancing ocean productivity by artificial upwelling is evaluated as a nature-based solution for food security and climate change mitigation. Fish production is intended through diatom-based plankton food webs as these are assumed to be short and efficient. However, our findings from mesocosm experiments on artificial upwelling in the oligotrophic ocean disagree with this classical food web model. Here, diatoms did not reduce trophic length and instead impaired the transfer of primary production to crustacean grazers and small pelagic fish. The diatom-driven decrease in trophic efficiency was likely mediated by changes in nutritional value for the copepod grazers. Whilst diatoms benefitted the availability of essential fatty acids, they also caused unfavorable elemental compositions via high carbon-to-nitrogen ratios (i.e. low protein content) to which the grazers were unable to adapt. This nutritional imbalance for grazers was most pronounced in systems optimized for CO2 uptake through carbon-to-nitrogen ratios well beyond Redfield. A simultaneous enhancement of fisheries production and carbon sequestration via artificial upwelling may thus be difficult to achieve given their opposing stoichiometric constraints. Our study suggest that food quality can be more critical than quantity to maximize food web productivity during shorter-term fertilization of the oligotrophic ocean.

Water quality drives the reconfiguration of riverine planktonic microbial food webs

The food web is a cycle of matter and energy within river ecosystems. River environmental changes resulting from human activities are increasingly threatening the composition and diversity of global aquatic organisms and the multi-trophic networks. How multiple environmental factors influence food web patterns among multi-trophic microbial communities in rivers remains largely unknown. Using water quality evaluation and meta-omics techniques, we investigated the composition, structure and interaction characteristics, and drivers of food webs of microorganisms (archaea, bacteria, fungi, protists, metazoa, viridiplantae and viruses) at multiple trophic levels in different water quality environments (Classes II, III, and IV). First, water quality deterioration led to significant changes in the composition of the microbial community at multiple trophic levels, which were represented by the enrichment of Euryarchaeota in the archaeal community, the increase of r-strategists in the bacterial community, and the increase of the proportion of predators in the protist community. Second, deteriorating water quality resulted in a significant reduction in the dissimilarity of community structure (homogenization of community structure in Class III and IV waters). Of the symbiotic, parasitic, and predatory networks, the community networks in Class II water all showed the most stable symbiotic, parasitic, and predatory correlations (higher levels of modularity in the networks). In Class III and IV waters, nutrient inputs have led to increased reciprocal symbiosis and decreased competition between communities, which may have the risk of a positive feedback loop driving a system collapse. Finally, inputs of phosphorus and organic matter could be the main drivers of changes in the planktonic microbial food web in the Fen River. Overall, the results indicated the potential ecological risks of exogenous nutrient inputs, which were important for aquatic ecosystem conservation.

Insights into planktonic food-web dynamics through the lens of size and season

Knowledge of the trophic structure and variability of planktonic communities is a key factor in understanding food-web dynamics and energy transfer from zooplankton to higher trophic levels. In this study, we investigated how stable isotopes of mesozooplankton species varied seasonally (winter, spring, autumn) in relation to environmental factors and plankton size classes in a temperate coastal ecosystem. Our results showed that spring is characterized by the strongest vertical and size-structured plankton food-web, mainly fueled by the phytoplankton bloom. As a result, spring displayed the largest isotopic niche space and trophic divergence among species. On the contrary, both pelagic and benthic-derived carbon influenced low productive seasons (winter and autumn), resulting in more generalist strategies (trophic redundancy). Stable isotope mixing models were used to explore how different seasonal structures influenced the overall food web up to predatory plankton (i.e., mysids, chaetognaths, and fish larvae). Different feeding strategies were found in spring, with predators having either a clear preference for larger prey items (> 1 mm, for herring and dab larvae) or a more generalist diet (sprat and dragonets larvae). During low productive seasons, predators seemed to be more opportunistic, feeding on a wide range of size classes but focusing on smaller prey. Overall, the food-web architecture of plankton displayed different seasonal patterns linked to components at the base of the food web that shaped the main energy fluxes, either from phytoplankton or recycled material. Additionally, these patterns extended to carnivorous plankton, such as fish larvae, emphasizing the importance of bottom-up processes.

Transcriptomic insights into the shift of trophic strategies in mixotrophic dinoflagellate Lepidodinium in the warming ocean.

The shift between photoautotrophic and phagotrophic strategies in mixoplankton significantly impacts the planktonic food webs and biogeochemical cycling. Considering the projected global warming, studying how temperature impacts this shift is crucial. Here, we combined the transcriptome of in-lab cultures (mixotrophic dinoflagellate Lepidodinium sp.) and the metatranscriptome dataset of the global ocean to investigate the mechanisms underlying the shift of trophic strategies and its relationship with increasing temperatures. Our results showed that phagocytosis-related pathways, including focal adhesion, regulation of actin cytoskeleton, and oxidative phosphorylation, were significantly stimulated in Lepidodinium sp. when cryptophyte prey were added. We further compared the expression profiles of photosynthesis and phagocytosis genes in Lepidodinium sp. in the global sunlit ocean. Our results indicated that Lepidodinium sp. became more phagotrophic with increasing temperatures when the ambient chlorophyll concentration was >0.3mg.m-3 (~20.58% of the ocean surface) but became more photoautotrophic with increasing temperatures when the chlorophyll concentration was between 0.2 and 0.3mg.m-3 (~11.47% of the ocean surface). Overall, we emphasized the crucial role of phagocytosis in phago-mixotrophy and suggested that the expression profile of phagocytosis genes can be a molecular marker to target the phagotrophic activity of mixoplankton in situ.

Trophic variability of inter- and intra-copepod species in the South Sea of Korea during summer

Spatial variation and interspecies differences in the trophic position (TP) of copepods were investigated using nitrogen isotope ratios of amino acids. In the summer of 2021, coastal waters and the Changjiang diluted water generated clear seawater temperature and salinity fronts in the South Sea of Korea. Paracalanus parvus s. l. was a dominant species in the copepod community, and the second dominant species differed among inshore, intermediate, and offshore sites. The TP of each copepod species was estimated in two ways, considering only metazoan diets (TPGlu, based on glutamic acid and phenylalanine nitrogen isotope ratios) and both metazoan and protistan diets (TPAla, based on alanine and phenylalanine nitrogen isotope ratios). Both TPGlu and TPAla indicated trophic variability among copepod species and the contribution of protistan diets as a food source in the study area. Calanus sinicus showed a similar herbivorous TP of 2.0 in both TPGlu and TPAla, suggesting little contribution from protistan diets. Two copepod species (P. parvus s. l. and Acartia omorii) exhibited TPGlu values of approximately 2.0 but their TPAla values increased from 0.1 to 0.5, indicating mixed diets of both primary producers and protists. The other three copepods (Pseudocalanus sp., Oithona similis, and O. atlantica) showed a wide range in TPGlu (2.4–3.1) and TPAla (2.7–3.4), suggesting that protistan trophic transfers enhance TPAla (by up to 0.5) in omnivorous copepods. We found a spatial variation in the TPs of copepods among water masses by various controlling factors including surface seawater temperature, salinity, and size-fractionated Chl-a. Our findings support that the TP values could be potential indicative of interspecies variability, providing useful information on the composition of the planktonic food web.

Long-term oscillations in the normalized biomass-size spectrum reveal the impact of oligotrophication on zooplankton trophic structure in the North Atlantic Subtropical Gyre

Ocean warming of the North Atlantic Subtropical Gyre (NASG) induced oligotrophication and a decrease in integrated net primary production during the 2010s, potentially affecting higher trophic levels. We analyzed long-term records (1994–2019) of daytime and nighttime zooplankton biomass in five size classes from the NASG. Daytime biomass decreased in the three largest size classes during the 2010s, while decrease in nighttime biomass was less evident due to the relative stability in diel vertical migrator biomass. We used the normalized biomass size spectrum (NBSS) to estimate the relative transfer efficiency between trophic levels. The steepness of the NBSS slope at the end of the time series increased by 14% (daytime) and 24% (nighttime) from the maximum observed annual average values (2011 and 2009, respectively). This suggests oligotrophication during the 2010s led to a significant reduction in the transfer of biomass across trophic levels, with negative impacts on the NASG planktonic food web.