Structure of the zooplankton community and trophic interactions in a pelagic system influenced by internal island-trapped waves

Zooplankton plays a crucial role in marine ecosystems by mediating trophic interactions, recycling nutrients, and attenuating carbon flux via the biological pump. Understanding zooplankton community structure and functioning is key to predicting ecosystem responses to environmental change and global carbon cycling. The eLTER network provides long-term ecological data to disentangle natural variability from directional environmental change. Here we analysed a 20-year (2003-2022) time-series of zooplankton abundance and community composition data (Povero et al. 2024) from the pelagic compartment of the eLTER site Promontory of Portofino (eLTER-IT15-001-M), station Punta Faro (44° 17.750’ N; 9° 13.050’ E) (Fig. 1). Our analysis aimed to: 1) assess changes in zooplankton community composition over time, 2) identify common trends across taxonomic descriptors, and 3) investigate the influence of environmental variables. To explain observed patterns, we combined time-series analysis with two complementary modelling approaches. An Ecopath model was use to compare community structure and functioning between early (2003-2005) and later (2018-2019) periods (Vassallo et al. 2022). Stable isotope mixing models based on carbon isotope fingerprints of amino acids (Larsen et al. 2013) estimated the proportional contributions of distinct production sources (i.e., marine autotrophs, heterotrophic bacteria, and terrestrially-derived organic matter). Our results revealed strong seasonal cycles and long-term changes in environmental variables, accompanied by shifts in zooplankton abundance and community composition. Total mesozooplankton and copepod abundance increased since 2019, particularly in summer and autumn (Fig. 1). Species richness increased from 2009, while the Simpson index declined, indicating a more even distribution of individuals across taxa. These trends were accompanied by increased contributions of carnivorous (Corycaeus spp.) and detritivorous (Oncaea spp.) copepods, other typically carnivorous groups (Chaetognatha), and gelatinous suspension-feeders (Thaliacea). In parallel, we observed decreased contributions of typically herbivorous (e.g., Calocalanus styliremis, Euterpina acutifrons, Paracalanus nanus, Clausocalanus parapergens) and small copepods (Oithona. nana), Appendicularia, and meroplankton. Zooplankton variability modestly correlated with environmental changes, including positive temperature seasonal anomalies since 2012 and increased oligotrophy. Our findings suggest a transition from a herbivore-dominated food web towards a system increasingly supported by detrital and microbial processes. The Ecopath model indicated increased consumption on detritus and heterotrophs (Fig. 2; Vassallo et al. 2022). Isotope models estimated highest proportional contributions from marine autotrophs but increased contributions from microbial secondary production to summer zooplankton, especially bacterivores and detritivores (Fig. 2). Thus, while stability in the role of zooplankton of capturing newly fixed carbon and exporting it to depth is confirmed, at least at the study site, these shifts may reflect the ecosystem’s capability to reorganise under changing environmental conditions, potentially at the cost of reduced energy transfer efficiency and higher metabolic demands.

Parallel structure among environmental gradients and three trophic levels in a subarctic estuary

Brown trout (Salmo trutta) are known to have effects on multiple trophic levels in New Zealand streams, but their impacts on lower trophic levels are less well understood within lentic systems. We examined the effects of brown trout removal using rotenone on zooplankton and phytoplankton community composition in the Upper Karori Reservoir, New Zealand. Significant shifts were observed in zooplankton and phytoplankton composition following removal of brown trout from the reservoir. Shifts in zooplankton community composition did not occur immediately following trout removal (February), but instead followed the likely timing of galaxiid spawning (July). The removal of brown trout likely resulted in reduced predation pressure on galaxiids. A major change occurred in the zooplankton community with the dominance shifting from larger crustaceans to smaller rotifers, indicating an increased predation pressure from the larval native galaxiid. A delayed response in zooplankton community composition change indicates rotenone was not the direct cause of this. A major shift in phytoplankton community composition occurred immediately following trout removal. This was not consistent with the trophic cascade hypothesis of reduced grazing pressure from larger zooplankton due to increased galaxiid predation as a result of brown trout removal.

Deforestation of riparian areas is a major driver of biodiversity loss in aquatic ecosystems. Thus, we investigated the influence of forest cover and physical and chemical characteristics of streams on zooplankton communities in the southeastern Amazon. We addressed the following questions: (1) Are environmental factors (water physical and chemical characteristics and landscape variables) and dispersive processes (reflected in the spatial structure among sampling sites) efficient predictors of zooplankton communities in different hydrologic seasons? (2) Can zooplankton species be indicators of watersheds’ forest-cover levels? We sampled 15 streams located in nine rural settlements in northern Mato Grosso, Brazil, in the dry (August) and rainy (March) seasons of 2017. The forest-cover level had a significant effect on the physical and chemical characteristics (conductivity, dissolved oxygen, and temperature) of streams and also on the structure and composition of zooplankton communities, mainly of rotifers and testate amoebae. Areas with low vegetation cover had seasonal changes in species richness, individuals density, and zooplankton community structure. Environmental and spatial variables had no significant effect on the structure of zooplankton communities, which may indicate the strong influence of stochastic factors. Species from three zooplankton groups (rotifers, microcrustaceans, and testate amoebae) were indicators of forest-cover classes. This study provided valuable contributions to the conservation of riparian ecosystems and the use of biological indicators in environmental monitoring programs.

We studied the zooplankton community structure in a set of 33 interconnected shallow ponds that are restricted to a relatively small area (‘De Maten’, Genk, Belgium, 200 ha). As the ponds share the same water source, geology and history, and as the ponds are interconnected (reducing chance effects of dispersal with colonisation), differences in zooplankton community structure can be attributed to local biotic and abiotic interactions. We studied zooplankton community, biotic (phytoplankton, macrophyte cover, fish densities, macroinvertebrate densities), abiotic (turbidity, nutrient concentrations, pH, conductivity, iron concentration) and morphometric (depth, area, perimeter) characteristics of the different ponds. Our results indicate that the ponds differ substantially in their zooplankton community structure, and that these differences are strongly related to differences in trophic structure and biotic interactions, in concordance with the theory of alternative equilibria. Ponds in the clear-water state are characterised by large Daphnia species and species associated with the littoral zone, low chlorophyll- a concentrations, low fish densities and high macroinvertebrate densities. Ponds in the turbid-water state are characterised by high abundances of rotifers, cyclopoid copepods and the opposite environmental conditions. Some ponds show an intermediate pattern, with a dominance of small Daphnia species. Our results show that interconnected ponds may differ strongly in zooplankton community composition, and that these differences are related to differences in predation intensity (top-down) and habitat diversity (macrophyte cover).

Hydrological conditions are responsible for the changes in lateral connectivity between the main river channel and the floodplain lakes, a factor controlling zooplankton abundance and diversity. We tested the hypothesis that the degree of connectivity between the aquatic habitats and the river channel governs the zooplankton densities and community structure. Abundances, community composition and species diversity of zooplankton were analysed against the gradient of lakes’ connectivity and the water quality parameters under a natural flood pulse in the Biebrza River (North-Eastern Poland). Our findings revealed that the water level fluctuations directly affect the availability of nutrients, aeration, what in turn controls the densities and biovolumes of zooplankton communities. Along with the increase in the lake isolation, the taxonomic diversity of zooplankton decreased, while the eudomination of taxa indicative of advanced trophy (Rotifera) was observed. Qualitative parameters, as number of species, diversity and richness, were significantly higher at mean water levels, which supports the intermediate disturbance hypothesis. The sensitivity of the zooplankton community to variable hydrological conditions and lateral connectivity gradient demonstrates its potential as an unexploited indicator of any habitat changes in the aquatic ecosystems.

ABSTRACTChanging Arctic climate may alter freshwater ecosystems as a result of warmer surface waters, longer open-water periods, reduced wintertime lake ice growth, and altered hydrologic connectivity. This study aims to characterize zooplankton community composition and size structure in the context of hydrologic connectivity and ice regimes in Arctic lakes. Between 2011 and 2016, we sampled the phytoplankton, zooplankton, and fish communities from a set of representative lakes on the Arctic Coastal Plain (ACP) of northern Alaska to determine potential food web responses to changing Arctic ecosystems. Multivariate analyses showed that time from ice-out had a strong influence on zooplankton community structure and that seasonal succession of zooplankton differed between lakes with varying hydrologic connectivity. Trends were observed suggesting that large-bodied zooplankton (Daphnia, calanoid copepods) may be more prevalent in poorly connected lakes with low fish diversity. Large-bodied zooplankton displayed higher biomass in lakes with high occurrences of bedfast ice, while small-bodied zooplankton (Bosmina, rotifers) displayed highest biomass in deeper lakes with low occurrences of bedfast ice. Our results contribute to limited knowledge of zooplankton in remote lakes of the ACP and suggest that the anticipated changes to aquatic ecosystems in the Arctic may include energetically less efficient plankton food webs.

Zooplankton community composition can be related to natural environmental factors such as lake morphology, lake landscape position, and water chemistry as well as anthropogenic factors such as agricultural and urban land-use. We hypothesized that within-lake factors, such as water chemistry, lake morphology, and human land-use would each be related to zooplankton community structure, but that watershed land-use would be the strongest correlate in southeast Wisconsin lakes. Zooplankton samples, collected every 3 months over a year, from 29 lakes were used to determine how lake and watershed morphology, water quality, and land-use were related to zooplankton community structure in the heavily developed Southeast Wisconsin Till Plain Ecoregion. Forward selection and a variation partitioning procedure were used to determine relative and shared contributions of each suite of variables in predicting zooplankton community structure. Redundancy analysis was used to characterize dominant gradients in pelagic zooplankton communities and related environmental factors and land-use. The major correlates of community structure included summer phosphorus, lake depth and surface area and urban and natural land. Variation partitioning illustrated that phosphorus alone accounts for the greatest part (12%) of community structure. Urban land-uses (residential, commercial and paved land) and lake morphology partially explain zooplankton community variation through combined effects with phosphorus. Small cladocerans and Skistodiaptomus pallidus were associated with higher phosphorus, shallow depth and higher urban land-use, while Daphnia pulicaria dominates in deep lakes with lower phosphorus and less urban land-use. This study contributes to the understanding of factors affecting zooplankton community structure in a largely human developed region and illustrates the importance of eutrophication in structuring zooplankton community composition.

Zooplankton community structure as determinant of toxic substances content along the European coast.

Zooplankton are a conduit of energy from autotrophic phytoplankton to higher trophic levels, and they can be a primary point of entry of microplastics into the aquatic food chain. Investigating how zooplankton communities are affected by microplastic pollution is thus a key step toward understanding ecosystem-level effects of these global and ubiquitous contaminants. Although the number of studies investigating the biological effects of microplastics has grown exponentially in the last decade, the majority have used controlled laboratory experiments to quantify the impacts of microplastics on individual species. Given that all organisms live in multispecies communities in nature, we used an outdoor 1130-L mesocosm experiment to investigate the effects of microplastic exposure on natural assemblages of zooplankton. We endeavored to simulate an environmentally relevant exposure scenario by manually creating approximately 270 000 0.015 × 1- to 1.5-mm polyester fibers and inoculating mesocosms with zero, low (10 particles/L), and high (50 particles/L) concentrations. We recorded zooplankton abundance and community composition three times throughout the 12-week study. We found no effect of microplastics on zooplankton abundance, Shannon diversity, or Pielou's evenness. Nonmetric multidimensional scaling plots also revealed no effects of microplastics on zooplankton community composition. Our study provides a necessary and realistic baseline on which future studies can build. Because numerous other stressors faced by zooplankton (e.g., food limitation, eutrophication, warming temperatures, pesticides) are likely to exacerbate the effects of microplastics, we caution against concluding that polyester microfibers will always have no effect on zooplankton communities. Instead, we encourage future studies to investigate the triple threats of habitat degradation, climate warming, and microplastic pollution on zooplankton community health. Environ Toxicol Chem 2024;43:418-428. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Seasonal variation of zooplankton abundance and community structure in Prince William Sound, Alaska, 2009–2016

Patterns of mesozooplankton community composition and vertical fluxes in the global ocean

Planktivorous fish predation directly affects zooplankton biomass, community and size structure, and may indirectly induce a trophic cascade to phytoplankton. However, it is not clear how quickly the zooplankton community structure and the cascading effects on phytoplankton recover to the unaffected state (i.e. resilience) once short-term predation by fish stops. The resilience has implications for the ecological quality and restoration measures in aquatic ecosystems. To assess the short-term zooplankton resilience against fish predation, we conducted a mesocosm experiment consisting of 10 enclosures, 6 with fish and 4 without fish. Plankton communities from a natural lake were used to establish phytoplankton and zooplankton in the mesocosms. High biomasses (about 20 g wet mass m-3) of juvenile planktivorous fish (perch, Perca fluviatilis) were allowed to feed on zooplankton in fish enclosures for four days. Thereafter, we removed fish and observed the recovery of the zooplankton community and its cascading effect on trophic interactions in comparison with no fish enclosures for four weeks. Short-term fish predation impaired resilience in zooplankton community by modifying community composition, as large zooplankton, such as calanoids, decreased just after fish predation and did not re-appear afterwards, whereas small cladocerans and rotifers proliferated. Total zooplankton biomass increased quickly within two weeks after fish removal, and at the end even exceeded the biomass measured before fish addition. Despite high biomass, the dominance of small zooplankton released phytoplankton from grazer control in fish enclosures. Accordingly, the zooplankton community did not recover from the effect of fish predation, indicating low short-term resilience. In contrast, in no fish enclosures without predation disturbance, a high zooplankton:phytoplankton biomass ratio accompanied by low phytoplankton yield (Chlorophyll-a:Total phosphorus ratio) reflected phytoplankton control by zooplankton over the experimental period. Comprehensive views on short and long-term resilience of zooplankton communities are essential for restoration and management strategies of aquatic ecosystems to better predict responses to global warming, such as higher densities of planktivorous fish.

Revisiting zooplankton as indicators in the Great Lakes: Which indicators detect temporal changes in the zooplankton community composition?

High-latitude oceans are among the most vulnerable regions to a warming climate. Historically covered by sea ice for much of the year, these areas are now experiencing accelerated environmental changes, such as increasing atmospheric and ocean temperatures, resulting in rapid glacial melt and calving. Sermilik Fjord, located in southeastern Greenland, is an Arctic fjord system influenced by marine-terminating glaciers and characterized by complex bathymetry and distinct water masses, including warm, saline Atlantic Water (AW), cold, saline polar water, and cold, fresh subglacial meltwater. These characteristics create a diverse and productive ecosystem, with unique physical characteristics among the inner fjord, outer fjord, shelf, and slope/off-shelf regions.This study aims to explore and compare zooplankton community assemblages across Sermilik Fjord, specifically focusing on the presence or absence of AW in the surface waters (40-200 m depth). This research will determine relationships between zooplankton community composition and AW, define community structure for each region, and identify indicator species associated with specific regions or water masses. Field collections were conducted using 29 samples from a bongo sampler deployed to a depth of 100 m. Vertical profiles of temperature, salinity, and other parameters were collected using a CTD, providing a detailed understanding of the water column structure and the presence of different water masses.About half of all stations that zooplankton were collected at had AW present in the surface waters. A hierarchical cluster analysis determined five key zooplankton community clusters in the fjord, and distinct separation between communities that had a presence or absence of AW in the surface waters. Zooplankton statistical analyses will be accomplished through a NMDS on community clusters. SIMPER analysis identified euphausiids and chaetognaths as key contributors (over 50%) to community dissimilarity. Diversity analysis using Shannon and Simpson indices revealed considerable variability in zooplankton community composition across samples. Low Shannon values (e.g., Upper fjord with H' = 0.032 and Mid-fjord with H’ = 0.011) and high Simpson values (e.g., Fjord Mouth, D = 0.65) indicated dominance by a single species, suggesting low community diversity. In contrast, samples near the glacier-terminus (H' = 1.10, D = 0.66) and coast (H' = 0.87, D = 0.55) exhibited higher diversity and evenness. These findings highlight areas with both high species dominance and more evenly distributed communities, reflecting spatial differences in zooplankton community structure. Preliminary results indicate potential gradients in community composition, with warmer water species and higher zooplankton abundance associated with AW presence in surface waters. Distinct communities were also observed between regions influenced by glacial meltwater and those characterized by warmer AW influx.This work contributes to our understanding of the ecological dynamics of Greenland's fjords in response to climate change, highlighting the importance of zooplankton as key players in Arctic marine ecosystems. It further emphasizes the challenges and urgency of studying these vulnerable regions, as they undergo profound shifts in their physical and biological environments.