Zooplankton Assemblages Research Articles

Winter distribution of zooplankton and ichthyoplankton assemblages in the North Sea and the English Channel.

Although zooplankton were extensively studied in the North Sea, knowledge about winter zooplankton assemblages is still scarce, despite potential influence of zooplankton overwintering stocks on seasonal plankton succession and productivity. Furthermore, several economically and ecologically important fish species reproduce during winter contributing to the zooplankton community as passive members (eggs) or predators (larvae). To elucidate on winter zooplankton distribution, abundance and composition in the Southern North Sea and Eastern English Channel, we defined assemblages based on mesozoo- and ichthyoplankton data sampled between January and February 2008 using fuzzy-clustering and indicator species. Mesozoo- and ichthyoplankton (eggs+larvae) were integrated in a common analysis by using a spatial grid adapted to the datasets and defined by means of a geostatistical method developed in agronomics. Potential environmental drivers of assemblage distribution were evaluated by means of GLMM and comparison with data from 2022 facilitated insight about the inter-annual representativeness of the assemblages. Five zooplankton assemblages were found varying with regard to total zooplankton abundance, dominant and indicator taxa. Spatial variability of abiotic (dissolved nutrients, salinity, depth, temperature, organic matter in suspension, chlorophyll a), biotic variables (phyto- and microplankton composition), water masses and fish spawning grounds were revealed as potential drivers of assemblage distribution. Assemblages off the Rhine-Scheldt estuary and in the German Bight harbored the biggest zooplankton overwintering stocks that might influence the grazing pressure on phytoplankton spring production. Assemblages off the Rhine-Scheldt estuary and covering the English Channel and the Southern Bight were found to be of high importance for herring and plaice larvae. Although further analyses suggested inter-annual representativeness of the assemblages found (2008 vs 2022), the assessment of further years would be necessary to account for potential inter-annual variability. Future studies could profit from the assessment of microzooplankton facilitating insight in fish larvae feeding potential and zooplankton overwintering strategies.

Latitudinal variation in zooplankton over the Emperor Seamounts (34°–44° N, 170°–171° E) during the summer of 2019

During a research expedition to the Northwest Pacific between July 13 and August 11, 2019, we collected oceanic zooplankton samples over the Emperor Seamounts (Koko, Jingu, Nintoku, and Suiko guyots). We analyzed the horizontal and vertical distributions of abundance and biomass and also the structure of the zooplankton in the highly productive but poorly studied ocean waters between 34° and 44° N. The zooplankton was represented by four genera of amphipods, seven genera of pteropods, 39 genera of copepods, and larvae of benthic invertebrates and fish. The abundance and biomass of zooplankton increased towards higher latitudes along the series Koko < Jingu < Nintoku < Suiko guyots. The highest index of diversity was recorded over the Koko and Jingu guyots; the highest species richness occurred over the Jingu and Nintoku guyots. Small-sized copepods, appendicularians, chaetognaths, euphausiids, and fish eggs and larvae concentrated in the epipelagic; large copepods and ostracods concentrated in the mesopelagic. We identified three types of zooplankton assemblages: a Subtropical assemblage between 34°–35° N characterized by tropical/subtropical forms; a Transition assemblage characterized by subtropical, subarctic, and widespread species between 38°–41° N; and a Subarctic assemblage characterized by subarctic and widespread species between 43°–44° N. The spatial distribution of zooplankton was a function of environmental variables such as surface salinity, temperature at 200-m, and, to a certain extent, by surface Chl-a concentrations.

Do red tide events promote an increase in zooplankton biodiversity?

Red tides occurring off the southern coast of Korea impact the marine ecosystem and aquaculture industries. Zooplankton are crucial in the food web, connecting primary producers to higher predators and interact diversely with red tide organisms. This study explores dynamics of the zooplankton community over seven years including three red tide and four non-red tide years in Tongyeong using metabarcoding. In non-red tide years, zooplankton diversity showed typical seasonal patterns, increasing from June to early October. However, during red tide years, diversity remained high, with a shift in species composition-decreased Copepoda and increased Branchiopoda, Echinodermata, Malacostraca, and Annelida. Diversity indices were significantly higher in red tide years across all periods except for the richness in “after” that showed an insignificant higher value. The differences in zooplankton assemblages across periods were influenced by surface temperatures and the density of the red tide-causing alga Margalefidinium polykrikoides. Eight species emerged as indicator species and showed direct correlations with M. polykrikoides and among them, seven species were indicator species for red tide occurrence years. The ecological characteristics of M. polykrikoides blooms and their recurrent occurrences over several decades suggest that zooplankton may adapt to the toxins and use these blooms as spawning cues. Overall, this study provides comprehensive understanding on changes in zooplankton communities during red tide events, offering novel insights into their ecology.

Effects of Yellow Sea Warm Current on zooplankton community composition and functional groups in winter

The Yellow Sea Warm Current (YSWC) constitutes a significant hydrological feature in the Yellow Sea, particularly prominent during winter, facilitating the transport of warm, saline waters and warm-water species from the open sea to the Bohai and Yellow Seas. The YSWC induces alterations in the community structure and function of zooplankton. However, the effects of the YSWC on the functional trait compositions and functional groups of zooplankton remain unclear. This study aimed to elucidate the influence of the YSWC on the community structure, functional trait composition, and functional groups of zooplankton during winter of 2016. The YSWC significantly impacted the zooplankton assemblage in the central Yellow Sea (CYSA), resulting in notable distinctions from the Shandong coastal assemblage (SCA) and Jiangsu coastal assemblage (JCA). Compared to the SCA and JCA (comprising 45 and 34 taxa, with abundances of 119.4 ± 114.6 ind·m-3 and 82.8 ± 62.1 ind·m-3, respectively), the CYSA exhibited higher species richness and abundance (with 51 taxa and 144.4 ± 103.4 ind·m-3, respectively). This study documented a total of 11 warm-water species, showing a decreasing trend in both species richness and abundance from south to north. The CYSA was characterized by the predominance of medium‒sized, current‒feeding, omnivorous‒herbivorous broadcast spawners, whereas the SCA and the JCA were predominantly dominated by giant‒sized, ambush‒feeding carnivores. The Qingdao-Shidao anticyclonic eddy in the southern of Shandong Province led to a significant increase in the abundance of zooplankton, potentially impacting Yellow Sea fishery resource. This research contributed to a deeper understanding of how YSWC influence the zooplankton community and offered fresh insights into the effects of YSWC on zooplankton function traits and functional groups.

Effect of an invasive species Mnemiopsis leidyi on the zooplankton community structure in the Caspian Sea

During the past three decades, the Caspian Sea (CS) ecosystem has undergone striking changes due to anthropogenic activities, particularly following the invasion of comb jelly Mnemiopsis leidyi that overfed on zooplankton in the late 1990 s. This study investigated the temporal and spatial patterns of the zooplankton community in CS over 25-year intervals using univariate and multivariate analyses. The objective was to assess the status of the zooplankton community through a comparison of the pre-invasion (1994–96) and post-invasion (2009–11 and 2018–19) of M. leidyi in the CS. The study revealed more zooplankton taxa for the pre-invasion period and fewer for the post-invasion period of 2018–19. Several key species such as Eurytemora grimmi, Polyphemus exicus, Synchaeta sp., and Calanipeda aquae dulcis disappeared post-invasion, and more than 50 taxa decreased to one-third in the post-invasion period. The abundance and diversity indices significantly declined following the invasion. The visualization of non-metric multidimensional scaling (nMDS) showed that periodic variations were more explanatory than assessments based on subareas and seasonal intervals. Significant differences were observed in the zooplankton assemblages across the three periods. In addition to highly affecting the zooplankton community, certain pelagic fish populations, notably those belonging to the kilka genus (Clupenella), have experienced significant decline as a result of M. leidyi invasion. In conclusion, the invasion of M. leidyi has caused major changes to the abundance and diversity of tropic levels in the CS, possibly through a combination of bottom-up and top-down forces at work in the ecosystem.

Ecological perspectives on water quality and zooplankton diversity in the Ravi River

AbstractFreshwater ecosystems are intricately linked to the diversity and abundance of zooplankton assemblages, which serve as vital indicators of environmental conditions. The aim of the present study was to investigate the habitat ecology in relation to zooplankton abundance of the Ravi River. Sampling was conducted at selected sampling sites along the Ravi River basin, encompassing various habitat types and environmental gradients. A total of 13 species of zooplankton were observed during the study in which Copepoda was most dominant followed by Cladocera. Cyclops sp. (152 ind/L) and Bosmina sp. (145 ind/L) were the dominant genera throughout the study. Asplanchana sp. (44 ind/L) was observed as a rare species during the study. Some habitat ecological parameters like water temperature, dissolved oxygen, and total dissolved solids show a declining trend in Zone 3. Turbidity was highest in Zone 3 and lowest in Zone 1, and almost all zooplankton genera showed a declining trend in Zone 3. Dissolved oxygen shows a significant positive correlation and directly influences the distribution of zooplankton assemblage structure. This study concludes that certain anthropogenic activities like municipal dumping, sand mining, and altered water flow have negatively impacted water quality and zooplankton diversity in the Ravi River. So, these types of activities should be monitored regularly and controlled immediately.

At the base of deep-sea food webs: Assemblage and trophic structure of suprabenthos and zooplankton in submarine canyons

Submarine canyons act as hotspots of biodiversity, hosting vulnerable marine ecosystems, and playing a fundamental role in bridging coastal zones with deeper areas. Here, we investigated the suprabenthic and Deep Scattering Layer (DSL) zooplankton fauna, that play a key role in deep-sea food webs, as main resources for both mobile and sessile megafauna, in two submarine canyons (Squillace and Amendolara) of the Ionian Sea (Central Mediterranean Sea). Our results highlighted different taxonomic and functional diversity between the two adjacent canyons: (i) biomass and abundance of suprabenthos followed an opposite trend in the two canyons, increasing both with depth in Amendolara (higher abundance and biomass in the lower part of the canyon), and decreasing with depth in Squillace (greater in the head of the canyon); (ii) DSL zooplankton abundance and biomass followed a spatial distribution, decreasing with increasing distance from the coast for both canyons (i.e. lower offshore than at the head of the canyon). Food-web structure investigated by means of stable isotope analysis of δ13C and δ15N showed a more diverse trophic niche for suprabenthos than for zooplankton. Furthermore, possible feeding modes of species with unknown feeding behaviour have been proposed. The results of the current article highlight the different ecological processes occurring within each canyon. Understanding the spatial variations of communities inhabiting submarine canyons, especially those at the base of deep-sea food webs which can act as driver of megafaunal communities (both sessile and mobile-commercial species), is essential to focalise future conservation efforts.

Larval coregonine density and diet composition along beaches of northern Lake Michigan

Recent oligotrophication in Lake Michigan has contributed to reduced biomass of spring zooplankton and a shift in the zooplankton assemblage toward more calanoid copepods. These changes have likely altered prey availability for first feeding native fish species that hatch in early spring. While spring zooplankton densities and assemblage compositions are routinely monitored in offshore areas of Lake Michigan, zooplankton in nearshore areas such as shallow beach environments are less studied; and associated descriptions of diet characteristics among larval coregonine species are limited. In this study, we a) describe the nearshore (<1 m depth) zooplankton assemblage at four sites in northeastern Lake Michigan during early spring 2015–2019 and b) compare diets and diet selectivity of larval lake whitefish (Coregonus clupeaformis) and Cisco (C. artedi). Zooplankton composition varied among years, but calanoid copepods and copepod nauplii consistently dominated the zooplankton assemblage. Cisco and lake whitefish larvae were captured regularly, with Elk Rapids containing the highest proportion of ciscoes each year. For both species, calanoid and cyclopoid copepods were common in diets and were positively selected as prey. Although previous research has indicated high consumption of cyclopoid copepods by larval coregonines, our results provide new evidence that larval lake whitefish and Cisco in northeastern Lake Michigan will also consume and select for calanoid copepods when they are abundant. As such, should calanoid zooplankton continue to dominate the copepod community in Lake Michigan, larval coregonines appear capable of exploiting this abundant prey resource to improve their likelihood of survival to later life stages.

Empirical verification of feeding selectivity of larval and juvenile pelagic fishes using in-situ zooplankton assemblages

Most studies on the feeding ecology of larvae and juveniles of commercially important pelagic fishes have used field-based approaches. However, due to possible biases related to net sampling, it is uncertain whether the results obtained from those studies truly represent the situation of live fish in the sea. Here we investigated the feeding ecology of pelagic fishes through a laboratory experiment minimizing the biases inherent in field net sampling. In the experiment, hatchery-reared juvenile chub mackerel (Scomber japonicus) and larval/juvenile Japanese anchovy (Engraulis japonicus) were fed with wild-caught zooplankton assemblages collected from around Hakatajima Island in the Seto Inland Sea, Japan. The relationships between fish size and prey number in the gut, and the selectivity on each prey organism were determined. As a result, in both species, prey number and size increased with body size, and the fish showed strong selectivity for crustaceans including copepodites and adults of copepods. Our data has also clearly indicated that both species can selectively prey on preferred foods that are rare while avoiding non-preferred foods that are abundant. These results, which substantially accord with reports from previous field studies, will not only help field scientists make a convincing interpretation of their data, but also open the possibility of further laboratory studies on detailed mechanisms of the feeding selectivity of larval/juvenile pelagic fishes.

Combined stress of an insecticide and heatwaves or elevated temperature induce community and food web effects in a Mediterranean freshwater ecosystem

Ongoing global climate change will shift nature towards Anthropocene's unprecedented conditions by increasing average temperatures and the frequency and severity of extreme events, such as heatwaves. While such climatic changes pose an increased threat for freshwater ecosystems, other stressors like pesticides may interact with warming and lead to unpredictable effects. Studies that examine the underpinned mechanisms of multiple stressor effects are scarce and often lack environmental realism. Here, we conducted a multiple stressors experiment using outdoor freshwater mesocosms with natural assemblages of macroinvertebrates, zooplankton, phytoplankton, macrophytes, and microbes. The effects of the neonicotinoid insecticide imidacloprid (1 µg/L) were investigated in combination with three temperature scenarios representing ambient, elevated temperatures (+4 °C), and heatwaves (+0 to 8 °C), the latter two having similar energy input. We found similar imidacloprid dissipation patterns for all temperature treatments with lowest average dissipation half-lives under both warming scenarios (DT50: 3 days) and highest under ambient temperatures (DT50: 4 days) throughout the experiment. Amongst all communities, only the zooplankton community was significantly affected by the combined treatments. This community demonstrated low chemical sensitivity with lagged and significant negative imidacloprid effects only for cyclopoids. Heatwaves caused early and long-lasting significant effects on the zooplankton community as compared to elevated temperatures, with Polyarthra, Daphnia longispina, Lecanidae, and cyclopoids being the most negatively affected taxa, whereas Ceriodaphnia and nauplii showed positive responses to temperature. Community recovery from imidacloprid stress was slower under heatwaves, suggesting temperature-enhanced toxicity. Finally, microbial and macrofauna litter degradation were significantly enhanced by temperature, whereas the latter was also negatively affected by imidacloprid. A structural equation model depicted cascading food web effects of both stressors with stronger relationships and significant negative stressor effects at higher than at lower trophic levels. Our study highlights the threat of a series of heatwaves compared to elevated temperatures for imidacloprid-stressed freshwaters.

Responses of zooplankton assemblages to environmental variability among brackish coastal ponds, New Zealand

Coastal brackish waters, including estuaries, lagoons and coastal ponds, experience fluctuations in physicochemical properties, particularly in salinity, due to the differential forcing of marine and freshwater inflows. Most research concerning brackish waters is skewed towards lagoons and estuaries, while relatively little exists on small coastal ponds. This study examined variability in zooplankton community composition relative to environmental conditions among brackish coastal ponds in Auckland, New Zealand. Eight coastal stormwater ponds, four freshwater ponds, and four marine sites were selected to represent a wide salinity gradient and sampled for zooplankton and environmental variables. Marked spatial and temporal variability in environmental conditions was observed among the coastal ponds. Salinity of the ponds was, on average, lower in winter, influenced by higher rainfall, while in summer reduced rainfall led to significantly lower freshwater inputs. Zooplankton communities within the coastal ponds were a mix of those in freshwater and marine sites, along with unique species not observed in each of the adjoining ecosystems. Salinity was the most important factor driving zooplankton community composition among sites, as widely reported for estuaries and lagoons. The degree of temporal change in salinity within ponds also affected community composition, with euryhaline copepods such as Sulcanus conflictus and Mesochra parva most abundant at sites with larger seasonal salinity changes. Salinity increases due to sea-level rises may lead to the disappearance of species that are less tolerant to salinity, with a move to dominance by euryhaline species.

Near-natural streams: Spatial factors are key in shaping multiple facets of zooplankton α and β diversity

In near-natural basins, zooplankton are key hubs for maintaining aquatic food webs and organic matter cycles. However, the spatial patterns and drivers of zooplankton in streams are poorly understood. This study registered 165 species of zooplankton from 147 sampling sites (Protozoa, Rotifers, Cladocera and Copepods), integrating multiple dimensions (i.e., taxonomic, functional, and phylogenetic) and components (i.e., total, turnover, and nestedness) of α and β diversity. This study aims to reveal spatial patterns, mechanisms, correlations, and relative contribution of abiotic factors (i.e., local environment, geo-climatic, land use, and spatial factors) through spatial interpolation (ordinary kriging), mantel test, and variance partitioning analysis (VPA). The study found that α diversity is concentrated in the north, while β diversity is more in the west, which may be affected by typical habitat, hydrological dynamics and underlying mechanisms. Taxonomic and phylogenetic β diversity is dominated by turnover, and metacommunity heterogeneity is the result of substitution of species and phylogeny along environmental spatial gradients. Taxonomic and phylogenetic β diversity were strongly correlated (r from 0.91 to 0.95), mainly explained by historical/spatial isolation processes, community composition, generation time, and reproductive characteristics, and this correlation provides surrogate information for freshwater conservation priorities. In addition, spatial factors affect functional and phylogenetic α diversity (26%, 28%), and environmental filtering and spatial processes combine to drive taxonomic α diversity (10%) and phylogenetic β diversity (11%). Studies suggest that spatial factors are key to controlling the community structure of zooplankton assemblages in near-natural streams, and that the relative role of local environments may depend on the dispersal capacity of species. In terms of diversity conservation, sites with high variation in uniqueness should be protected (i) with a focus on the western part of the thousand islands lake catchment and (ii) increasing effective dispersal between communities to facilitate genetic and food chain transmission.

Anthropogenic Eutrophication Drives Major Food Web Changes in Mwanza Gulf, Lake Victoria

Discerning ecosystem change and food web dynamics underlying anthropogenic eutrophication and the introduction of non-native species is necessary for ensuring the long-term sustainability of fisheries and lake biodiversity. Previous studies of eutrophication in Lake Victoria, eastern Africa, have focused on the loss of endemic fish biodiversity over the past several decades, but changes in the plankton communities over this same time remain unclear. To fill this gap, we examined sediment cores from a eutrophic embayment, Mwanza Gulf, to determine the timing and magnitude of changes in the phytoplankton and zooplankton assemblages over the past century. Biogeochemical proxies indicate nutrient enrichment began around ~ 1920 CE and led to rapid increases in primary production, and our analysis of photosynthetic pigments revealed three zones: pre-eutrophication (prior to 1920 CE), onset of eutrophication with increases in all pigments (1920–1990 CE), and sustained eutrophication with cyanobacterial dominance (1990 CE–present). Cladoceran remains indicate an abrupt decline in biomass in ~ 1960 CE, in response to the cumulative effects of eutrophication and lake-level rise, preceding the collapse of haplochromine cichlids in the 1980s. Alona and Chydorus, typically benthic littoral taxa, have remained at relatively low abundances since the 1960s, whereas the abundance of Bosmina, typically a planktonic taxon, increased in the 1990s concurrently with the biomass recovery of haplochromine cichlid fishes. Overall, our results demonstrate substantial changes over the past century in the biomass structure and taxonomic composition of Mwanza Gulf phytoplankton and zooplankton communities, providing a historical food web perspective that can help understand the recent changes and inform future resource management decisions in the Lake Victoria ecosystem.

Dynamic oceanographic influences on zooplankton communities over the northern Gulf of Mexico continental shelf

Dynamic influences of ocean processes on distribution, abundance, and diversity of zooplankton communities were studied over the continental shelf in the northern Gulf of Mexico (GoM) from 2015 to 2017. Zooplankton sampling was conducted on four summer cruises in the northcentral GoM. Sampling was designed in waters potentially influenced by the Loop Current (LC) and/or Mississippi River discharge to assess the impacts of these two mesoscale features on the abundance and diversity of zooplankton. During the three-year study, the LC displayed distinct spatial-temporal variations in penetration and occurrence in the northern GoM. Environmental conditions (i.e., sea surface temperature, salinity, and dissolved oxygen) varied between months and years sampled, and were significantly different among cruises (ANOVA, p < 0.001). The majority of zooplankton consisted of calanoid copepods (65% ± 7.2%, mean ± SD), while non-copepod taxa were primarily chaetognaths, polychaetes, tunicates, and ostracods (23 ± 9.2%). Species abundance and diversity of zooplankton were significantly correlated with sea surface temperature, salinity, and dissolved oxygen (p < 0.05). Canonical correspondence analysis displayed significant associations between mesoscale features and dominant zooplankton groups among cruises and by taxa (Monte Carlo Permutation Test, p < 0.001). In addition, non-metric multidimensional scaling indicated that zooplankton assemblages were distinct, likely caused by Mississippi River plumes during the study period. As one of the few efforts to examine zooplankton dynamics at a low taxon level over the GoM continental shelf regarding the impact of mesoscale features, this study revealed seasonal (i.e. summer) and spatial patterns in distribution, abundance, and diversity of zooplankton communities subjected to the dynamic physicochemical conditions in the northern GoM, which will continue in a changing climate.

Divergence of zooplankton assemblages in unstable fishless and stable fish ponds

This study aimed to achieve a better understanding of the features of zooplankton assemblages in small water bodies and their biodiversity and composition in relation to the presence of fish and accompanying environmental characteristics. This study was conducted in 16 mid-field ponds. Compositional and biodiversity indexes and ordination methods were used to analyze the relationship between zooplankton assemblages in unstable fishless and stable fish ponds. A total of 121 zooplankton taxa were identified. Compositional indicators revealed significant differences in zooplankton assemblages between fish ponds (FPs) and fishless ponds (FLPs). Canonical correspondence analysis indicated that variation in zooplankton assemblages depended on the ponds’ features. Most of the high trophic state indicator species were present only in FPs or occurred sporadically in FLPs. Rarefaction and extrapolation indicated a higher number of zooplankton taxa in FPs than in FLPs. The stability of the ecosystem was essential for maintaining the high species richness of zooplankton. Diversity indices were not influenced by variations in species composition or environmental differences among ponds. Diversified pond types are necessary to maintain the heterogeneity of mid-field ponds, which support the high regional biodiversity of zooplankton assemblages.

Continuous Zonal Gradients Characterize Epipelagic Plankton Assemblages and Hydrography in the Subtropical North Atlantic

The subtropical North Atlantic is a key region for understanding climate impact in the ocean. Plankton studies in this region have been generally framed in biogeographic provinces or focused on latitudinal gradients. In this study, we demonstrate the benefits of using empirically constructed continuous gradients versus the use of average values for biogeographical provinces to characterize plankton assemblages along a longitudinal transect at 24.5° N using an unprecedented array of stations including hydrographic observations, abundance of phytoplankton and zooplankton, and plankton size spectra in the epipelagic layer (0–200 m). In addition, the variability of zooplankton assemblages was analyzed using detailed taxonomic identification at selected stations. We found significant gradients in most hydrographic and plankton variables. The former, including surface temperature and salinity, the depth of the upper mixing layer, and the depth of the chlorophyll maximum, displayed non-linear gradients with maximum or minimum values near the center of the transect. In contrast, most plankton variables showed linear zonal gradients. Phytoplankton, microzooplankton (&lt;100 µm), and the slope and the intercept of the size spectra increased (and Trichodesmium decreased) to the west. Total mesozooplankton (&gt;200 µm) did not show any significant zonal pattern, but the taxonomic assemblages were characterized by a gradual replacement of large Calanoids by small-bodied Cyclopoid copepods from east to west. The use of continuous gradients provides more detailed information on the zonal structure of subtropical plankton than the classical approach using discrete areas.

Common bully (Gobiomorphus cotidianus) and mosquitofish (Gambusia affinis) shift littoral zooplankton community composition to smaller bodied taxa

ABSTRACT We investigated the effects of native common bully (Gobiomorphus cotidianus) and non-native western mosquitofish (Gambusia affinis) on zooplankton assemblages in an outdoor mesocosm experiment. Twenty-one 100 L mesocosms containing zooplankton were assigned either four G. cotidianus, four G. affinis, or were left as a control group (i.e. with no fish) and monitored for 14 days. Zooplankton community composition in the fish treatments was significantly different from the control tanks by day 14, but did not differ from one another. Both G. cotidianus and G. affinis reduced crustacean abundances, including Bosmina meridionalis, Daphnia pulex and Mesocyclops australiensis, relative to the controls. This reduced the effects of predation, competition, and physical interference by the crustaceans on rotifers, allowing rotifers to reach numerical dominance in the fish treatments. In contrast, zooplankton assemblages in the control mesocosms became dominated by cladocerans, indicating that these had a competitive advantage in the absence of the zooplanktivorous fish species. Our results suggest that G. cotidianus and G. affinis have similar preferences for zooplankton taxa and that G. affinis can alter littoral zooplankton assemblages in a comparable manner to G. cotidianus.

Heatwave‐mediated decreases in phytoplankton quality negatively affect zooplankton productivity

Abstract Climate change is expected to increase the frequency of extreme temperature events. The effect of heatwaves on phytoplankton is of particular concern because they are a key source of C, N, P and essential fatty acids to aquatic ecosystems. Laboratory studies have demonstrated that phytoplankton grown at warmer temperatures are a lower quality food source, but how heatwaves affect phytoplankton quality at the community scale is currently unclear. Here we address this knowledge gap by growing natural assemblages of freshwater phytoplankton at “ambient”, “constant warming” or “heatwave” conditions. We next fed these phytoplankton communities to natural assemblages of zooplankton to test the prediction that zooplankton that consume heatwave‐exposed phytoplankton will exhibit reductions in biomass. Our experiment demonstrated that zooplankton that consumed “heatwave” phytoplankton attained lower community biomass than those fed “constant warming” or “ambient” phytoplankton. Additionally, despite receiving similar total heat input, phytoplankton exposed to “heatwave” conditions contained lower C, N, P and fatty acid concentrations compared to phytoplankton grown in “constant warming” conditions. Correlations between zooplankton biomass and all measured phytoplankton traits revealed that decreases in zooplankton biomass were best explained by low quantities of C, N and monounsaturated fatty acids in “heatwave” phytoplankton. Our study demonstrates that the effects of heatwaves on phytoplankton quality are clearly distinct from those caused by constant warming temperatures and that heatwave‐mediated decreases in resource quality have immediate effects on consumer productivity. Read the free Plain Language Summary for this article on the Journal blog.

Seston biomass in plankton assemblages in the Southwestern Atlantic Ocean: spatial, vertical, and temporal variations.

Bioseston is a heterogeneous assemblage of bacterioplankton, phytoplankton, zooplankton, and planktonic debris. A detailed knowledge of biosestons is essential for understanding the dynamics of trophic flows in marine ecosystems. The distributional features of seston biomass in plankton (micro- and mesoplankton) in the Southwest Atlantic Ocean (Rio de Janeiro State, Brazil) were analyzed using stratified samples gathered to a depth of 2,400 m during night time. The horizontal pattern of biomass distribution was analyzed vis-a-vis station depth during both wet and dry periods, with higher values recorded in the continental shelf than in the slope, confirming the terrestrial contribution of nutrient sources to the marine environment. This horizontal variation reinforces the occurrence of seasonal vortices in Cabo Frio and Cabo de São Tomé on the central coast of Brazil. Environmental variables reflect the hydrological signatures of the water masses along the Brazilian coast. The largest seston biomass was related to high temperatures, salinities, and low inorganic nutrient concentrations in tropical and South Atlantic central waters. The observed distribution patterns suggest that seston biomass in plankton in the region may be structured based on partitioned horizontal and vertical habitats and food resources.

Biotic assemblages of gelatinous zooplankton in the Gulf of Mexico and adjacent waters: An evolutionary biogeographic approach.

Gelatinous zooplankton constitutes a polyphyletic group with a convergent evolutionary history and poorly known biogeographical patterns. In the Gulf of Mexico, a region with complex geological, hydrological, and biotic histories, the study of this group has been limited to taxonomical and ecological aspects. In this study, we implemented a track analysis to identify distributional patterns of gelatinous zooplankton in the Gulf of Mexico and adjacent waters based on a dataset of 6067 occurrence records corresponding to Hydrozoa, Scyphozoa, Cubozoa, Ctenophora, Chaetognatha, Thaliacea, and Appendicularia. Information was compiled from the Global Biodiversity Facility Information (GBIF) and Ocean Biodiversity Information System (OBIS) databases and peer-reviewed literature. Individual tracks were constructed by joining the minimum distance between the occurrence localities of each taxon using a minimum spanning tree algorithm. We identified generalized tracks using parsimony analysis of endemicity with progressive character elimination (PAE-PCE). The areas where different generalized tracks overlapped were considered to represent panbiogeographical nodes. Seven generalized tracks (two with nested patterns) and six panbiogeographical nodes were recognized, mainly in neritic zones. The distributional patterns of gelatinous zooplankton allowed us to identify four biogeographic areas, supporting previously proposed biogeographic schemes. Gelatinous zooplankton in the Gulf of Mexico showed a convergent spatial distribution that can be explained by vicariant and dispersal events. The historical biogeography of the gelatinous biotas of the Gulf of Mexico has been little studied compared to ecological approaches, and the lack of integrative studies considering historical patterns is evident. This type of research is fundamental to understanding the evolutionary history of natural resources from a spatial perspective, identifying sites of biodiversity and endemism, and establishing a biogeographic baseline of the region for further studies.