Mesozooplankton Community Research Articles

Spatiotemporal changes in the community and demography of mesozooplankton in the eastern Indian sector of the Southern Ocean during austral summer 2018/2019

The Southern Ocean is facing rapid environmental changes. However, few studies have been conducted on the spatiotemporal variability of mesozooplankton communities under recent climatic conditions, particularly in the eastern Indian sector. This study describes the spatiotemporal variability of the mesozooplankton community and the demographics of large copepods and krill in this sector, sampled through a Rectangular Mid-Water Trawl with 1 m2 mouth area (RMT1) during the austral summer of 2018/2019 as part of the KY1804 survey. Cluster analysis indicated that the mesozooplankton community was divided into five groups that showed only small longitudinal differences, as they were affected by oceanic fronts. Part of the variability was explained by physical (local upwelling) and biological features (e.g., the occurrence of species showing a specific spatial distribution, such as Euphausia crystallorophias). Horizontal changes in the zooplankton community structure were not attributed to temporal changes during the 2-month sampling period. The demographics of the dominant species, Calanoides acutus, Calanus propinquus, Metridia gerlacheri, and Thysanoessa macrura, exhibited significant temporal differences in abundance or mean stage index (MSI) between the early and late seasons. These differences matched the growth rates estimated in previous studies, suggesting that their growth during the study period was constant without regional differences. There were no evident changes in the abundance or demographics of Rhinalanus gigas, suggesting that they were in their reproductive season. These species-specific demographics could be explained by the species life cycles: growth in C. acutus and C. propinquus and reproduction in R. gigas during the austral summer. Abundances and MSIs confirmed the growth of dominant copepods and krill during the sampling period; however, no evident seasonal changes were observed in the zooplankton community structure. The findings of this study contribute to the understanding of lower trophic levels in marine ecosystems and the present carbon cycle in the eastern Indian sector of the Southern Ocean.

Sea Ice Dynamics and Planktonic Adaptations: A Study of Terra Nova Bay’s Mesozooplanktonic Community during the Austral Summer

Phytoplankton and zooplanktonic communities form the base of the Antarctic food web. This study examines the evolution of the mesozooplanktonic system in Terra Nova Bay during the austral summer (December–February), focusing on the impact of sea ice dynamics and the resulting phytoplankton blooms. Terra Nova Bay (Ross Sea) offers a valuable context given its high productivity and ecological variability. Using a diachronic approach, we analyzed data spanning twelve years to understand how the system’s structure and functionality change over time. A novel key metric, Days since Sea Ice Melting, was employed to track shifts in phytoplankton community development and trophic dynamics. The results indicate that the system enters the summer season increasing primary productivity and creating the support for the development of a more complex and organized system during the season. The phytoplankton bloom recorded during mid-season, coped by an increase in biomass, is followed by the establishment of a well-organized grazing system. A secondary phytoplankton bloom is observed towards the end of the summer, but it does not significantly affect mesozooplankton communities. Overall, this study highlights the dynamic nature of Terra Nova Bay’s mesozooplanktonic community and evaluates the influence of climate change on Antarctic marine ecosystems.

Environmental drivers of mesozooplankton dynamics in the Munroe Island, adjacent to Ashtamudi Estuary, India.

The response of mesozooplankton is critical in assessing the health of an estuarine ecosystem. Reports on the spatial and temporal dynamics of mesozooplankton community in estuarine and backwaters of the Southern parts of India are scanty. In this scenario, we appraised the community structure of mesozooplankton and their spatio-temporal dynamics based on various multivariate statistical assessments. A total of 31 taxa were identified and the abundance was principally dominated by Copepoda followed by Luciferidae during three sampling seasons. The most abundant species were: Paracalanus parvus parvus, Pseudodiaptomus aurivillii, Temora stylifera, and Pseudodiaptomus serricaudatus. Canonical correspondence analysis and Spearman's correlation coefficients underlined that salinity, turbidity, conductivity, temperature, dissolved oxygen, chlorophyll a (Chl-a), and nutrients were the principal environmental variables strongly linked with mesozooplankton dynamics in Munroe Island. The highest abundance of mesozooplankton was recorded in MoN (monsoon), followed by PrM (pre-monsoon), and PoM (post-monsoon). Hierarchical clustering confirmed that the grouping of sampling stations is based on the estuarine and freshwater influences on mesozooplankton abundance. During the entire investigation, various ecological indices were observed in good condition. Moreover, the optimum environmental conditions during the PoM season are marked with the highest indices values. Overall, multivariate investigations undoubtedly proved the suitability of mesozooplankton communities as potential bioindicators for spatial and seasonal ecological assessments. Our investigation emphasizes the high assemblages of mesozooplankton and their responses to various environmental variables and highlights the significance of long-term ecological monitoring in a threatened ecosystem like Munroe Island.

Responses of the mesozooplankton community to marine heatwaves: Challenges and solutions based on a long-term time series.

Marine heatwaves (MHWs) are extreme weather events that have major impacts on the structure and functioning of marine ecosystems worldwide. Due to anthropogenic climate change, the occurrence of MHWs is predicted to increase in future. There is already evidence linking MHWs with reductions in biodiversity and incidence of mass mortality events in coastal ecosystems. However, because MHWs are unpredictable, the quantification of their effects on communities is challenging. Here, we use the Helgoland Roads long-term time series (German Bight, North Sea), one of the richest marine time series in the world, and implement a modified before-after control-impact (BACI) design to evaluate MHW effect on mesozooplankton communities. Mesozooplankton play an essential role in connecting primary producers to higher trophic levels, and any changes in their community structure could have far-reaching impacts on the entire ecosystem. The responses of mesozooplankton community to MHWs in terms of community structure and densities occurred mainly in spring and autumn. Abundances of seven taxa, including some of the most abundant groups (e.g. copepods), were affected either positively or negatively in response to MHWs. In contrast, we observed no clear evidence of an impact of summer and winter MHWs; instead, the density of the most common taxa remained unchanged. Our results highlight the seasonally dependent impacts of MHWs on mesozooplankton communities and the challenges in evaluating those impacts. Long-term monitoring is an important contributor to the quantification of effects of MHWs on natural populations.

Hidden underlying mechanisms for changes in mesozooplankton communities: Transport and eddy driven changes

Mesozooplankton communities have been used extensively as reliable climate change indicators, mainly because of their rapid growth and sensitivity to environmental changes. This study explored the modifications in the taxonomic composition of the mesozooplankton community and the associated physical changes of transport-driven, eddy-driven, and marine heatwaves in the summers of the last 14 years (2009–2022) within the mixed layer of the Ulleung Basin in the East Sea/Japan Sea, where surface waters have rapidly warmed in recent decades. A slight increase was observed in the abundance of mesozooplankton from 2009 (3709 inds.m−3) to 2022 (4231 inds.m−3), with two notable peaks in 2015 (11,377 inds.m−3) and 2020 (11,184 inds.m−3), which was mainly attributed to the prevalence of Noctiluca scintillans. The first peak in 2015 showed thaliaceans to be the next dominant taxa, in which the southward direction of meandering in East Korea Warm Current (EKWC), presence of the Ulleung warm eddy, lower volume of the Western Channel (V-west) of the Korea Strait, and marine heatwaves (MHWs) did not occur. In contrast to the first peak, the second peak in 2020 showed Pyrocystis pseudonoctiluca to be the next dominant species, which may have been transported and advected by the strong V-west and eastward direction of the EKWC and the occurrence of MHWs that allowed the persistence of the subtropical species P. pseudonoctiluca. Overall, the significant increases in the second dominant mesozooplankton taxa appeared to be affected by physical changes, including transport or eddy-driven changes, along with the occurrence of strong V-west, the direction of the EKWC, and the occurrence of MHWs, which may synergistically influence the increase in the second dominant taxa during summer. This study highlights the complex interplay between notable variations in mesozooplankton communities and environmental factors, highlighting the potential consequences of different physical changes (transport-driven and eddy-driven) in this regional ocean.

Spatio-temporal variation of mesozooplankton in the northern coastal waters of Bay of Bengal

Mesozooplankton (MSP) are significant primary and secondary consumers in aquatic ecosystems that influence biogeochemical cycles and are considered important bioindicators of ecosystem functioning. Data on the spatio-temporal variability of the MSP population from the estuarine and coastal waters of northwestern Bay of Bengal (BoB) is scarce. The present study is the first detailed investigation on the spatio-temporal dynamics of MSP along the estuarine and coastal waters of northwestern BoB from two contrasting seasons over a period of three years based on univariate and multivariate statistical analyses. The crucial drivers for spatio-temporal variability in the MSP community were water temperature, salinity, dissolved oxygen (DO) concentration, nutrient (nitrate, phosphate, silicate) concentrations, and Chlorophyll (Chl) a concentration. The highest mean MSP abundance was obtained during summer monsoon (SM). The results of this study state that there is a significant seasonal difference in the hydrographical parameters that change the community dynamics of MSP inhabiting the estuarine and coastal waters of BoB. The MSP species assemblages during winter monsoon (WM) were governed by high salinity and DO levels while those during SM were associated with high water temperature, nutrients, and Chl a concentration. A clear seasonal shift from the gelatinous community in WM to the crustacean community in SM was documented. A total of 172 MSP species were identified during WM, and 200 MSP species during SM, with 37 species being exclusively detected during WM and 65 species being exclusively recorded during SM. The most abundant species during WM were Chrysaora caliparea, Cyanea nozakii, Sagitta bipunctata, Pleurobrachia pileus, and Acromitus flagellatus to name a few and those during SM consisted of Undinula vulgaris, Acartia (Odontacartia) spinicauda, Oithona simplex, Oithona similis, and Oncaea venusta. Copepoda was the most dominant taxa during all sampling seasons, while ichthyoplankton and meroplankton fauna were low during WM and comparatively higher during SM, indicating the top-down control of their population by gelatinous taxa. Biodiversity indices reflected well to excellent diversity patterns of MSP. In the current scenario of global warming and climate change, our research highlights the need for comprehensive long-term monitoring of estuarine and coastal waters of western BoB, which harbour a rich marine faunal diversity, to identify the bioindicators of seasonal variability among major MSP groups.

The impact of advection on a Subarctic fjord food web dominated by the copepod Calanus finmarchicus

Fjord and shelf food webs are frequently supplemented by the advection of external biomass, which in high-latitude seas often comes in the form of lipid-rich copepods that can support a wide range of fish species, including Northeast Arctic cod (Gadus morhua). A seasonal match or mismatch at the lower trophic levels (phytoplankton and zooplankton) is central in determining how much energy and biomass is available for higher trophic levels (fish). Here, we quantify the inflow of the copepod Calanus finmarchicus into the Vestfjorden fjord system using high-resolution measurements of ocean currents and zooplankton (laser optical plankton counter). We evaluate a spatio-temporal match/mismatch between the phytoplankton bloom and Calanus and assess the input of advected copeods at the lower trophic level fjord and shelf food web based on an integrative approach employing stable isotope analyses (C, N), fatty acid trophic marker analyses, and biovolume spectrum analyses. Our results suggest two different sources of the Calanus population in the fjord/shelf system: one fraction overwintered locally and started ascending early to feed on the phytoplankton bloom that peaked around April 11. The other fraction had only recently (end of April) been and still was being advected from the oceanic overwintering habitats. Ca. 119 g C/s of Calanus were advected into the fjord, comparable to the biomass of Calanus advected into an Arctic fjord, and the mesozooplankton community was dominated by the copepod. The fjord food web was tightly coupled between the phytoplankton spring bloom, the local part of the Calanus population (trophic level 1.8–2.4) and cod larvae (high levels of wax esters). On the shelf, our results suggest that the impact of advected Calanus in the food web is at its starting point (low trophic level, large difference of δ13C of POM and Calanus). We highlight important factors that can contribute to the successful spawning of Northeast Arctic cod: an extended phytoplankton bloom that can support both locally and advected Calanus, which in turn can supply the essential nauplii prey for first-feeding cod larvae.

Secondary production at the Barents Sea polar front in summer: contribution of different size classes of mesozooplankton

The Barents Sea polar front is characterized by high primary production following the retreat of the ice edge during spring. However, secondary production estimates of mesozooplankton across the front are scarce, despite being essential for understanding energy flow through the food web. We investigated mesozooplankton community composition and production across the Barents Sea polar front (75°-78°N) in June, covering both Atlantic and Arctic water masses with high spatial and taxonomic resolution. We highlight the contribution of small and large groups of mesozooplankton and estimate secondary production by comparing and evaluating 4 commonly used growth rate models. The zooplankton community composition and size distribution changed across the polar front. In the Atlantic region, Rotifera, Chaetognatha and Appendicularia were common, while copepods and their nauplii contributed most across the polar front and in Arctic water masses. Mesozooplankton secondary production took place mainly in the surface and was highest south of the front, declining towards Arctic waters. Considering production by copepods alone, highest values were found in the northern sector of the polar front and in the Arctic region. Young developmental stages (CI-CIV) of Calanus spp. and small-sized taxa contributed most to copepod production in Atlantic waters, while calanoid copepod nauplii contributed considerably to copepod production in Arctic waters. We emphasize that the production estimates were strongly influenced by the growth rate model and conclude that copepod secondary production in a summer situation with non-limiting food concentration was best described using a model that solely considers water temperature and copepod body weight.

Changes in Atlantic climatic regulation mechanisms that underlie mesozooplankton biomass loss in the northern Baltic Sea

The effects of climate-induced, long-term changes on mesozooplankton biomasses were studied based on monitoring data collected since 1966 in the northern Baltic Sea. We found that the biomasses of marine and brackish mesozooplankton had decreased significantly from 1966 to 2019, and a remarkable biomass and functional biodiversity loss took place in the mesozooplankton community. Our results put emphasis on the impact of two climate-driven regime shifts for the region's mesozooplankton community. The regime shifts took place in 1975 and 1976 and in 1989 and 1990, and they were the most important factor behind the abrupt biomass changes for marine mesozooplankton and total and marine Copepoda. Only the latter regime shift influenced the biomasses of brackish Copepoda, marine Cladocera, and total Rotifera. The decreasing length of the ice-cover period drove the decrease of the biomass of limnic Limnocalanus macrurus (Copepoda), while the winter North Atlantic Oscillation was behind biomass changes in the total and the brackish Cladocera. These findings may have important implications for planktivorous fish, such as Baltic herring, particularly in terms of their impact on commercial fishing.

Multi-year mesozooplankton flux trends in Kongsfjorden, Svalbard

AbstractWe conducted this study to investigate the relationship between environmental stressors and mesozooplankton fluxes in inner Kongsfjorden, Svalbard. The ongoing Arctic amplification, characterized by phenomena such as increased temperatures, glacial and watershed runoff, and diminishing ice cover, poses significant challenges to marine ecosystems. Our multi-year time-series analysis (2010–2018) of mesozooplankton, collected from a moored automatic sediment trap at approximately 87 m depth, aims to elucidate seasonal and interannual variations in fluxes within this Arctic fjord. We integrate meteorological, hydrological, and chemical datasets to assess their influence on zooplankton populations. Principal component analysis reveals the impact of seawater characteristics on mesozooplankton fluxes and composition, while two-way ANOVA highlights the role of seasonality in driving variations in our dataset. We observe a decrease in swimmer fluxes following the maxima mass flux event (from 2013 onwards), coupled with an increase in community diversity, possibly attributed to copepod decline and functional diversity. Notably, sub-Arctic boreal species such as Limacina retroversa have been detected in the sediment trap since 2016. Our continuous multi-year dataset captures the physical, chemical, and biological dynamics in this extreme environment. With Arctic amplification in Kongsfjorden and increasing submarine and watershed runoff, we anticipate significant shifts in mesozooplankton communities in the medium to long-term. This underscores the urgency for further research on their adaptation to changing environmental conditions and the potential introduction of alien species.

Spatial Distribution Pattern of the Mesozooplankton Community in Ross Sea Region Marine Protected Area (RSR MPA) during Summer

The Ross Sea region Marine Protected Area (RSR MPA) is one of the most productive regions in the Southern Ocean. Mesozooplankton intermediates the primary product to the higher predators, such as penguins and seals, in this ecosystem. In this study, the mesozooplankton community structure and spatial pattern in the RSR MPA in January were investigated by using 505 μm-mesh-size bongo net samples. As a result, 37 mesozooplankton taxa with a total mean abundance of 35.26 ind./m3, ranging from 2.94 to 139.17 ind./m3, were confirmed. Of the 37 taxa, 7 occupied almost 84% of the total abundance, with copepods being the main dominant taxa. As shown by our hierarchical analysis, the mesozooplankton community was divided into four groups, each associated with a specific geographical distribution. Group A was composed of stations around Terra Nova Bay and showed relatively low abundance. Group B included stations around the continental slope region. Group D was composed of the Ross Sea continental shelf stations, while group C consisted of stations geographically located between those of groups B and D. These four groups were influenced by various environmental factors, such as water temperature, salinity, and nutrients. In summary, the mesozooplankton community can be separated according to geographical pattern. This pattern is related to several environmental factors.

Seasonal variation in the mesozooplankton community structure and indicator species of the Yellow Sea

We investigated the mesozooplankton community structure of the Yellow Sea in four seasons, one season for each year during 2019–2022. Total abundance and biomass were highest in autumn and spring, respectively. The mesozooplankton community was dominated by small copepods such as Acartia, Paracalanus, and Oithona species throughout all seasons. Euphausiid larvae were particularly abundant in spring, whereas tunicates appeared only in autumn. In spring, summer, and winter, mesozooplankton were clustered into groups from coastal waters to the open ocean. However, in autumn, the mesozooplankton community was divided by latitude and indicator species analysis suggested the influence of a warm current in this season. A comparison of our findings with those of previous studies indicated no clear change in zooplankton species composition or its seasonality in the Yellow Sea within the past 30 years.

Romanian Black Sea Mesozooplankton’s Seasonal Dynamics and Distribution during 2013-2020

The analysis of the mesozooplankton community from the Romanian Black Sea waters revealed variations both from quantitative and qualitative point of view. A decrease in the number of identified taa according to the season was observed, with a maimum of 25 taa in the warm season and a minimum of 15 taa in the cold season. The nonfodder component represented by Noctiluca scintillans recorded the highest density and biomass values in the warm season, in the cold season being less represented, fact highlighted by Simper analysis. The mesozooplanktonic fodder component showed variations of density and biomass, copepods and the meroplanktonic component representing the bulk of the community in the warm season. In the warm season, the group of Cladocera’s recorded high density and biomass values, unlike in the cold season where they were very poorly represented. Other groups were better represented also in the warm season, showing a decrease in terms of abundance and biomass in the cold season.

Global mesozooplankton communities show lower connectivity in deep oceanic layers.

Mesozooplankton is a key component of the ocean, regulating global processes such as the carbon pump, and ensuring energy transfer from lower to higher trophic levels. Yet, knowledge on mesozooplankton diversity, distribution and connectivity at global scale is still fragmented. To fill this gap, we applied DNA metabarcoding to mesozooplankton samples collected during the Malaspina-2010 circumnavigation expedition across the Atlantic, Indian and Pacific oceans from the surface to bathypelagic depths. We highlight the still scarce knowledge on global mesozooplankton diversity and identify the Indian Ocean and the deep sea as the oceanic regions with the highest proportion of hidden diversity. We report no consistent alpha-diversity patterns for mesozooplankton at a global scale, neither across vertical nor horizontal gradients. However, beta-diversity analysis suggests horizontal and vertical structuring of mesozooplankton communities mostly attributed to turnover and reveals an increase in mesozooplankton beta-diversity with depth, indicating reduced connectivity at deeper layers. Additionally, we identify a water mass type-mediated structuring of mesozooplankton bathypelagic communities instead of an oceanic basin-mediated as observed at upper layers. This suggests limited dispersal at deep ocean layers, most likely due to weaker currents and lower mixing of water mass types, thus reinforcing the importance of oceanic currents and barriers to dispersal in shaping global plankton communities.

Mesozooplankton communities in deep-water areas of the Black Sea: are their composition and biomass regulated by the Ctenophore Mnemiopsis leidyi A. Agassiz, 1865?

The abundance, biomass, distribution and feeding patterns of the ctenophoreMnemiopsisleidyiA. Agassiz 1865 were studied in the deep-water Black Sea in October 2019. The biomass of the ctenophore in the open areas of the sea was 100–200 g m–2, the abundance was 16–38 ind. m–2. With the dominance of large individuals (≥30 mm) in the population, the activity of their reproduction remained extremely weak. The specific daily ration varied from ~4% to 13% of body carbon in adults and juveniles, respectively, and exceeded the minimum food requirements of the ctenophore. The daily consumption of three species of copepods (Acartiaspp.,Calanus euxinus,Oithona davisae) byM. leidyireached 4.5–11% of their biomass, while that of appendiculariansOikopleura(Vexillaria)dioica– 1.6%. The predatory pressure ofM. leidyion the entire prey zooplankton attained 2–4% of the biomass of organisms per day.

A space‐for‐time framework for forecasting the effects of ocean stratification on zooplankton vertical habitat use and trait composition

AbstractThe effects of environmental change on zooplankton communities, and more broadly, pelagic ecosystems are difficult to predict due to the high diversity of ecological strategies and complex interspecific interactions within the zooplankton. Trait‐based approaches can define zooplankton functional groups with distinct responses to environmental change. Analyses across multiple mesozooplankton groups can help identify key organizing traits. Here, we use the pronounced cross‐shore environmental gradient within the California Current Ecosystem in a space‐for‐time substitution to test potential effects of ocean warming and increased stratification on zooplankton communities. Along a horizontal gradient in sea‐surface temperature, water column stratification, and light attenuation, we test whether there are changes in zooplankton species composition, trait composition, and vertical habitat use. We employ DNA metabarcoding at two loci (18S‐V4 and COI) and digital ZooScan imaging of zooplankton sampled in a Lagrangian manner. We find that vertical distributions of many mesozooplankton taxa shift to deeper depths in the cross‐shore direction, and light attenuation is the strongest predictor of magnitude of change. Vertical habitat shifts vary among functional groups, with changes in vertical distribution most pronounced among carnivorous taxa. Herbivorous taxa remain associated with the chlorophyll maximum, especially in clear offshore waters. Our results suggest that increased stratification of this ocean region will lead to deeper depths occupied by some components of epipelagic mesozooplankton communities, and may result in zooplankton communities with more specialized feeding strategies, increased egg brooding, and more asexual reproduction.

Seasonal upwelling influence on trophic indices of mesozooplankton in a coastal food web estimated from δ15N in amino acids

Mesozooplankton communities in upwelling ecosystems are known to be influenced by seasonal upwelling enhancing primary productivity. However, the extent to which changes in trophic dynamics of mesozooplankton are driven by variations in the baseline of nitrogen or in the trophic position (TP) is poorly understood. We used nitrogen stable isotopes (δ15N) in bulk and in specific amino acids (CSIA-AA), as well as the taxonomic composition of mesozooplankton samples collected monthly during two full years, to investigate whether the upwelling intensity affects the isotopic N baseline and other trophic indices at the community level in the coastal system off NW Iberian Peninsula. We hypothesized that enhanced phytoplankton productivity during upwelling events would lead to a shorter and more efficient food web. Upwelling induced mesozooplankton herbivory (by increasing the abundance and biomass of omnivorous-herbivorous copepods and other taxa), resulting in a linear decrease of TPGlx (estimated using Glx = glutamic acid + glutamine, which mainly involves metazoan links) from downwelling to upwelling situations. In contrast, TPAla (estimated using alanine, which also reflects microbial trophic pathways) was highest in low-moderate upwelling and decreased with upwelling strength. Thus, an optimal coupling between metazoan and microbial food webs was reached at low-moderate upwelling intensities when the microbial contribution was ca. 25 %, while this contribution decreased (<15 %) either during downwelling or during strong upwelling. However, the significance of differences in mean values of TP and zooplankton composition was small due to the rapid succession of upwelling and downwelling events. Our results illustrate how a combination of CSIA-AA and community data can provide valuable information on rapid changes in mesozooplankton food web structure in highly dynamic upwelling systems.

Diel vertical migration of copepods in the tropical and subtropical Atlantic Ocean

The vertical distribution of the zooplankton community was analyzed from the surface to 800 m depth in the tropical-subtropical Atlantic Ocean in a transect of 12 stations during day and nighttime from Brazilian waters (10°S) to the Canary Islands (27°N). A depth stratified opening-closing 1 m2 mouth MOCNESS was used and 8 layers analyzed. Besides this net, the epipelagic layer (0–200 m) was sampled for microzooplankton (53 µm Calvet net) and mesozooplankton (200 µm WP2 net) communities. An increasing gradient of mesozooplankton abundance was observed from the oligotrophic waters off Brazil to the oceanic upwelling off Northwest Africa. Copepods were always the most abundant group (86 %) and 241 species identified, although only 60 were >1 %. Smaller species predominated and Oncaea was the dominant group (18 %) followed by Clausocalanus (14 %) with C. furcatus dominating. Paracalanus and Oithona were also quite abundant (12 and 9 % respectively). Small copepods such as C. furcatus and Euchaeta marina predominated in the oligotrophic waters while Calanus helgolandicus, Calanoides carinatus, Temora stylifera, Paracalanus parvus, and Pleuromamma robusta dominated in the upwelling area. In the epipelagic layer large copepods (e.g., Rhincalanus, Pleuromamma, Euchirella, and Metridia) were numerous during nighttime and in the mesopelagic zone during day were quantitatively less abundant. The vertical distribution of the dominant copepods (>1 %) showed multiple diel patterns but at least five were distinguished and their dominant taxa defined: (1) Species living in the twilight zone with slight up and down motion. (2) Species dominating the epipelagic layers with weak vertical movements. (3) Strong migrants living in the mesopelagic zone during day and moving upward towards the surface or (4) migrating to the subsurface layer at nighttime, and (5) those organisms characterizing the oceanic upwelling where the calanoids predominated (e.g., C. helgolandicus and C. carinatus). Our findings highlight the complexity in the diel vertical migration patterns of copepods in the tropical-subtropical Atlantic Ocean and the relevant role of the deep-sea copepods and their diel vertical migration. These results could be used as baseline information for low latitudes of the Atlantic Ocean in future studies of the biological carbon pump.

Atlantification influences zooplankton communities seasonally in the northern Barents Sea and Arctic Ocean

The Barents Sea is undergoing rapid ocean warming with less sea ice and increased Atlantic inflow, shifting the pelagic ecosystem towards a more boreal one, a process referred to as Atlantification. While such changes have already been documented in the southern and central Barents Sea, less is known about the degree of Atlantification in the northern Barents Sea and Arctic Ocean. In this seasonal study, we identified the mesozooplankton biodiversity, abundance and biomass in the Northern Barents Sea along a transect with seven stations stretching from the central Barents Sea (76°N) across the shelf break and into the Arctic Ocean (82°N) in August and December 2019, and March, May and July 2021. The broad range of mesozooplankton taxa and sizes were collected by conducting duplicate depth-stratified tows using alternate nets of mesh-sizes 64 µm and 180 µm. The majority of zooplankton taxa were ubiquitous in the study area, but the abundances and life stages varied depending on the season, region and the dominant water mass. We identified three distinct biogeographical regions with different zooplankton diversity and seasonal dynamics; (i) south of the Polar Front, (ii) northern Barents Sea shelf, and (iii) shelf slope and Arctic Ocean. During summer, high abundances of Atlantic/boreal and cosmopolitan zooplankton, mainly Calanus finmarchicus, Metridia longa, Oithona similis and Microsetella norvegica were found just south of the Polar Front in the central Barents Sea. On the shelf, Arctic species, such as Calanus glacialis, Pseudocalanus spp., and Limacina helicina dominated year-round with relatively high and stable biomass. At the northernmost stations, peaks of C. finmarchicus and Oncaeidae (Triconia borealis and Oncaea spp.) occurred in winter, combined with bathypelagic species such as Paraeuchaeta spp., Scaphocalanus brevicornis, Spinocalanus spp., Gaetanus brevispinus and Heterorhabdus norvegicus. Hence, when comparing the mesozooplankton communities at the different locations and seasons, four distinct communities were identified: shelf winter, shelf spring, shelf summer, and Arctic Ocean. Stronger advection and increased northward expansion of Atlantic zooplankton species are anticipated in the future, which could impact the diversity of the more endemic and energy-richer Arctic zooplankton communities.

Phototropic Behavioral Responses of Zooplankton in Lake Baikal In Situ and during the Anthropogenic Impact Modeling

Earlier, we showed that the registration of the behavioral responses of autochthonous mesozooplankton communities in situ is a more dynamic methodological approach in the biological assessment of the environmental well-being of aquatic ecosystems, as well as an alternative method to generally accepted tests on mortality and immobilization. The change in behavioral responses, including phototropic responses, may occur at lower concentrations of pollutants, leading to the inhibition of the risk-avoidance response of predatory fish attack and, ultimately, to the change in zooplankton abundance and biodiversity. The biological significance of such changes is quite high since zooplankters form the basis of food chains. This work studies the possibility of biomonitoring the quality of fresh water in Lake Baikal according to the state of the autochthonous mesozooplankton community in summer and winter using a digital holographic camera developed and tested by us in laboratory conditions. This method makes it possible to determine the concentration of plankters in the controlled volume of the DHC and perform photostimulation with different levels of illuminance. The depth profilometry of the phototropic response was compared with the profilometry of plankton concentration, intraspecific diversity of crustaceans according to the Pielou index, and the results of catching using the Juday net in the natural environment of the lake and during the modeling of the anthropogenic impact (introduction of table salt solution into the local area close to the registration probe). The circadian rhythm parameters were determined by the spectral analysis of the long-term registration of the phototropic response dynamics. It was noted that the inhibition of the phototropic response was the most adequate marker of the exogenous impact and the appearance of an alternating factor among the studied indicators of the state of the plankton community, namely, intraspecific diversity, synchronism of circadian rhythms, and response to paired photostimulation. The revealed patterns of behavioral responses of autochthonous zooplankton in natural and artificially modified conditions will allow for the implementation of long-term continuous control over the environmental well-being of water areas, including the collection ponds of treatment facilities, cooling ponds of nuclear power plants, and other water areas in contact with potentially hazardous facilities. The comparison of the identified patterns with the behavioral responses of euryhaline mesozooplankton will expand this method to assess the well-being of salt-water and marine reservoirs under the anthropogenic impact and will make it possible to create a continuous monitoring system.