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Food quality (e.g. the presence of cyanobacteria) may affect the behavior, morphology and life history of cladocerans. However, little is known about how diet quality may affect the effectiveness of Daphnia 's behavioral defenses against predators. Here we test the hypothesis that the presence of non-toxic cyanobacteria in Daphnia 's diet reduces its ability to escape fish attack. The animals cultured under two different food quality regimes (green alga vs. cyanobacteria) were subjected to two types of predation experiments. In the first, we tested Daphnia 's ability to avoid a predator whose feeding was simulated using a pipette, while in the second we tested Daphnia 's ability to escape when confronted with a foraging fish (Poecilia reticulata). In both types of experiments, the ability to avoid predator attack was greater in Daphnia fed high quality green algae than in Daphnia fed low quality cyanobacteria.. We conclude that low food quality can affect the fitness of Daphnia not only by slowing their growth and reproduction rates, but also by increasing their susceptibility to predation. [ABSTRACT FROM AUTHOR]

Daphnia longispina and other large cladocerans are active filterers, consuming algae and other suspended matter. Undigested particles sink to the bottom and are further involved in the biotic cycle. The absence of planktivorous fish favors the development of large zooplanktonic species showing high filtration activity. Owing to filtration activity, zooplankton can clarify water masses. [ABSTRACT FROM AUTHOR]

Cyanobacteria blooms occurred in the pond in early August; Microcystis aeruginosa was constantly present in the plankton. Its biomass reached its peak of 24.1–28.6 mg/L in mid-August. The proportion of individual cells (4 µm in size) and colonies up to 50 µm in size in the M. aeruginosa biomass increased to 68–85%. In late August, the proportion of 50–100-µm fraction did not exceed 5–7%. The proportion of a larger fraction >100 µm decreased from 24 to 8%. Rotifers, represented by 14 taxonomic units recorded in the plankton, dominated in the water body. The abundance of Rotifers reached 51–96% of that of zooplankton. Dominating in abundance was Keratella cochlearis cochlearis. It accounted for 44 to 86% of rotifer population, and its abundance varied from 500 to 2000 ind./L. The rotifer abundance during the M. Aeruginosa bloom period reached its highest of 4.5 thous. ind./L and at the end of this period decreased to 580 ind./L. In the high-trophicity pond, rotifers play the leading role in the water body trophodynamics. [ABSTRACT FROM AUTHOR]

AbstractNumerous studies have analyzed the distribution and concentration of microplastic particles (MPs) in various aquatic habitats, and estimated how they affect residual biota. Decidedly less research concerns how organisms (e.g. ichthyofauna) affect the quantitative and qualitative characteristics of MPs in the water. Therefore, the aim of our study was to test, if fish can increase the vertical flux of MPs to sediments. This would be the result of the combined effect of increased sedimentation rate and the active transport resulting from vertical migration of the fish in the simultaneous presence of a thermal gradient (and in turn water density gradient). We conducted eight 5-day-long experiments in thermally stratified twin vertical columns filled with a suspension of polystyrene beads of 25 and 250 µm in size, with initial density of 1000 and 200 × L−1, respectively. In each of the experiments, we placed sediment traps at three different depths in each of the columns, and four planktivorous fish in one of them. During the experiments, we monitored the distribution of fish. At the end of the experiment, we determined the number of MPs and the amount of detritus in both sediment traps and water. We found that the presence of planktivorous fish significantly increased the vertical flux of MPs to sediments and decreased the density of MPs in the water column. These effects seem to be more likely due to the increased sedimentation rate of MPs due to the fish activity, rather than to their active transport, as MPs in the experimental variant were covered with detritus to a greater extent, than in the control, and the fish remained in the subsurface layer in the water column. Our study seems to be the first one showcasing the effect of fish on the concentration and distribution of MPs in aquatic ecosystems.

Numerous studies have analyzed the distribution and concentration of microplastic particles (MPs) in various aquatic habitats, and estimated how they affect residual biota. Decidedly less research concerns how organisms (e.g. ichthyofauna) affect the quantitative and qualitative characteristics of MPs in the water. Therefore, the aim of our study was to test, if fish can increase the vertical flux of MPs to sediments. This would be the result of the combined effect of increased sedimentation rate and the active transport resulting from vertical migration of the fish in the simultaneous presence of a thermal gradient (and in turn water density gradient). We conducted eight 5-day-long experiments in thermally stratified twin vertical columns filled with a suspension of polystyrene beads of 25 and 250 µm in size, with initial density of 1000 and 200 × L−1, respectively. In each of the experiments, we placed sediment traps at three different depths in each of the columns, and four planktivorous fish in one of them. During the experiments, we monitored the distribution of fish. At the end of the experiment, we determined the number of MPs and the amount of detritus in both sediment traps and water. We found that the presence of planktivorous fish significantly increased the vertical flux of MPs to sediments and decreased the density of MPs in the water column. These effects seem to be more likely due to the increased sedimentation rate of MPs due to the fish activity, rather than to their active transport, as MPs in the experimental variant were covered with detritus to a greater extent, than in the control, and the fish remained in the subsurface layer in the water column. Our study seems to be the first one showcasing the effect of fish on the concentration and distribution of MPs in aquatic ecosystems. Fish foraging activity increased the sedimentation of PS MPs. Fish foraging activity decreased the PS MP concentration in the water column. The study seems to provide the first experimental evidence on the effect of fish on the distribution of MPs in aquatic environments. [ABSTRACT FROM AUTHOR]

We studied abundance and dynamics of zooplankton in the tidal freshwater Cache Slough Complex (CSC) in the northern Delta of the San Francisco Estuary during June, July, and October 2015. We asked whether the CSC was an area of high zooplankton production that could act as a source region for open waters of the estuary. Abundance of the copepod Pseudodiaptomus forbesi was similar to that in freshwater reaches of the central and eastern Delta and higher than that in the adjacent Sacramento River. Growth rate of P. forbesi was higher than previously measured in large estuarine channels because of higher temperature and phytoplankton biomass in the CSC. Samples of P. forbesi examined with molecular techniques contained an unexpectedly high proportion of DNA from cyanobacteria and little DNA from more nutritious phytoplankton. We also examined tidal exchanges of phytoplankton biomass and copepods between Liberty Island, a shallow tidal lake within the CSC, and the adjacent southern Cache Slough, which links the CSC to the Sacramento River. We calculated zero net flux of phytoplankton over 127 days between June and October. The tidal flux of copepods, calculated using tidal flow from an in situ flow station and half-hourly sampling over three 24.8-hr tidal cycles, varied a great deal because of temporal patchiness and day/night variation in abundance. Overall, the tidal flux was indistinguishable from zero, while the tidally-averaged water flow (and therefore the net copepod flux) was always into the wetland. Our results show some promise for the CSC as a productive habitat for planktivorous fishes and as a laboratory for learning how to design future wetland restoration. However, we remain cautious about whether wetlands such as the CSC may export large quantities of food organisms that can support fishes in other regions of the estuary.

Euphausiids (hereafter “krill”) are one of the main components of the pelagic communities of the Humboldt Current System (HCS). Their community dynamics have been well studied in central-southern Chile where upwelling is strongly seasonal, but little is known about the permanent-upwelling area of the HCS, which yields the largest fishery in the world, the Peruvian anchovy. We applied hierarchical generalized additive models with environmental and biological predictors to determine the main drivers of krill abundance, adjusting species-specific functions. We used a time series of 16 bi-annual surveys to study annual, seasonal, and spatial scales of variability of the four numerically dominant taxa: Euphausia mucronata (Humboldt krill), E. eximia, Stylocheiron affine, and Nematoscelis spp. The spatial pattern of the Humboldt krill (the dominant species) proved it is an upwelling-associated species, with higher abundances within 10 km from the coast. The other 3 taxa showed opposite spatial patterns with higher abundances offshore. The main covariates explaining krill abundances were the depth of the upper limit of the oxygen minimum zone (dOMZ) and the mean temperature of the water column. Humboldt krill was negatively correlated to both drivers, and the opposite effect was observed for the other taxa. Although many krill species are metabolically adapted to cope with the severe hypoxic conditions of this system, the Humboldt krill was the only species with higher modeled abundances when dOMZ was shallower. Chlorophyll-a remained high during all sampling periods, and it was an insignificant predictor for all taxa, suggesting food is not a limitation for krill in this highly productive system. The acoustic biomass of the Peruvian anchovy had a negative non-linear effect on the abundances of the Humboldt krill, and higher Humboldt krill abundances were found in areas with no anchovy hotspots. Our results indicate that krill in this system are susceptible to changes in temperature, oxygen, and upwelling conditions. Extreme events (e.g. heatwaves and ENSO events) are expected to increase in frequency and intensity, while climate change scenarios show a potential intensification of upwelling. These conditions could lead to distribution displacements and alter trophic interactions by modifying the distribution and biomass of the predator.

Zooplankton are a vital component of the food web as the major conduit of energy from primary producers to planktivorous fishes. Therefore, changes in the structure of the zooplankton community affect how energy moves through aquatic ecosystems. Changes in the zooplankton community structure are typically documented through detailed community analysis, which can be difficult to interpret and communicate to non-experts. Alternatively, a few key summary indicators of community structure may better disseminate this information to a broader audience. In this study we analyzed zooplankton data from 1997 to 2019 from the US Environmental Protection Agency's Great Lakes National Program Office to select indicators that summarized changes in the zooplankton community. Two indicators (percent of calanoids by biomass and areal density of herbivorous cladocerans) detected known changes in lakes Huron, Michigan, and Ontario, correlated with the crustacean zooplankton areal biomass above (herbivorous cladocerans) or in the hypolimnion (percent calanoids), and were related to the lake or basin-wide Carlson's Trophic State Index based on chlorophyll (TSI chl) (percent calanoids by biomass). Our findings suggest that employing one or both selected indicators in addition to TSI chl and total crustacean zooplankton areal biomass, an already reported indicator in the Great Lakes that captures overall food availability for fish, would provide additional key information about zooplankton community structure in simple metrics that could be effectively communicated to stakeholders. [ABSTRACT FROM AUTHOR]

Coral reef mutualisms involve complex trophic ecological relationships that produce indirect effects. Excluding mutualistic cleaner fish Labroides dimidiatus from reefs indirectly increases the abundance of many fishes and reduces demersal stages of their ectoparasitic prey (gnathiid isopods). Whether cleaners affect populations of planktivorous fishes that consume demersal zooplankton, and consequently indirectly affect the rest of the demersal zooplankton community — via presumed changes in planktivory — is unknown. Therefore, using a long-term cleaner fish manipulation on patch reefs (July 2000 to December 2012, Great Barrier Reef), we tested whether cleaner treatment (removal or control) affects planktivorous fish abundance and diversity, and demersal zooplankton biomass, abundance, and diversity. Fish surveys, 9 and 12 years after removing cleaners revealed fewer fish on removal compared to control reefs for one of the three most abundant planktivores, but not total abundance (Pomacentridae, 26 species), diversity, and composition. Emerging zooplankton were sampled during the day and night over nine sampling trips across 12 years. There was no effect of cleaner treatment on post-removal values, compared with pre-removal values in July 2000, for zooplankton biomass, abundance, diversity, and composition (34 taxa). Zooplankton abundance showed no diel differences, but diversity, and occasionally biomass, were higher at night. Zooplankton composition also showed diel differences, with three taxa contributing the most to this dissimilarity [Cirripeda nauplii, Facetotecta (Arthopoda), Oikopleuridae (Chordata)]. Zooplankton diversity did not differ among times, abundance was higher in January 2002 relative to July 2000, and composition differed among all times. The lack of detectable indirect effects of cleaner fish presence on zooplankton (non-gnathiid) may partly be due to cleaners’ variable effect on planktivorous fish abundance, but also the result of invertebrate planktivory and other processes that affect zooplankton populations not investigated here. Nevertheless, the pronounced diel and temporal changes in zooplankton observed likely influence coral reef trophic interactions.

Large zooplankton can efficiently graze on algae and thereby improve water quality. However, zooplankton body size is considered to decrease with decreasing latitude because of the high fish predation and warm temperatures at lower latitudes. To explore how fish stocking has destabilized the trophic cascade and influenced water quality along a latitudinal gradient of reservoirs, we compared zooplankton body size and trophic cascades to water quality indicators in the subtropical Lake Qiandaohu and in tropical Guangdong reservoirs. The results showed that the body length and total biomass of the dominant zooplankton in Lake Qiandaohu were much larger than those of zooplankton in the Guangdong reservoirs. Moreover, fish predation was the key factor influencing the changes in zooplankton body size and total biomass in Lake Qiandaohu. In the Guangdong reservoirs, water temperature and total phosphorus were the pivotal drivers of zooplankton body size and biomass, respectively. In addition, structural equation models showed that the decreasing zooplankton body size and biomass under fish pressure weakened the grazing pressure on phytoplankton and indirectly contributed to the low water clarity in Lake Qiandaohu. However, fish had little influence on the inefficient algal grazing of zooplankton in the Guangdong reservoirs. Overall, zooplankton can function as an indicator of water quality in fish-controlled subtropical reservoirs but not in nutrient-controlled tropical reservoirs. [ABSTRACT FROM AUTHOR]

We studied zooplankton distributions in the upper San Francisco Estuary at nested scales of tens to thousands of meters. The purposes of the study were to assess how well the Interagency Ecological Program (IEP) zooplankton monitoring represents abundance, and to investigate the variability of plankton on scales similar to those of foraging by fish. Samples were taken at three sites in the western Sacramento–San Joaquin Delta. We took 18 sets of six samples each with a plankton net along transects from near shore to center channel, and six sets of ten samples in the vicinity of a drifter either in mid-channel or near shore. Sampling took place in June–July 2014 during neap and spring tides, ebb and flood, day and night (transects only). Analysis focused on three common copepod species. Transect samples showed little consistent variation along transects, except that Pseudodiaptomus forbesi was less abundant nearshore than offshore by day at Big Break, the most landward site. The ratio of adults to adults + copepodites was strongly and positively related to turbidity by day but not by night, indicating demersal behavior. Drifter samples showed a minimum standard deviation of log10 sample counts of about 0.1, indicating that about two-thirds of replicate abundance values were within 80 % to 125% of the mean. A measure of difference between plankton samples at pairs of sample points was unrelated to distance between sample points for drifter samples, weakly related along transects for Limnoithona spp. stages, and strongly related for P. forbesi mainly because of the along-transect gradients at Big Break. The IEP sampling program is representative of plankton abundance except for demersal organisms, which can be ten-fold more abundant by night than by day. Small planktivorous fish could forage in patches of up to ~25% higher abundance than the mean.

Microplastic pollution of inland environments has been receiving increased publicity over the last few years. For the first time, this study reports on the presence of microplastics (0.1 mm to <5 mm) in the gastrointestinal tracts of Alburnus tarichi in Lake Van from January to April 2020. In total, 101 fishes were obtained from Citoren, Edremit, Gevas and Dagonu fisheries regions. A total of 3338 pieces of plastic, ranging from 8 to 124 samples per fish with averaging 34 ± 13 MPs/individual, were extracted from gastrointestinal tracts of fish. The majority of the ingested particles consisted of fibres (74%), and blue coloured material was the most consumed microplastic. The polymers identified by Fourier Transform Infrared Spectroscopy (FTIR) were polyethylene and polypropylene. Consequently, this study suggested that the vast majority of A. tarichi were contaminated with microplastics and showed higher abundance in comparison with other worldwide studies so far. [ABSTRACT FROM AUTHOR]

Zooplankton play a key role in energy transfer within lake food webs, but we have a poor knowledge concerning their role as phytoplankton grazers in shallow subtropical lakes. In this study, we aimed to determine how zooplankton grazing upon phytoplankton is altered in different scenarios of fish predation and turbidity, and we explored the relevance of grazing compared to other environmental variables, to explain phytoplankton biomass changes. A mesocosm experiment was conducted by including the following treatments: fish, turbidity, fish + turbidity, and a control (without fish or varying turbidity). The experiment lasted 21 days, and samples were taken four times. Zooplankton grazing was only effective for the microphagous group upon Cryptophyceae, while large Chlorophyceae and small pennate Bacillariophyceae biomass were benefited in the presence of copepods and cladocerans, being negatively affected by depletions in nitrogen availability. In the turbidity treatment, a reduction in phytoplankton biomass was obtained, artificially increasing zooplankton grazing on phytoplankton, while fish presence inhibited grazing of adult copepods and cladocerans. The other groups of phytoplankton were only influenced by the environment. This experiment suggests that phytoplankton biomass variations would be more affected by the environment than by zooplankton grazing in shallow lakes from the Paraná River. [ABSTRACT FROM AUTHOR]

Zooplankton were 3–8 times more abundant during the day near the surface than elsewhere in the water column over a 1–2.4 m deep back reef in Moorea, French Polynesia. Zooplankton were also significantly more abundant near the surface at night although gradients were most pronounced under moonlight. Zooplankton in a unidirectional current became concentrated near the surface within 2 m of departing a well-mixed trough immediately behind the reef crest, indicating that upward swimming behavior, rather than near-bottom depletion by reef planktivores, was the proximal cause of these gradients. Zooplankton were highly enriched near the surface before and after a full lunar eclipse but distributed evenly throughout the water column during the eclipse itself supporting light as a proximal cue for the upward swimming behavior of many taxa. This is the first investigation of the vertical distribution of zooplankton over a shallow back reef typical of island barrier reef systems common around the world. Previous studies on deeper fringing reefs found zooplankton depletion near the bottom but no enrichment aloft. In Moorea, where seawater is continuously recirculated out the lagoon and back across the reef crest onto the back reef, selection for upward swimming behavior may be especially strong, because the surface serves both as a refuge from predation and an optimum location for retention within the reef system. Planktivorous fish and corals that can forage or grow even marginally higher in the water column might have a substantial competitive advantage over those nearer the bottom on shallow reefs. Zooplankton abundance varied more over a few tens of centimeters vertical distance than it did between seasons or even between day and night indicating that great care must be taken to accurately assess the availability of zooplankton as food on shallow reefs.

Planktivorous fishes are known to consume coral larvae due to their high nutritional value that can benefit both the individual and their progeny. However, how the consumption of coral larvae varies among coral and fish taxa, between day and night and with the density of coral larvae is not well understood. Here, we used a series of laboratory feeding assays to determine how the consumption of coral larvae by five planktivorous damselfish species (Abudefduf sexfasciatus, Amblyglyphidodon curacao, Dascyllus trimaculatus, D. reticulatus, and Chromis viridis) differed (1) between larvae of the broadcast-spawning scleractinian coral Acropora tenuis and the brooding non-scleractinian coral Heliopora coerulea when offered at a constant density (100 larvae/50 L), (2) between day and night (A. tenuis only), and (3) among larval densities (A. tenuis: 5–500 larvae/50 L; H. coerulea: 5–250 larvae/50 L). When coral larvae were offered at 100 larvae/50 L, all five fish species examined consumed A. tenuis (78–97 larvae h−1) at a greater rate than H. coerulea (23–57 larvae h−1), with these differences likely related to the behavior and/or pigmentation of the larvae. Consumption also varied among the fishes examined, though the differences were small when compared to differences between coral species. Consumption of A. tenuis was twofold higher at day than night likely reflecting the diurnally active feeding behavior of the fishes examined. When the density of coral larvae offered to the fishes was varied, three of five species exhibited a type II functional response (decelerating intake rate) for A. tenuis, while four out of five damselfish species exhibited a type III (sigmoidal intake rate) or type I (linear intake rate) for H. coerulea. This study demonstrates that predation by fishes contributes significantly to coral larvae mortality and that differences in consumption rates among coral species may affect the composition of coral recruits and ultimately coral assemblages. [ABSTRACT FROM AUTHOR]

Increased temperature in the epilimnion and hypoxia in the metalimnion of a lake would result in an increase of positive-size-selective fish predation on zooplankton and in turn in a decrease of mean body size in zooplankton populations and communities. We tested this hypothesis in four types of experiments with juvenile rudd (Scardinius erythrophthalmus) foraging on Daphnia longispina in an indoor twin column tank system. In each experiment of the first three types, one column contained one of three types of experimental treatments differing from the control treatment (in the other column) by the following: (i) elevated temperature in the epilimnion, (ii) hypoxia in the metalimnion and (iii) simultaneous elevated temperature in the epilimnion and hypoxia in the metalimnion. In the fourth type of experiment, the gradients of temperature and oxygen concentration in both columns were the same, but prior to the experiments, Daphnia and fish in the control treatment were acclimated to normoxia and, in the experimental treatment, to hypoxia. The results confirmed our hypothesis, since the predation rate of fish was greater in each of the first three experimental treatments than in the control. We did not detect an effect of the acclimation to hypoxia on the predation rate of the fish. [ABSTRACT FROM AUTHOR]

Abstract Endocrine disrupting compounds (EDCs) are becoming an increasing concern regarding bioaccumulation in aquatic biota. However, the effects of regional pollution levels and specific feeding habits on the bioaccumulation of EDCs in fish are rarely reported. 4-Nonylphenol (4-NP), bisphenol A (BPA), 4-tert-octylphenol (4-t-OP), triclocarban (TCC) and triclosan (TCS) were determined in abiotic compartments [water, sediment, suspended particulate matter (SPM)] and fish with different feeding habits along the Pearl River, China. EDCs in abiotic compartments exhibited significant (p < 0.05) spatial variations, forming five zones clustered based on site-specific EDC concentrations. 4-NP was the dominant compound, contributing 58–98% of the EDCs in fish, followed by BPA (<41%), 4-t-OP (<13%), and TCC and TCS (<4.7%). The concentrations of 4-NP and 4-t-OP, BPA, and TCC and TCS were the highest in brackish carnivorous, planktivorous, and detritivorous fish, respectively. The bioaccumulation factors (BAFs) showed that 4-NP accumulated (BAF > 5000) in all fish except for suck-feeding detritivores, while 4-t-OP and TCC accumulated in filter-feeding planktivores. The concentration of 4-NP in carnivores was significantly higher than that in detritivores, indicating the potential biomagnification of 4-NP along food chains. EDCs in sediment and SPM and those in water were most positively correlated with those in detritivores and planktivores, respectively, suggesting the potential of fish with these two feeding habits to act as bioindicators of EDC pollutants. Graphical abstract Image 1 Highlights • Endocrine disrupting compounds (EDCs) in fish varied greatly across spatial zones. • EDCs in fish followed 4-nonylphenol > bisphenol A >4-tert-octylphenol > two biocides. • Xenoestrogen in carnivores/planktivores and biocide in detritivores accumulated most. • 4-Nonylphenol with bioaccumulation factors > 5000 in fish exhibited biomagnification. • EDCs in detritivores and planktivores could indicate those in environmental media. Endocrine disrupting compounds in fish varied significantly across feeding habits and spatial zones, with the greatest 4-nonylphenol accumulation in brackish carnivores near the Pearl River Estuary. [ABSTRACT FROM AUTHOR]

Abstract Millimetre-sized fragments have been documented in many fish species, but their transfer through food webs is still poorly understood. Here we quantified and described plastic fragments in the digestive tracts of 43 Easter Island flying fish (Cheilopogon rapanouiensis) and 50 yellowfin tunas (Thunnus albacares) from coastal waters around Rapa Nui (Easter Island) in the South Pacific subtropical gyre, and of fish preyed upon by T. albacares. Overall, seven C. rapanouiensis (16%) individuals had ingested microplastics, most of which resembled the common planktonic prey of the fish. One microplastic was found in the gut of a fish ingested by a tuna, which indicates that trophic transfer may occur between tuna and prey. A single T. albacares (2%) had ingested five mesoplastics (15.2–26.3 mm) that were probably not mistaken for prey items, but rather accidentally ingested during foraging on fish prey. The absence of microplastics in T. albacares suggests that such small particles, if transferred from the prey, do not accumulate in the relatively large digestive tract of large predators. On the other hand, larger plastic items may accumulate in the gut of tunas, to which they may induce deleterious effects that still need to be examined. However, only a small portion of the fish had ingested mesoplastics. The results of this study suggest that microplastic contamination is not an immediate threat to large predatory fish, such as T. albacares , along the coast of Easter Island within the South Pacific subtropical gyre. Graphical abstract Image 1 Highlights • Microplastics were found in flying fish and fish preyed upon by yellowfin tunas. • Microplastics may be transferred from the prey, but do not remain in the tunas' guts. • Microplastic contamination might not be an immediate threat to large predatory fish. • Large plastic fragments may accumulate in tunas, but were found in only few (2%) of the fish. Microplastics may be transferred from fish prey to their predators, but do not remain in the predators' guts. Mesoplastics may accumulate in the guts of large predators, but are only ingested by few fish. [ABSTRACT FROM AUTHOR]

In recent decades, a marked decrease in planktivorous fish (Osmerus eperlanus eperlanus m. spirinchus Pallas and Coregonus albula (L.)) in Lake Peipsi has stimulated research into the potential effect of fish predation on zooplankton. The abundance of planktivorous fish and the abundance and biomass of zooplankton were studied from 1986 to 2014, and the diet of plankton-eating fish was analysed from 2007 to 2013. A linear regression model was used to establish possible trends in the zooplankton assemblages (biomass, abundance, mean weight of individuals) and in the abundance of zooplankton-eating fish. The ANOVA test was used to evaluate differences in the zooplankton assemblages and in fish consumption in the years with large and small fish cohorts. Despite the collapse of the smelt and vendace populations, the abundance of plankton-eating fish remained high due to an increase in the abundance of juvenile fish. Fish juveniles consumed as much as or even more zooplankton than adult planktivores. Since 1986, a significant decrease was observed in the cladoceran and copepod biomass and in the mean body weight of cladocerans. Large cladocerans such as Leptodora kindtii (Focke) and Bythotrephes longimanus Leydig were rare in zooplankton samples and the biomass of Bosmina spp. decreased. Changes in the structure of the zooplankton community were most likely caused by the feeding of juvenile fish as the calculated consumption by fish was high, especially in years with particularly large fish cohorts. [ABSTRACT FROM AUTHOR]

The trophic position of sardine ( Sardina pilchardus ) in spring was determined from the analysis of the natural abundance of stable nitrogen isotopes in the southern Bay of Biscay between 1998 and 2014. Mesozooplankton (200–500 µm) was employed as the reference baseline, as its isotopic signature tracked the seasonal development of phytoplankton blooms. Two methods for estimating trophic position were compared. Scaling the isotopic enrichment per trophic level to the isotopic signature of the reference baseline produced lower trophic position estimates than those using the traditional assumption of a constant enrichment per trophic level. Indeed, the studied sardine population is placed as a mid trophic level consuming prey near the base of the food web. Zonal averages of trophic position increased with decreasing size of the sardine stock and increasing values of the upwelling index, suggesting the interaction of trophic relationships and population dynamics. The effects of changes in the timing of phytoplankton blooms and the release of competition for prey at low population densities were discussed as potential factors causing an increase in mean trophic position of sardine in recent years. The application of the scaled trophic position estimates to the monitoring of sardine populations will facilitate the comparison of different studies and provide new insights on the factors regulating these populations. [ABSTRACT FROM AUTHOR]

Microplastics (<5 mm) have been found in many fish species, from most marine environments. However, the mechanisms underlying microplastic ingestion by fish are still unclear, although they are important to determine the pathway of microplastics along marine food webs. Here we conducted experiments in the laboratory to examine microplastic ingestion (capture and swallowing) and egestion by juveniles of the planktivorous palm ruff, Seriolella violacea (Centrolophidae). As expected, fish captured preferentially black microplastics, similar to food pellets, whereas microplastics of other colours (blue, translucent, and yellow) were mostly co-captured when floating close to food pellets. Microplastics captured without food were almost always spit out, and were only swallowed when they were mixed with food in the fish's mouth. Food probably produced a ‘gustatory trap’ that impeded the fish to discriminate and reject the microplastics. Most fish (93% of total) egested all the microplastics after 7 days, on average, and 49 days at most, substantially longer than food pellets (<2 days). No acute detrimental effects of microplastics on fish were observable, but potential sublethal effects of microplastics on the fish physiological and behavioural responses still need to be tested. This study highlights that visually-oriented planktivorous fish, many species of which are of commercial value and ecological importance within marine food webs, are susceptible to ingest microplastics resembling or floating close to their planktonic prey. [ABSTRACT FROM AUTHOR]

To understand dissolved oxygen deficiency in Chesapeake Bay and its direct impact on zooplankton and planktivorous fish communities, six research cruises were conducted at two sites in the Chesapeake Bay from spring to autumn in 2010 and 2011. Temperature, salinity, and dissolved oxygen were measured from hourly conductivity, temperature, and depth (CTD) casts, and crustacean zooplankton, planktivorous fish and gelatinous zooplankton were collected with nets and trawls. CTD data were grouped into three temperature groups and two dissolved oxygen-level subgroups using principal component analysis (PCA). Species concentrations and copepod nonpredatory mortalities were compared between oxygenated conditions within each temperature group. Under hypoxic conditions, there usually were significantly fewer copepods Acartia tonsa and bay anchovies Anchoa mitchilli, but more bay nettles Chyrsaora chesapeakei and lobate ctenophores Mnemiopsis leidyi. Neutral red staining of copepod samples confirmed that copepod nonpredatory mortalities were higher under hypoxic conditions than under normoxia, indicating that the sudden decline in copepod concentration in summer was directly associated with hypoxia. Because comparisons were made within each temperature group, the effects of temperature were isolated, and hypoxia was clearly shown to have contributed to copepod decreases, planktivorous fish decreases, and gelatinous zooplankton increases. This research quantified the direct effects of hypoxia and explained the interactions between seasonality and hypoxia on the zooplankton population.

The gut contents of 292 planktivorous fish, from four families (Atherinopsidae, Clupeidae, Engraulidae and Scombridae) and seven species, captured along the coast of the southeast Pacific, were examined for microplastic contamination. Only a small fraction of all studied fish (2.1%; 6 individuals) contained microplastic particles in their digestive tract. Microplastics found were degraded hard fragments and threads, ranging from 1.1 to 4.9 (3.8 ± SD 2.4) mm in length, and of various colours, which suggests that the planktivorous fish species examined herein did not capture microplastics on the basis of their colour. The low prevalence of microplastic contamination in planktivorous fishes found in this study suggests that the risk of accidental ingestion by these species might be limited in the coastal upwelled waters of the southeast Pacific, perhaps due to small human population and highly dynamic oceanographic processes. [ABSTRACT FROM AUTHOR]

Climate change projections indicate increased precipitation in northern Europe, leading to increased inflow of allochthonous organic matter to aquatic systems. The food web responses are poorly known, and may differ depending on the trophic structure. We performed an experimental mesocosm study where effects of labile dissolved organic carbon (DOC) on two different pelagic food webs were investigated, one having zooplankton as highest trophic level and the other with planktivorous fish as top consumer. In both food webs, DOC caused higher bacterial production and lower food web efficiency, i.e., energy transfer efficiency from the base to the top of the food web. However, the top-level response to DOC addition differed in the zooplankton and the fish systems. The zooplankton production increased due to efficient channeling of energy via both the bacterial and the phytoplankton pathway, while the fish production decreased due to channeling of energy mainly via the longer and less efficient bacterial pathway. We conclude that the added DOC either acted as a subsidy by increasing the production of the top trophic level (mesozooplankton), or as a sink causing decreased top consumer production (planktivorous fish). [ABSTRACT FROM AUTHOR]

An increasing number of studies have described the presence of microplastics (≤ 5 mm) in many different fish species, raising ecological concerns. The factors influencing the ingestion of microplastics by fish remain unclear despite their importance to a better understanding of the routes of microplastics through marine food webs. Here, we compare microplastics and planktonic organisms in surface waters and as food items of 20 Amberstripe scads ( Decapterus muroadsi ) captured along the coast of Rapa Nui (Easter Island) to assess the hypothesis that fish ingest microplastics resembling their natural prey. Sixteen (80%) of the scad had ingested one to five microplastics, mainly blue polyethylene fragments that were similar in colour and size to blue copepod species consumed by the same fish. These results suggest that planktivorous fish, as a consequence of their feeding behaviour as visual predators, are directly exposed to floating microplastics. This threat may be exacerbated in the clear oceanic waters of the subtropical gyres, where anthropogenic litter accumulates in great quantity. Our study highlights the menace of microplastic contamination on the integrity of fragile remote ecosystems and the urgent need for efficient plastic waste management. [ABSTRACT FROM AUTHOR]

Food web configuration is shaped by many factors, including primary production patterns and oceanographic features, such as upwelling events. In this study, we investigate variability in the trophic position, food web interlinks and energy pathways of four planktivorous demersal fish in the Southern Bay of Biscay- NE Atlantic. The study area is exposed to upwelling events of varying intensity and shows a significant spatial gradient along the coast. The two sampling years were characterized by markedly different conditions, with weak summer upwelling in 2012 and an intense upwelling season in 2013. We used a complementary approach based on stomach content analysis (SCA) and stable isotope analysis (SIA) to test the effects of upwelling intensity and persistence on the food-web. In particular we investigated whether different intensities shift the main flow of energy between the pelagic and benthic energy pathways. We found conspicuous interannual changes in the isotopic POM, whose δ 15 N was 2.5‰ lower during the productive 2013 season. Interannual changes in the isotopic signature of the fish species were also detected, although their values did not mirror variability at the basal level. The SCA results did not match the isotopic changes, which likely reflected dietary adjustments of the species during summer. The upwelling intensity gradient along the coast did not affect the nitrogen isotopic ratio of any group, however there was a significant effect of such gradient on the carbon isotopic ratio of the fish and euphausiid species. This effect was likely related to the higher primary production associated with intense upwelling conditions. [ABSTRACT FROM AUTHOR]

There is accumulating evidence of the significant role of three-spined sticklebacks (Gasterosteus aculeatus) for ecosystem structure and functioning in coastal areas of the Baltic Sea, but little is known about the role of the species in the pelagic foodwebs of the Baltic and its interaction with other planktivorous fish species. In this study, we assess the feeding niche, diet overlap, and prey selectivity of sticklebacks, herring (Clupea harengus), and sprat (Sprattus sprattus) in an area of the southwest Baltic Sea (Kalmar Sound) in relation to the seasonal abundance of zooplanktonic prey during 2009-2011. The main prey items for all fish species studied were the calanoid copepods Eurytemora affinis, Temora longicornis, and Acartia spp. during spring and summer, and the cladoceran Bosmina spp in autumn. The diet of all the three fish species overlapped considerably, especially during summer and autumn. A substantial diet overlap was even present between sticklebacks and larger (>10 cm) herring and sprat. We also found evidence for an overlap in feeding preferences, i.e. certain zooplankton species were selected by all the three fish species in each given season. Overall, these results indicate potential resource competition between sticklebacks and clupeids. With an increasing abundance of sticklebacks in the Baltic Sea, their role should not be neglected when investigating pelagic foodweb dynamics, and management of herring and sprat may have to account for sticklebacks by considering that sticklebacks, via food competition, might affect clupeid stocks. [ABSTRACT FROM AUTHOR]