Pelagic Food Web Research Articles

Movement ecology of a deep-pelagic mesopredator, the bigscale pomfret: implications for pelagic food web connectivity and fishery susceptibility

Movement ecology of a deep-pelagic mesopredator, the bigscale pomfret: implications for pelagic food web connectivity and fishery susceptibility

Hidden Complexities in the Base of the Pelagic Food Web: Correlates Between Zooplankton and Chlorophyll Vary by Functional Feeding Group

Zooplankton in estuaries provide an important link between primary production and fish. Resource managers in the San Francisco Estuary have several initiatives designed to increase phytoplankton production, expecting zooplankton increases to follow. However, it is not always clear if an increase in phytoplankton biomass will lead to an increase in zooplankton biomass. We used data from 20 years of zooplankton and chlorophyll-a monitoring in the estuary to create linear models of zooplankton abundance versus chlorophyll-a concentration, salinity, turbidity, and microzooplankton biomass (rotifers and copepod nauplii) for 12 of the most abundant zooplankton taxa, categorized by functional feeding guilds (herbivores, omnivores, predators). We then used 50 years of zooplankton data to assess changes in relative abundance of the three feeding guilds, over time and by salinity. We found that herbivorous taxa were positively related to chlorophyll-a concentration, whereas predatory taxa were not, and omnivorous taxa had mixed results. There were positive correlations between microzooplankton and abundance of most of the target taxa. We also documented dominance of herbivores in freshwater regions while omnivorous and predatory taxa dominate in brackish water. There has been an increase in abundance of omnivorous taxa in all salinity zones over time, and an increase in predatory taxa in brackish water. Taken together, these results indicate that management actions designed to increase phytoplankton abundance may be effective in freshwater areas where herbivorous taxa dominate but may be less effective in brackish-water areas where predatory and omnivorous taxa dominate. The increase in predatory copepods in brackish-water areas may increase food chain length and therefore decrease trophic efficiency in the transfer of carbon to fishes at the top of the food web.

Shifts in the feeding ecology of the endemic and Endangered Sardinella tawilis in Lake Taal, Philippines

Understanding fish feeding ecology may provide key ecological information for fisheries conservation and management. In Lake Taal, Philippines, the Endangered status of the endemic Sardinella tawilis, the world’s only freshwater Sardinella species, underscores the threats and vulnerability of its remaining population due to the recent eruption of Taal Volcano and ongoing anthropogenic pressures. In this study, we examined possible shifts in the feeding ecology of S. tawilis by assessing its trophic position through the evaluation of the pelagic food web structure in Lake Taal and analyzing changes in its prey preferences, diet composition, morphometrics, and gonadal maturity between 2 collection periods: pre- and post-eruption of Taal Volcano. Stable isotope analysis (SIA) quantitatively confirmed that S. tawilis primarily feeds on zooplankton due to its size-selective zooplanktivorous feeding behavior. This was further confirmed by gut content analysis (GCA) revealing the dominance of copepods and cladocerans in its diet. Temporal comparison of diet composition and the selectivity index indicated a shift in the most preferred prey item from calanoids to cyclopoid copepods and a noticeable increased preference for Moina micrura, Ceriodaphnia cornuta, and Bosmina fatalis. However, morphometry and gonadal maturity showed no significant differences between the collection periods. Together, these findings provide a 15 yr update on the feeding ecology of S. tawilis, and demonstrate the capacity of combined SIA and GCA to elucidate trophic interactions. These could aid stakeholders and policymakers in creating ecosystem-based conservation and management strategies to aid recovery of this Endangered species.

Genetic diversity of hyperiid amphipods along a meridional transect in the Atlantic Ocean

Abstract Detecting plankton community responses to a changing ocean environment requires knowledge of the diversity and distribution of species. Hyperiid amphipods are an important component of pelagic food webs as prey and as commensals and parasitoids of gelatinous zooplankton, but their species and genetic diversity remains incompletely known. Using mitochondrial cytochrome c oxidase subunit 1 (CO1) gene sequences, we assessed the genetic diversity of hyperiids collected along a meridional transect from 39 °N to 45 °S in the Atlantic Ocean in order to evaluate the congruence of genetic clades with morphologically-identified species, examine phylogeographic patterns within broadly-distributed species, and to contribute vouchered DNA barcodes into the public domain. CO1 sequences were obtained from 273 specimens representing 63 morphologically-identified hyperiid species belonging to 34 genera and 16 families, including sequences from 26 species with no prior reports in publicly-available databases. We recovered substantial amounts of genetic variation within nine nominal species, which were composed of two to four clades with 11–36% average sequence divergence between clades (range 10–45%). These are probably distinct or unrecognized biological species, however, additional sampling of morphological and/or genomic diversity is necessary to confirm this. Additionally, we examined the geographic distribution of genetic variation within Lycaea pulex, Phronimopsis spinifera, Phrosina semilunata, Tryphana malmii, and Vibilia armata, all species with broad or disjunct distributions across the Atlantic Ocean. We found that these species are composed of lineages with apparent geographic specializations to tropical, subtropical or temperate waters, suggesting more restricted biogeographic distributions than described for the inclusive morphospecies. This new collection of CO1 barcodes furthers our understanding of the diversity and distribution of hyperiids in the Atlantic Ocean and will support future research on this ecologically significant plankton group.

Changes in the swimming behavior of Temora turbinata (Copepoda, Calanoida) in response to sub-lethal concentrations of caffeine and triclosan.

Changes in the swimming behavior of Temora turbinata (Copepoda, Calanoida) in response to sub-lethal concentrations of caffeine and triclosan.

Drivers of Mercury Accumulation in Juvenile Antarctic Krill, Epipelagic Fish and Adélie Penguins in Different Regions of the Southern Ocean

Antarctica and the Southern Ocean are important sinks in the global mercury (Hg) cycle, and in the marine environment, inorganic Hg can be converted by bacteria to monomethylmercury (MeHg), a highly bioavailable and toxic compound that biomagnifies along food webs. In the Southern Ocean, higher concentrations of Hg and MeHg have typically been reported in the coastal waters of the Ross and Amundsen Seas, where katabatic winds can transport Hg from the Antarctic Plateau and create coastal polynyas, which results in spring depletion events of atmospheric Hg. However, some studies on MeHg biomagnification in Antarctic marine food webs have reported higher Hg concentrations in penguins from sub-Antarctic waters and, unexpectedly, higher levels in juvenile krill than those in adult Antarctic krill. In light of recent estimates of the phytoplankton and zooplankton biomass and distribution in the Southern Ocean, this review suggests that although most studies on MeHg biomagnification refer to the short diatom–krill–vertebrate food chain, alternative and more complex pelagic food webs exist in the Southern Ocean. Thus, juvenile krill and micro- and mesozooplankton grazing on very small autotrophs and heterotrophs, which have high surface-to-volume ratios for MeHg ad-/absorption, may accumulate more Hg than consumers of large diatoms, such as adult krill. In addition, the increased availability of Hg and the different diet contribute to a greater metal accumulation in the feathers of Adélie penguins from the Ross Sea than that of those from the sub-Antarctic.

Trophic ecologies and dietary niches of Arctic charr (Salvelinus alpinus) and three coastal forage fishes in Frobisher Bay, Nunavut

Here, the trophic ecology of four mid-trophic level fishes is described for an Arctic coastal marine habitat near Iqaluit, Nunavut. Arctic charr (Salvelinus alpinus), Arctic cod (Boreogadus saida), Fish Doctor (Gymnelus viridis), and sculpins (Cottidae) diet and feeding strategies were estimated using gut content; dietary niches were compared using stable isotopes (δ13C, δ15N); and relationships between diet indices and metrics of fish condition, including calorie content, were assessed. While the four taxa differed in foraging strategy, targeted prey, and the strength of associations with benthic and pelagic food web pathways, niche overlap occurred among the benthic and pelagic taxa. Pelagic Arctic charr and Arctic cod specialized in hyperbenthic amphipods and copepods, respectively, with evidence that selectivity was flexible. Fish Doctor and sculpins were benthic generalists with evidence for inter-individual and population-level specialization. Arctic charr occupied a central isotopic niche, resulting in a high probability of dietary niche overlap with the other three taxa. Fish Doctor and sculpins were likely to overlap with each other, and both had a low probability of overlap with Arctic cod. Isotopic diet indicators did not significantly explain variation in fish condition or calorie content. Findings reiterate the complex trophic ecologies that allow Arctic fish to divide resources and respond to changes in prey availability.

Chronic Broadband Noise Increases the Fitness of a Laboratory-Raised Freshwater Zooplankton.

Although there is an increasing interest in the effects of anthropogenic noise on underwater wildlife, most studies focus on marine mammals and fish, while many other taxa of substantial ecological importance are still overlooked. This is the case for zooplankton species, which ensure the coupling between primary producers and fishes in pelagic food webs. Here, we measured lifespan, reproduction, and mobility of laboratory-raised water fleas Daphnia magna, a widespread freshwater zooplankton species, in response to continuous broadband noise. Surprisingly, we found a significant increase in survival and fecundity, leading to a higher individual fitness when considering total offspring production and a slight increase in the population growth rate according to the Euler-Lotka equation. Exposed water fleas were found to be slower than control individuals, and we discussed potential links between mobility and fitness. Our results can have implications in aquaculture and for in-lab studies (e.g., in ecotoxicology) where the acoustic environment receives little attention. Chronic broadband noise can be associated with certain human activities, but the consequences for natural Daphnia populations might differ as reduced velocity could have negative outcomes when considering competition and predation. Our work is one of the few showing an effect of noise on individual fitness and suggests that noise should be better accounted for in laboratory studies.

Spring Food Spectrum of Two Planktivores (Sardina pilchardus Walbaum 1792 and Sardinella aurita Valenciennes 1847) From the Central Algerian Coast (3° E to 4° E—South West Mediterranean Sea) in Relation to Sexes and Sizes

ABSTRACTSardina pilchardus, Walbaum 1792, and Sardinella aurita, Valenciennes 1847, are the fish most consumed by the Algerian population. Sardine has become increasingly rare in commercial fishing catches, whereas round sardinella is being sold at excessively high prices. Given the importance of trophic interactions between clupeids and the low levels of the pelagic food web (plankton and others), the stomach contents of the two species of S. pilchardus and S. aurita are studied. Samples are collected from the spring commercial fishery (April and May 2023) associated with the two main ports on the central coast (Algiers and Bou Ismail) justified by the intense trophic activity of these two predators. High feeding intensity (or stomach vacuity coefficient) is recorded in both males and females. Our results support the hypothesis that the diet of species in specific zones remains consistent due to stable local trophic resources. This is underscored by the lack of variation in prey quality and quantity across size and sex in both clupeid species, emphasizing the role of these interactions within the pelagic food web. Using the morphological identification and counting technique, the preys are linked to 12 taxonomic units belonging to the zooplankton, phytoplankton, and miscellaneous communities. The analysis of the diet is carried out by classifying the prey on the basis of the frequency index (If), taking into account the sex and the size where three categories of prey are defined. Copepods are the preferred prey with rates that did not differ significantly by sex and place of origin of the two clupeids. These findings extended to secondary prey (other zooplankton and phytoplankton). Also, the composition of the diet remained homogeneous in the Gulfs of Algiers and Bou Ismail. Quantitative and qualitative results on diet composition of the two clupeids and their trophic interactions within the two target ecosystems will serve as a basis for fisheries management (i.e., Ecosystem Approach to Fisheries Management [EAFM] and Ecosystem‐Based Fisheries Management [EBFM]). The modeling of all the variables will allow decision makers to predict the level of exploitable biomass in the medium and long term.

Reviews and syntheses: On increasing hypoxia in eastern boundary upwelling systems – zooplankton under metabolic stress

Abstract. Eastern boundary upwelling systems (EBUSs) are ecologically and economically important marine regions of the world ocean. In these systems, zooplankton play a pivotal role in transferring primary production up through the food web. Recent studies show that global warming is causing a gradual deoxygenation of the world ocean, while in EBUSs, an expansion and intensification of the subsurface oxygen minimum zone (OMZ) is taking place, further exacerbating hypoxic conditions for zooplankton inhabiting the upwelling zone. Hypoxia can affect zooplankton by limiting their aerobic respiration and constraining the migration, energy budget, reproduction, and development. These effects, however, depend on some specific adaptations evolved in habitats, permanently or episodically, subjected to low-oxygen waters. Various metabolic, physiological, behavioural, and morphological adaptations have been described in zooplankton interacting with the OMZ. Adjustment of the aerobic respiration under variable oxygen levels deserves special attention since such adaptive responses to endure mild or severe hypoxia may involve trade-offs in energy usage that impact other metabolic functions or energy-demanding processes. In addition, the oxidative stress resulting from exposure to highly fluctuating oxygen conditions in the upwelling zone can impose further energy expenses. New demands imply a reduction in the energy budget otherwise available for escape, migration, growth, feeding, and reproduction with further ecological consequences for population and community dynamics. This paper reviews and explores the existence or lack of such adaptive metabolic responses along with potential effects of oxidative stress and their role in zooplankton dynamics in EBUSs with major consequences for the pelagic food web and biological productivity.

You Can't Reach the Lost Valley by Boat: Navigating Bottom‐Up Restoration Pathways

ABSTRACTWe explore bottom‐up restoration pathways on the West Florida Shelf using an Atlantis end‐to‐end biogeochemical food web model. We simulate three ecosystem states, the 1990s, the present day, and a restored future ecosystem. The restored ecosystem is informed by current restoration efforts by the Florida Trustee Implementation Group, responsible for administering restoration projects totaling $680 million. These will restore nearshore habitat, improve water quality and promote user access. We consider four bottom‐up drivers of productivity: seagrass coverage, mangrove presence, wave energy and nutrient inputs. We developed a series of generalised additive models to analyse Florida Wildlife Commission fisheries independent monitoring fish abundance data from inshore sites in Florida. After correcting for confounding environmental variables using statistical models, we describe the relationship between relative abundance and habitat. These relationships were used to parameterize recruitment and feeding effects in Atlantis to reflect the ecology of seagrass‐associated fish. Model results suggest restoration efforts in inshore areas yield a 2%–3% increase in small‐bodied forage fish and support a more robust pelagic food web. Restoration increases demersal fish abundance, and there is a suggestion that mangroves may promote this via a habitat mosaic effect. Cross‐shelf ontogenetic migration offers an express route by which terrigenous production is exported to offshore food webs, but benefits to species of interest are variable. There were increases of 1%–3% for pelagic fish groups and 1%–5% for demersal fish groups.

Hypoxia zone and functional group losses in zooplankton community structure

The oxygen in the oceans is decreasing, and this topic is little commented on in scientific studies. The reduction in oxygen occurs mainly in places that receive a greater availability of nutrients, either naturally or anthropically, which increases the excessive proliferation of phytoplankton that develop the hypoxic zones. Hypoxic zones are increasing with the effects of land use and fertilizers, global warming, and climate change among other reasons. In this review, the following were analyzed: 1) hypoxia zone increase associated with natural anthropogenic effects such as: eutrophication, global warming and climate change, 2) the correlation found in pelagic food web with loss of functional group with emphasis on zooplankton community as a response to adaptations in the hypoxia zone. Hypoxia zones have been causing changes in the food web on a global scale, and this effect becomes even more evident if steps are not taken to reduce effluents and environmental imbalances.

Observation‐Based Estimate of Net Community Production in Antarctic Sea Ice

Abstract Antarctic sea ice is one of the largest biomes on Earth providing a critical habitat for ice algae. Measurements of primary production in Antarctic sea ice remain scarce and an observation‐based estimate of primary production has not been revisited in over 30 years. We fill this knowledge gap by presenting a newly compiled circumpolar data set of particulate and dissolved organic carbon from 362 ice cores, sampled between 1989 and 2019, to estimate sea‐ice net community production using a carbon biomass accumulation approach. Our estimate of 26.8–32.9 Tg C yr−1 accounts for at least 15%–18% of the total primary production in the Antarctic sea‐ice zone, less than a previous observation‐based estimate (63–70 Tg C yr−1) and consistent with recent modeled estimates. The results underpin the ecological significance of sea‐ice algae as an early season resource for pelagic food webs.

Ocean color anomaly detection to estimate surface Calanus finmarchicus concentration in the Gulf of Maine

The planktonic copepod, Calanus finmarchicus, plays a pivotal role in the Gulf of Maine (GoM) pelagic food web as a primary food source for many species, including the critically endangered North Atlantic right whale (NARW). Thus, observing C. finmarchicus on a Gulf-wide scale via satellite could be beneficial for understanding changes in the migration patterns of the NARW. This study investigated the application of ocean color remote sensing to detect the surface population levels of C. finmarchicus in the GoM. Using remote sensing reflectance data from the MODIS Aqua sensor, we processed enhanced RGB (eRGB) imagery to detect and quantify the presence of C. finmarchicus, which is identifiable by its red astaxanthin pigment. This study employed a refined approach from the method originally developed off the coast of Norway, which integrates eRGB imagery and radiative transfer modeling to generate optical anomaly maps that are used for quantifying surface C. finmarchicus concentrations in the GoM. We detected surface swarms of C. finmarchicus in the ocean color imagery and estimated their concentrations. However, due to the method’s reliance on astaxanthin/red pigment-based detection, other astaxanthin-rich red/brown plankton were misidentified as C. finmarchicus. While the approach presented is effective for identifying astaxanthin anomalies in ocean color and holds potential for quantifying the surface populations of C. finmarchicus, it requires local knowledge to accurately quantify the C. finmarchicus abundances.

Investigating plankton size spectra, biomass, abundance, and community composition in the Subtropical Convergence Front in the Southern Ocean

Phytoplankton community structure is crucial to pelagic food webs and biogeochemical processes. Understanding size-based biomass distribution and carbon dynamics is essential for assessing their contributions to oceanic carbon cycling. This study quantifies plankton carbon (C) based size spectra, community composition, living to total particulate organic carbon (POC) and C:Chlorophyll a (C:Chla) ratios across biogeographical provinces in the Pacific sector of the Southern Ocean near the Subtropical Front (Chatham Rise, Aotearoa-New Zealand). We analyzed phytoplankton community composition using epifluorescence microscopy and flow cytometry, while quantifying size-fractionated Chl-a and POC to estimate normalized biomass, abundance size spectra, and C:Chla ratios. On average, subtropical-influenced waters had lower macronutrients, higher total Chla (1.1 ± 0.2 μg Chla L-1) and were dominated by nanoplankton, which accounted for 45% of the total plankton community (35.2 ± 4.6 μg C L-1). In contrast, picoplankton dominated plankton communities within the subantarctic-influenced and accounted for 35% of the total plankton community (18.5 ± 0.9 μg C L-1) in these water with higher macronutrient concentrations and lower total Chla concentrations (0.32 ± 0.06 μg Chla L-1). Subantarctic-influenced regions had steeper (more negative) slopes for the normalized biomass size spectrum (average = -1.00) compared to subtropical-influenced waters (average = -0.78) indicating greater relative dominance of small taxa. The subantarctic-influenced region had ~2-fold higher surface average C:Chla ratios compared to the subtropical-influenced region with picoplankton consistently having lower C:Chla ratios, due to low Chla values, than larger nano- or microplankton. Live plankton carbon contributed a median of 67% of total particulate organic carbon in the euphotic zone (non-living detritus comprises the remaining ~1/3), which is indicative of substantial primary production and rapid recycling by a strong microbial loop. Our study provides important insights into phytoplankton community structure, biomass distribution and their contribution to carbon sequestration in this region, highlighting the important roles of nanoplankton in subtropical productive waters and picoplankton in offshore subantarctic waters as well as a strong variation of C:Chla across different phytoplankton size classes.

Little seasonal variation of mercury concentrations and biomagnification in an Arctic pelagic food web

Despite numerous studies on mercury in Arctic biota, data from inaccessible, ice-covered regions − especially during the polar night and late winter − remain scarce. This scarcity results in poor understanding of the seasonal dynamics of mercury within the food web. From the Northern Barents Sea, we quantified total mercury and the dietary descriptors δ15N and δ13C as long-term dietary signals (weeks to months) in biota to a) investigate the seasonal pelagic food web structure, b) seasonality in total mercury concentration, c) and its biomagnification in the food web. Mercury and dietary descriptors were analyzed in copepods, macrozooplankton (krill, amphipods, arrow worms and pteropods) and the fishes, Atlantic cod (Gadus morhua), polar cod (Boreogadus saida) and capelin (Mallotus villosus) during spring, late summer, early,and late winter. Seasonal changes were observed in δ15N values in some macrozooplankton and capelin, and some seasonal variation was observed across the food web with depleted δ13C values in spring and enriched δ13C values in late summer. Mercury concentrations were lower (range: 2.49 ng/g dw in the krill Thyssanossa sp. – 70.55 ng/g dw in the pelagic pteropod Clione limacina) than reported from other parts of the Arctic. We found a positive linear relationship between mercury and relative trophic position represented by δ15N, i.e., biomagnification, during all seasons, except in early winter. As Clione limacina likely had different turnover rates for mercury and stable isotopes resulting in low δ15N, but high mercury concentrations in early winter, compared to the other species in the food web, the pteropod was omitted from the regression. By omitting Clione limacina, biomagnification was similar across all seasons (R2adj = 0.45). Thus, we saw clear mercury biomagnification with consistent and little seasonal variation in this high Arctic marine food web despite large seasonal fluctuations in abiotic and biotic conditions.

Carbon and Nitrogen Isoscapes of Particulate Organic Matter in the Pacific Ocean

Abstract Large‐scale geographical distributions in nitrogen and carbon stable isotope ratios (δ15N and δ13C) of particulate organic matter (POM) are essential to understand the variation in the baseline of pelagic food webs in the Pacific Ocean, where phytoplankton production and biological N2 fixation are highly variable because of heterogeneity of nitrate and iron supply. Here, we determined their isoscapes during summer and discussed potential factors characterizing regional ecosystems from the viewpoint of nitrogen cycling. We collected a total of 2,289 and 2,278 isotope values for δ13C and δ15N, respectively, by synthesizing previously published data with our newly measured data, and analyzed their relationships with temperature, concentrations of nitrate and chlorophyll‐a, and N2 fixation activity, obtained from databases. POM δ13C and δ15N regionally varied in ranges of −30 to −18‰ and −4 to 14‰, respectively. POM δ13C was correlated positively with temperature throughout the ocean. In contrast, POM δ15N was negatively correlated with nitrate concentration at high latitudes and with N2 fixation activity at low latitudes. High values (>8‰) of POM δ15N were identified mainly in the marginal area of equatorial upwelling; the highest values (10–14‰) were in the subtropical Southeastern Pacific. Using the isotopic values and nitrate concentration, we classified the ecosystems into 10 groups. Our data demonstrated the distribution patterns of ecosystems with different degrees of nitrate utilization, which are presumably associated with iron supply, and ecosystems sustained by different nitrogen sources: diazotrophic nitrogen and nitrate supplied below the nitracline and/or horizontally advected.

Mercury bioaccumulation and assimilation in marine plankton in meltwater influenced fjords and shelf waters along the east coast of Greenland.

The rapid melting of the Arctic cryosphere due to climate change will result in significant freshwater input into Arctic marine ecosystems. This might also cause the release of legacy mercury (Hg) stored in the cryosphere, increasing Hg concentration and its subsequent effects on the marine biota. However, there is scarce knowledge on the concentration of Hg in the lower trophic level organisms at the base of the Arctic pelagic food web. This is particularly important since these organisms modulate the transfer of Hg to higher trophic levels, including fish and marine mammals. We quantified the Hg concentration in two plankton size classes (> 200 and 50 – 200 μm) in coastal waters along the east Greenland coast and investigated the potential assimilation efficiency of both inorganic Hg (IHg) and methyl Hg (MeHg) in mesozooplankton and their faecal pellets in experimental incubations. The concentration of Hg in plankton ranged from 12 to 109 ng (g dw)-1 without clear trends between geographic locations or between fjords and coastal areas. Also, the concentrations did not vary between the different plankton size fractions. MeHg concentrations were lower in the mesozooplankton faecal pellets than IHg, which may be due to the higher assimilation of MeHg than IHg in mesozooplankton tissue. Our results confirm that Arctic zooplankton assimilates MeHg more efficiently than IHg and may contribute significantly to the partitioning and cycling of different Hg types in Arctic marine ecosystems.

Trophic ecology of squids in the Benguela Upwelling System elucidated by combining stomach content, stable isotope and fatty acid analyses

Squids occur worldwide in marine ecosystems and play a major role in pelagic food webs by connecting lower and higher trophic levels. Their high feeding intensities and pronounced diel vertical migrations make squids important components of nutrient cycles in marine ecosystems. This study assessed the trophic position and nutritional ecology of a squid species assemblage in the Benguela Upwelling System (BUS) by combining stomach content, stable isotope and fatty acid trophic marker analyses. Samples were collected in the northern (nBUS) and southern (sBUS) BUS in austral spring 2021. A total of 20 squid species from eleven families were identified. Stomach content analyses showed that squid species preyed on a variety of organisms ranging from crustaceans to lanternfishes (Myctophidae) and flying squids (Ommastrephidae). Stable isotope analyses revealed significant differences in δ15N values of small squids (dorsal mantle length < 10 cm) between the two subsystems, but none in those of large squids (dorsal mantle length > 10 cm). Trophic levels ranged from 2.2 to 5.0. Isotopic niche width was widest in the families Ommastrephidae and Cranchiidae. Principal component analyses of fatty acid compositions displayed distinct clusters separating squid families and different prey taxa. This study shows that nBUS and sBUS squids exploit a large variety of pelagic prey organisms and that trophic differences are primarily dependent on squid species and size. The results emphasize the squids’ importance as interlinking element in pelagic food webs and their key function in energy transfer between epi- and mesopelagic layers.

Bivalves as a Mercury Bioindicator: A National Isotopic Survey along the Coast of South Korea.

Mercury (Hg) is a contaminant that poses health risks for human populations relying on seafood consumption. To mitigate its impact, identifying and monitoring Hg sources have become priorities, notably under the Minamata Convention. Bivalves are commonly used as sentinels in contaminant biomonitoring but can accumulate Hg from diverse environmental media. To investigate their Hg sources, bivalves (blue mussel, Mytilus edulis, and Pacific oyster, Crassostrea gigas) and their associated sediment were sampled along the coast of South Korea and analyzed for Hg concentration and isotopic values. Oysters displayed low Δ199Hg (0.19 ± 0.19‰) and δ202Hg (-0.35 ± 0.55‰), highlighting a sedimentary source, whereas mussels exhibited higher values (0.72 ± 0.87 and 0.09 ± 0.72‰, respectively) indicating bioaccumulation from the water column. Sulfur, carbon, and nitrogen stable isotopes suggested that such a difference was not due to feeding niches. However, Hg isotopic trajectory analysis showed that environmental conditions controlling the desorption and remobilization of sediment Hg, notably via tidal flows, were likely to drive the observed source(s). While sediment Hg is not systematically reflected in biota, bivalves, which typically display Hg pools accumulated by benthic or pelagic food webs, appear valuable for Minamata Convention biomonitoring, though their fluctuating Hg sources require careful interpretation.