Pelagic Food Chain Research Articles

A giant stem-group chaetognath.

Chaetognaths, with their characteristic grasping spines, are the oldest known pelagic predators, found in the lowest Cambrian (Terreneuvian). Here, we describe a large stem chaetognath, Timorebestia koprii gen. et sp. nov., from the lower Cambrian Sirius Passet Lagerstätte, which exhibits lateral and caudal fins, a distinct head region with long antennae and a jaw apparatus similar to Amiskwia sagittiformis. Amiskwia has previously been interpreted as a total-group chaetognathiferan, as either a stem-chaetognath or gnathostomulid. We show that T. koprii shares a ventral ganglion with chaetognaths to the exclusion of other animal groups, firmly placing these fossils on the chaetognath stem. The large size (up to 30 cm) and gut contents in T. koprii suggest that early chaetognaths occupied a higher trophic position in pelagic food chains than today.

Impacts of autochthonous dissolved organic matter on the accumulation of methylmercury by phytoplankton and zooplankton in a eutrophic coastal ecosystem

The bioaccumulation of methylmercury (MeHg) within the pelagic food webs is a crucial determinant of the MeHg concentration in the organisms at higher trophic levels. Dissolved organic matter (DOM) is recognized for its influence on mercury (Hg) cycling in the aquatic environment because of its role in providing metabolic substrate for heterotrophic organism and serving as a strong ligand for MeHg. However, the impact of DOM on MeHg bioaccumulation in pelagic food chains remain controversial. Here, we explored MeHg bioaccumulation within a pelagic food web in China, in the eutrophic Bohai Sea and adjacent seas, covering a range of DOM concentrations and compositions. Our findings show that elevated concentrations of dissolved organic carbon (DOC) and phytoplankton biomass may contribute to a reduction in MeHg uptake by phytoplankton. Moreover, we observe that a higher level of autochthonous DOM in the water may result in more significant MeHg biomagnification in zooplankton. This can be explained by alterations in the structure of pelagic food webs and/or an increase in the direct consumption of DOM and particulate organic matter (POM) containing MeHg. Our study offers direct field monitoring evidence of dual roles played by DOM in regulating MeHg transfers from water to phytoplankton and zooplankton in coastal pelagic food webs. A thorough understanding of the intricate interactions is essential for a more comprehensive evaluation of ecological risks associated with MeHg exposure in coastal ecosystems.

Delineating the food web structure in an Indian estuary during tropical winter employing stable isotope signatures and mixing model.

The food and feeding links and sources in an impacted tropical estuary situated along India's western coast, the Ulhas River Estuary (URE) was analyzed employing the stable carbon and nitrogen isotopic signatures (δ13C and δ15N). Three basal carbon sources, such as mangrove leaves, particulate organic matter (phytoplankton), and detritus, were analyzed together with eight consumer groups from various trophic guilds. The δ13C varied from - 19.67 to - 24.61‰, whereas δ15N ranged from 6.31 to 15.39‰ from the primary consumer to the top predator species. The stable isotope mixing model developed for URE revealed a phytoplankton based pelagic food chain and detritus based benthic food chain in URE. The fairly larger value of SEA (Standard Ellipse Area) in the URE suggest a much broader food web structure and high trophic diversity in the ecosystem. Higher influence of detritus on the assimilated diet of majority of consumers and evidences of nitrogen enrichment in the basal sources such as detritus and particulate organic matter by anthropogenic activities in URE point towards nitrogen pollution and subsequent trophic disturbance in this tropical estuarine ecosystem.

Main functional groups of the plankton in front of the fishing port of Anconcito, Santa Elena-Ecuador

Coastal marine plankton is an indicator of the link in the pelagic food chains of the first level (phytoplankton) and the second trophic level (zooplankton); therefore, it is of great ecological, fisheries and economic importance. In this context, the objective of this study was to describe the main components of two plankton communities (phytoplankton and zooplankton) and environmental variables (temperature, salinity, oxygen) at three distances (1, 5 and 10 miles) off the fishing port of Anconcito from May 2019 to February 2020, as an indicator of the ecosystem health of a fishing sector. Samples were collected horizontally using plankton nets with 55 and 300 μm mesh eyes for 10 minutes. Temperature, salinity, pH and dissolved oxygen were measured in situ with a multiparameter probe at the surface level. Mean temperature values were recorded among the three study areas of 25°C, salinity of 31 ups, dissolved oxygen of 4.6 mg.L-1 and pH of 8.The results showed that the first trophic level recorded the abundance of centric diatoms, considered indicators of productive zones mainly in August, while their density in the other months was lower. The phytoplankton composition recorded 167 species; only 12 species of centric diatoms and 13 species of dinoflagellates recorded higher algal density (> 2%), with 142 species present/scarce, the same that evidenced slight differentiation in the three zones. Therefore, diatoms and dinoflagellates were the phytoplankton functional groups that contributed a heterogeneous diversity index in the three sites. Twenty-three zooplanktonic groups were identified, with an average of six species of jellyfish, twelve siphonophores, two cladocerans, two appendicularians and various larval forms of different taxa. Copepods presented the highest mean abundance (48639.3 ind. (100m3 )-1 and 33.3 % respectively), followed by invertebrate eggs (48434.3 ind. (100m3 )-1 and 36. The ANOSIM analysis showed that there is no significant difference between the population densities of the three fixed stations, and in the Bonferroni pairwise comparison between stations 1 and 10 miles, there is no significant difference. In contrast, between stations (10 and 5), (5 and 1) miles there are significant differences, through the SIMPER application, it was found that the group that contributed to the dissimilarity was the copepods, with 38.29% between stations of 10 and 5 miles. The principal component analysis reported that the groups of cladocerans, crustaceans, mollusks, quetognathans, jellyfish and ichthyoplankton were associated with temperature. In conclusion, this line of research on phytoplankton and zooplankton recorded nutritional indicators such as diatoms and copepods very important in the trophic web, combined with local ecological and oceanographic aspects, indicate that the state of health of the coastal margin in front of Anconcito Bay is optimal.

Pelagic Food Chain of the Bering Sea

Pelagic Food Chain of the Bering Sea

Transport and Fate of 137Cs Released From Multiple Sources in the North Atlantic and Arctic Oceans

The North Atlantic and Arctic oceans, along with the North Pacific, are the main reservoirs of anthropogenic radionuclides introduced in the past 75 years. The POSEIDON-R compartment model was applied to the North Atlantic and Arctic oceans to reconstruct 137Cs contamination in 1945–2020 due to multiple sources: global fallout, exchange flows with other oceans, point-source inputs in the ocean from reprocessing plants and other nuclear facilities, the impact of the Chernobyl accident and secondary contamination resulting from river runoff and redissolution from bottom sediments. The model simulated the marine environment as a system of 3D compartments comprising the water column, bottom sediment, and biota. The dynamic model described the transfer of 137Cs through the pelagic and benthic food chains. The simulation results were validated using the marine database MARIS. The calculated concentrations of 137Cs in the seaweed and non-piscivorous and piscivorous pelagic fish mostly followed the concentration of 137Cs in water. The concentration in coastal predator fish lagged behind the concentration in water as a result of a diet that includes both pelagic and benthic organisms. The impact of each considered source on the total concentration of 137Cs in non-piscivorous fish in the regions of interest was analyzed. Whereas the contribution from global fallout dominated in 1960–1970, in 1970–1990, the contribution of 137Cs released from reprocessing plants exceeded the contributions from other sources in almost all considered regions. Secondary contamination due to river runoff was less than 4% of ocean influx. The maximum total inventory of 137Cs in the Arctic Ocean (31,122 TBq) was reached in 1988, whereas the corresponding inventory in the bottom sediment was approximately 6% of the total. The general agreement between simulated and observed 137Cs concentrations in water and bottom sediment was confirmed by the estimates of geometric mean and geometric standard deviation, which varied from 0.89 to 1.29 and from 1.22 to 1.87, respectively. The approach used is useful to synthesize measurement and simulation data in areas with observational gaps. For this purpose, 13 representative regions in the North Atlantic and Arctic oceans were selected for monitoring by using the “etalon” method for classification.

Colony Formation in Three Species of the Family Scenedesmaceae (Desmodesmus subspicatus, Scenedesmus Acutus, Tetradesmus Dimorphus) Exposed to Sodium Dodecyl Sulfate and its Interference with Grazing of DAPHNIA Galeata.

By mimicking the info-chemicals emitted by grazers, the common anionic surfactant sodium dodecyl sulfate (SDS) can induce colony formation in the green algal genus Scenedesmus at environmentally relevant concentrations. The morphometric effects can hinder the feeding efficiency of grazers, reducing energy flow along the pelagic food chain from Scenedesmus to consumers. Despite this potential ecological risk, few studies exist on whether the SDS-triggered induction of colonies is common in other species of the family Scenedesmaceae. Here, we investigated the effects of SDS on the growth and morphology of three species of Scenedesmaceae (Desmodesmus subspicatus, Scenedesmus acutus, and Tetradesmus dimorphus) and on the clearance rates of Daphnia galeata grazing on the SDS-induced colonies. SDS triggered colony formation in all algal species at concentrations nonlethal to them (0.1-10mg L-1) in 72h; however, the induction levels of colony formation were generally lower than for those in the Daphnia culture medium. We also found that the SDS-induced colonial algae reduced D. galeata clearance rates. Our results highlight the potential effect of SDS on the Daphnia-Scenedesmaceae system by triggering the morphological response of Scenedesmaceae at concentrations below those that exert toxicity. Such disruptive effects of pollutants on predator-prey interactions should be considered within the framework of ecological risk assessments.

Disentangling effects of multiple stressors on matter flow in a lake food web.

Understanding the relative importance of multiple stressors is valuable to prioritize conservation and restoration measures. Yet, the effects of multiple stressors on ecosystem functioning remain largely unknown in many fresh waters. Here, we provided a methodology combining ecosystem modeling with linear regression to disentangle the effects of multiple stressors on matter flow, an important ecosystem function. Treating a shallow lake as the model ecosystem, we simulated matter flow dynamics during 1950s–2010s with different combinations of stressors by Ecopath with Ecosim (EwE) modeling and determined the relative importance of each stressor by generalized linear mixed models. We found that matter flow of the lake food web was highly dynamic, attributing to effects of multiple anthropogenic stressors. Biological invasion played the strongest role in driving the matter flow dynamics, followed by eutrophication, while biomanipulation (i.e., phytoplankton control by planktivorous fish stocking) was of little importance. Eutrophication had a stronger role on primary producers, pelagic food chain, and top predators, while biological invasion on consumers in the middle of food chains. The former was more important in driving the quantity of matter flow, while the latter on trophic transfer efficiencies. Scenario forecasting showed that reducing nutrients contents could largely shape the matter flow pattern, while biomanipulation had little effect. Our findings provided new insights into understanding the mechanistic links between anthropogenic stressors and ecosystem functioning by combining ecosystem modeling with linear regression.

Profiling metal contamination from ultramafic sediments to biota along the Albanian shoreline of Lake Ohrid (Albania/Macedonia)

Ultramafic sediments exhibit high levels of geogenically-derived and potentially toxic metals, with Ni, Cr and Co often exceeding benchmark values. As yet, a holistic understanding of the bioavailability, mobility, potential ecotoxicity and trophic transfer of trace elements in both benthic and pelagic food chains in aquatic ultramafic environments (UME) is lacking. We investigated potential environmental health issues due to metal contamination by jointly implementing chemical, ecological and toxicological tools, along the Albanian shoreline of Lake Ohrid. It is an aquatic system of worldwide importance, representative of temperate UME with a legacy of Ni and Cr contamination from mining activities. Levels of Ni, Cr, Cd, Cu, Co, Fe, Mn and Zn were determined in waters, sediments and native biota. The potential environmental mobility of sediment-bound elements was further assessed using CaCl2, EDTA and acetic acid extractions. Whole-sediment ecotoxicity tests were also carried out using ostracods and chironomids, according to standardized procedures. Despite Ni and Cr concentrations above the sediment quality guidelines for probable effect levels, we did not observe ecotoxic effects in laboratory tests. However, these elements were bioavailable to native organisms under field conditions, especially to benthic primary producers in direct contact with sediments (up to 139 mg Cr kg−1 and 785 mg Ni kg−1). Although biomagnification was not observed, further investigations of metal translocation, metabolism and elemental trophic transfer along benthic food webs appears to be a general research priority in the management of temperate UME. The present study shows that proper management of temperate UME requires not only the integration of data from different lines of evidence, but also laboratory vs. field approaches to understand the subtler, long-term effects of increased elemental body burdens in native organisms.

Sustained productivity and respiration of degrading kelp detritus in the shallow benthos: Detached or broken, but not dead

Temperate kelp forests contribute significantly to marine primary productivity and fuel many benthic and pelagic food chains. A large proportion of biomass is exported from kelp forests as detritus into recipient marine ecosystems, potentially contributing to Blue Carbon sequestration. The degradation of this organic material is slow and recent research has revealed the preservation of photosynthetic functions over time. However, the physiological correlates of detrital breakdown in Laminaria spp. have not yet been studied. The warming climate threatens to reshuffle the species composition of kelp forests and perturb the dynamics of these highly productive ecosystems. The present study compares the physiological response of degrading detritus from two competing North East Atlantic species; the native Boreal Laminaria hyperborea and the thermally tolerant Boreal-Lusitanian L. ochroleuca. Detrital fragment degradation was measured by a mesocosm experiment across a gradient of spectral attenuation (a proxy for depth) to investigate the changes in physiological performance under different environmental conditions. Degradation of fragments was quantified over 108 days by measuring the biomass, production and respiration (by respirometry) and efficiency of Photosystem II (by PAM fluorometry). Data indicated that whilst degrading, the photosynthetic performance of the species responded differently to simulated depths, but fragments of both species continued to produce oxygen for up to 56 days and sustained positive net primary production. This study reveals the potential for ostensibly detrital kelp to contribute to Blue Carbon fixation through sustained primary production which should be factored into Blue Carbon management. Furthermore, the physiological response of kelp detritus is likely dependent upon the range of habitats to which it is exported. In the context of climate change, shifts in species composition of kelp forests and their detritus are likely to have wide-reaching effects upon the cycling of organic matter in benthic ecosystems.

The potential of arsenic biomagnification in marine ecosystems: A systematic investigation in Daya Bay in China

In this study, we systematically investigated the bioaccumulation and trophic transfer of arsenic (As) in a typical semi-enclosed gulf, Daya Bay. Ten categories of organisms and environmental samples for As, δ13C, and δ15N analyses were collected from 14 sampling sites in all four seasons. The results demonstrated that As concentrations in the organisms and environmental samples were within the normal range of As levels in other uncontaminated marine ecosystems. Arsenic concentrations were generally lower in the pelagic organisms than in the benthic organisms. Arsenic concentrations in the organisms at higher trophic levels (fish, crabs, shrimp, and cephalopods) were lower in summer and higher in winter, while As in the environments was stable in all seasons. The results of δ13C and δ15N analysis indicated that this ecosystem had a marine-derived food web with approximately 3.5 trophic levels. The positive correlation of As and δ15N in the organisms demonstrated that As was biomagnified along trophic transfer in the whole gulf food web in winter and spring. Specifically, As was biomagnified in the benthic food chains in all four seasons and in the pelagic food chains in winter and spring. These trends were consistent with the analysis of As transfer among the categories within the empirical food web. The trophic magnification factors (TMFs) of As were generally higher among the benthic categories than the pelagic categories. In addition, As transfer from stomach content to muscle was positively correlated to δ13C in fish, suggesting that As transfer was enhanced by a benthic habit. These results demonstrated that As could be biomagnified in marine food webs for specific organism compositions and seasonal variations, and a benthic habit was an important promoter for As biomagnification. Therefore, this study partially explained previous investigations in which As trophic transfers were diverse among marine ecosystems.

Contribution of Dietary Uptake to PAH Bioaccumulation in a Simplified Pelagic Food Chain: Modeling the Influences of Continuous vs Intermittent Feeding in Zooplankton and Fish.

Dietary uptake is important for trophic transfer of polycyclic aromatic hydrocarbons (PAHs) in the freshwater pelagic ecosystem. In this study, we hypothesized that both the dietary uptake rate and interval significantly influenced its relative contribution to bioaccumulation. We developed a toxicokinetic model framework for the bioaccumulation of deuterated PAHs (PAHs-d10) in aquatic organisms considering different feeding intervals ranging from none for phytoplankton to approximately continuous for zooplankton to discrete for fish and built a simple artificial freshwater pelagic food chain composed of algae Chlorella vulgaris, zooplankton Daphnia magna, and zebrafish. We conducted bioaccumulation experiments and simulations for Daphnia magna and zebrafish under different algal densities based on our model. The results showed that intermittent feeding led to a large fluctuation in the PAH-d10 concentrations in zebrafish compared to a leveled-off pattern in Daphnia magna because of approximately continuous feeding. Trophic dilution of PAHs-d10 occurred in the food chain when there was waterborne-only uptake, but dietary uptake largely mitigated its extent that depended on dietary uptake rates. The assimilation efficiency, dietary uptake rate, and its relative contribution to bioaccumulation of PAHs-d10 in zebrafish were all higher than those in Daphnia magna, suggesting that dietary uptake played a more important role in bioaccumulation of PAHs at higher trophic-level organisms.

Preliminary Development of an Expert System for the Fate Prediction of Marine Radioactivity

Preliminary version of an expert system for the initial prompt assessment of environmental impacts of marine radioactivity materials released routinely and/or accidently has been developed for the technical support of decision makers. The concept of the system is similar to the CORMIX system for the thermal discharge prediction developed by professor Jirka from US. The system uses the multi-compartment model POSEIDON-R equipped with dynamic food chain model so called extended BURN-POSEIDON (Bezhenar et al, 2016) which takes into account the transfer of marine radioactivity through pelagic and benthic marine organisms. In detail, phytoplankton, zooplankton, fish (two types: piscivorous and non-piscivorous), crustaceans, and molluscs are considered for the pelagic food chain, deposit-feeding invertebrates, demersal fish and bottom predators are considered for the benthic food chain and coastal predators feeding by both pelagic and benthic organisms. The modeled region is assumed to have a straight coastline and is divided into a set of horizontal rectangular boxes (or compartments) and vertical layers. Sizes of boxes can vary horizontally but are kept constant. At the outset water depth, currents and mixing without temporal variation are specified in each box. The specified values are constant to the direction parallel to the coastline while they can vary to the direction normal to the coastline. Preliminary experiments have been performed according to a range of hypothetical scenarios where routine and accidental releases of radioactivity in the coastal environment are included. It is expected that the expert system will help to provide decision makers with prompt information about the marine radioactivity contamination level in marine organisms in coastal sea regions where routine and accidental releases of marine radioactivity occur.

Turning up the heat: warming influences plankton biomass and spring phenology in subtropical waters characterized by extensive fish omnivory.

Understanding how biological communities respond to climate change is a major challenge in ecology. The response of ectotherms to changes in temperature depends not only on their species-specific thermal tolerances but also on temperature-mediated interactions across different trophic levels. Warming is predicted to reinforce trophic cascades in linear aquatic food chains, but little is known about how warming might affect the lower trophic levels of food webs involving extensive fish omnivory, a common scenario in subtropical and tropical waterbodies. In this study, a mesocosm warming experiment was conducted involving a pelagic food chain (fish-zooplankton-phytoplankton) topped by the omnivorous bighead carp [Aristichthys nobilis (Richardson)]. We found that temperature elevation significantly enhanced the growth of fish and suppressed zooplankton, including both metazooplankton and ciliates, while abundances of phytoplankton, despite disruption of temporal dynamics, did not increase correspondingly-likely due to fish predation. Our results suggest that trophic cascades are less unlikely to be reinforced by warming in food chains involving significant omnivory. Moreover, we found that warming advanced the spring abundance peak of phytoplankton abundance and that of the parthenogenetic rotifer Brachionus quadridentatus; whereas, it had no effect on the only sexually reproducing copepod, Mesocyclops leuckarti, presumably due to its prolonged life history. Our study also confirmed that warming may lead to a phenological mismatch between some predators and their prey because of the distinct life histories among taxa, with potentially severe consequences for resource flow in the food chain, at least in the short term.

Dog overpopulation on Santa Cruz Island, Galapagos 2018

AbstractIn addition to El Niño events, which can alter the pelagic food chain that supports growth and survival of sea lions, dog overpopulation, and diseases can cause further population declines in sea lions on the Galapagos. Knowledge of baseline data and evaluation of existing strategies to control dog reproduction are required for effective management of dog overpopulation on the archipelago. The main objective of the study reported here was to estimate the human: dog ratio on Santa Cruz Island, Galapagos in September 2018. In addition, dog demographic data were used to model the expected annual dog population growth in the next 3 years. The observed human: dog ratio was 756:187 (or 4.04:1) which extrapolates to 3,886 dogs; an increase of 55% in the dog population on Santa Cruz Island from 2014 to 2018. Study results show that current spay‐neuter efforts (about 180 dogs per year in the last 5 years; 57% females, 43% males) are not enough to keep the dog population stable (i.e., current baseline of 3,886 dogs in 2018). Our results justify a revision of current management action plan to control dog reproduction on Santa Cruz Island, Galapagos.

Food Web Structure and Trophic Dynamics of a Fish Community in an Ephemeral Floodplain Lake

In Africa, wetlands such as shallow, ephemeral lakes provide ecosystem services such as water purification, food supply and flood control but are subject to dyanmic flooding/drying cycles which vary in duration from years to decades. The stochastic nature of drying events subjects ephemeral lake fauna to persistent disturbance regimes, therefore understanding how biota respond to flooding and drying events is essential for their conservation and management. Primary production sources supporting consumer biomass in the shallow ephemeral Lake Liambezi (upper Zambezi Ecoregion), were investigated using stable isotope analysis, mixing models and stomach content analysis to investigate the following hypotheses: (1) algal primary production supports a higher consumer biomass than aquatic macrophytes; (2) the lake food chain is short, because the majority of fish fauna are detritivorous/herbivorous cichlids that are consumed by top predators; (3) fish community trophic structure will be similar between years; and (4) with short food chains and stochastic resource availability, there will be substantial competition for food among fish species. Results showed that phytoplankton production supported substantial consumer biomass in Lake Liambezi, with important contributions from macrophytes and associated detritus and/or periphyton. While particulate organic matter (POM) contributed substantially to the diet of herbivorous/detritivorous tilapiine cichlids (the backbone of Lake Liambezi’s commercial fishery), considerable dietary carbon was likely also derived from aquatic plants and associated detritus and/or periphyton compared to other fishes. Three major food chains were identified in the lake. The phytoplankton-based pelagic food chain was longest, involving up to four trophic transfers. The benthic food chain based primarily on detritus of planktonic origin (but may also include macrophyte associated detritus/periphyton) was characterised by high levels of omnivory and involved up to three trophic transfers. The macrophytic detritus-based food chain was shortest, involving just two trophic transfers. Predators fed across all three food chains, but predominantly on the two benthic food chains. A combination of dietary overlap (amongst piscivores/predators, amongst insectivores), dietary specialisation (tilapiine cichlids, alestids), the integration of multiple food chains and behavioural adaptation to changing dietary resources underpins the ability of Lake Liambezi’s fish community to thrive under the stochastic nature of ephemeral lake ecosystems.

Structural Equation Modelling Reveals That Nutrients and Physicochemistry Act Additively on the Dynamics of a Microcosm-Based Biotic Community.

Anthropogenic eutrophication has caused widespread environmental problems in freshwater lakes, reducing biodiversity and disrupting the classic pelagic food chain. Increasing our understanding of the exact role of nutrients and physicochemical variables on microbial dynamics, and subsequent microalgal and cyanobacterial blooms, has involved numerous studies ranging from replicate microcosm-based studies through to temporal studies of real lake data. In a previous experimental microcosm study, we utilised metaproteomics to investigate the functional changes of a microalgal-bacterial community under oligotrophic and eutrophic nutrient levels. Here, we analyse the time series data from this experiment with a combination of typically used univariate analyses and a more modern multivariate approach, structural equation modelling. Our aim was to test, using these modern methods, whether physicochemical variables and nutrient dynamics acted additively, synergistically, or antagonistically on the specific biotic community used in the microcosms. We found that nutrients (nitrogen and phosphorus) and temperature acted additively on the interactions between the microalgae and bacteria present, with the temperature effects elevated in the eutrophic conditions we applied. The data suggests that there may be no synergistic interaction between nutrients and temperature in the tested microcosms. Our approach demonstrates how the application of multivariate methods to existing datasets, in our case from nutrient-enriched freshwater microcosms, enables new information to be extracted, enhancing interpretations as well as allowing more reliable comparisons to similar published studies.

Effects of zooplankton structure and phosphorus concentration on phytoplankton biomass in a freshwater pelagic food chain

In Lake Lugano (Switzerland and Italy), temporal variation in the structure of herbivore zooplankton (a shift to greater dominance by large-bodied species and higher biomass) and phosphorus (soluble reactive phos- phorus, SRP) concentrations provided useful settings to examine the effects of these changes on phytoplankton biomass. Results from this study indicate that, within an overlapping range of SRP concentrations (14.2–24.5 μg P L –1, which indicate a P-enriched lake), phytoplankton biomass (measured as Chlorophyll-a, CHL) was substan- tially lower after the shift to larger-bodied zooplankton. In addition, after the shift, CHL displayed an unforeseen negative association with SRP, which was parelleled by a decrease in consumer control (indicated by the ratio between herbivore and phytoplankton production) toward lower SRP concentrations. The suppression of phytoplankton indicates that the shift to greater biomass and larger-bodied zooplankton increased consumer-control on phytoplankton biomass, even at relatively high SRP concentrations. The negative relationship between SRP and CHL after the shift might have stemmed from reduced phytoplankton edibility and/or reduced quality at low SRP concentrations, although establishing whether the association is causal will require further research. The results suggest that zooplankton structure and SRP concentrations influenced phytoplankton biomass, although through complex and unanticipated effects.

Comparing the roles of Pacific halibut and arrowtooth flounder within the Gulf of Alaska ecosystem and fishing economy

AbstractThe fishing industry of the western and central regions of the coastal Gulf of Alaska (CGoA) directly employs over 17,000 people and processes fish with a wholesale value of US$618 million annually. Pacific halibut (Hippoglossus stenolepis) are a valued groundfish species because of the high quality of their flesh. In contrast, arrowtooth flounder (Atheresthes stomias) are much more abundant but of low value because their flesh degrades upon heating. Both are high trophic level predators but play different roles in the ecosystem because of differences in abundance and diet. Using an end‐to‐end ecosystem model, we evaluate the impact of alternate levels of fishing effort and large‐scale changes in oceanographic conditions upon both species, the ecosystem, and the fishing economy. Reduction of longline efforts to reduce Pacific halibut mortality led to reduction in total value of all CGoA landings but increase in value landed by sport fisheries, trawl fleets, and fish pot vessels as they exploit a greater share of available halibut, sablefish, and Pacific cod. Increased trawl effort to raise arrowtooth flounder mortality led to increase in total value of all landings but large reductions in value landed by longline, jig, fish pot, and sport fleets with greater competition for available Pacific cod, halibut, and sablefish. Oceanographic conditions that enhance pelagic food chains at the expense of benthic food chains negatively impact groundfish in general, though Pacific halibut and arrowtooth flounder are resilient to these effects because of the high importance of pelagic fish in their diets.

Insufficient evidence for BMAA transfer in the pelagic and benthic food webs in the Baltic Sea

The evidence regarding BMAA occurrence in the Baltic Sea is contradictory, with benthic sources appearing to be more important than pelagic ones. The latter is counterintuitive considering that the identified sources of this compound in the food webs are pelagic primary producers, such as diatoms, dinoflagellates, and cyanobacteria. To elucidate BMAA distribution, we analyzed BMAA in the pelagic and benthic food webs in the Northern Baltic Proper. As potential sources, phytoplankton communities were used. Pelagic food chain was represented by zooplankton, mysids and zooplanktivorous fish, whereas benthic invertebrates and benthivorous fish comprised the benthic chain. The trophic structure of the system was confirmed by stable isotope analysis. Contrary to the reported ubiquitous occurrence of BMAA in the Baltic food webs, only phytoplankton, zooplankton and mysids tested positive, whereas no measurable levels of this compound occurred in the benthic invertebrates and any of the tested fish species. These findings do not support the widely assumed occurrence and transfer of BMAA to the top consumers in the Baltic food webs. More controlled experiments and field observations are needed to understand the transfer and possible transformation of BMAA in the food web under various environmental settings.