Zooplankton indicator-based assessment in relation to site location and abiotic factors: a case study from the Gulf of Riga.

There are a number of zooplankton parameters proposed as indicators to evaluate environmental status of marine ecosystems. Mean size and total stock (MSTS) is the only zooplankton-based and HELCOM (Baltic Marine Environment Protection Commission - Helsinki Commission)-approved core indicator. MSTS was developed to evaluate the environmental status of the Baltic Sea based on total biomass (or abundance) and mean body weight of mesozooplankton. This indicator reflects status of the food web and zooplankton biodiversity. Both are qualitative descriptors for determining good environmental status (GES) as defined by the EU Marine Strategy Framework Directive 2008/56/EC. However, the existing indicator concept is applicable to the extent that it characterizes off-shore pelagic habitats, while use of MSTS for coastal habitats remains challenging. In this case study, we aimed to assess and discuss performance of MSTS applied to mesozooplankton data from the shallow Gulf of Riga. Both off-shore and coastal communities were included in the study. MSTS responses to variable environmental factors (temperature, salinity and riverine runoff) were analysed. Temporal variations in temperature revealed response in mean size, whereas salinity covaried with total stock (both - biomass and abundance). However, spatial variations of MSTS parameters stayed unexplained. The results demonstrate difficulties with and provide possible solutions for MSTS-based assessment, with a particular emphasis on coastal waters. The use of mesozooplankton abundance as a determinant parameter for assessment of coastal waters and substitution of the corresponding 99% confidence interval of the mean as an assessment value instead of the mean have been considered.

Stratification, nitrogen fixation, and cyanobacterial bloom stage regulate the planktonic food web structure.

Changes in the complexity of planktonic food webs may be expected in future aquatic systems due to increases in sea surface temperature and an enhanced stratification of the water column. Under these conditions, the growth of unpalatable, filamentous, N2 -fixing cyanobacterial blooms, and their effect on planktonic food webs will become increasingly important. The planktonic food web structure in aquatic ecosystems at times of filamentous cyanobacterial blooms is currently unresolved, with discordant lines of evidence suggesting that herbivores dominate the mesozooplankton or that mesozooplankton organisms are mainly carnivorous. Here, we use a set of proxies derived from amino acid nitrogen stable isotopes from two mesozooplankton size fractions to identify changes in the nitrogen source and the planktonic food web structure across different microplankton communities. A transition from herbivory to carnivory in mesozooplankton between more eutrophic, near-coastal sites and more oligotrophic, offshore sites was accompanied by an increasing diversity of microplankton communities with aging filamentous cyanobacterial blooms. Our analyses of 124 biotic and abiotic variables using multivariate statistics confirmed salinity as a major driver for the biomass distribution of non-N2 -fixing microplankton species such as dinoflagellates. However, we provide strong evidence that stratification, N2 fixation, and the stage of the cyanobacterial blooms regulated much of the microplankton diversity and the mean trophic position and size of the metabolic nitrogen pool in mesozooplankton. Our empirical, macroscale data set consistently unifies contrasting results of the dominant feeding mode in mesozooplankton during blooms of unpalatable, filamentous, N2 -fixing cyanobacteria by identifying the at times important role of heterotrophic microbial food webs. Thus, carnivory, rather than herbivory, dominates in mesozooplankton during aging and decaying cyanobacterial blooms with hitherto uncharacterized consequences for the biogeochemical functions of mesozooplankton.

A regional algorithm to model mesozooplankton biomass along the southwestern Bay of Bengal.

A three-dimensional regression analysis attempted to model mesozooplankton (MSP) biomass using sea surface temperature (SST) and chlorophyll-a (Chl-a). The study was carried out from January 2014 to July 2015 in the southwestern Bay of Bengal (BoB) and sampling was carried out on board Sagar Manjusha and Sagar Purvi. SST ranged from 26.2 to 33.1 °C while Chl-a varied from 0.04 to 6.09 µg L-1. During the course of the study period, there was a weak correlation (r = 0.32) between SST and Chl-a statistically. MSP biomass varied from 0.42 to 9.63 mg C m-3 and inversely related with SST. Two kinds of approaches were adopted to develop the model by grouping seasonal datasets (four seasonal algorithms) and comprising all datasets (one annual algorithm). Among the four functions used (linear, paraboloid, the Lorentzian and the Gaussian functions), paraboloid model was best suited. The best seasonal and annual algorithms were applied in the synchronous MODIS-derived SST and Chl-a data to estimate the MSP biomass in the southwestern BoB. The modelled MSP biomass was validated with field MSP biomass and the result was statistically significant, showing maximum regression coefficient for the seasonal algorithms (R2 = 0.60; p = 0.627; α = 0.05), than the annual algorithm (R2 = 0.52; p = 0.015, α = 0.05).

Planktonic community structure during a harmful bloom of Phaeocystis globosa in a subtropical bay, with special reference to the ciliate assemblages.

Planktonic community structure was investigated during outbreak of harmful Phaeocystis globosa bloom in a subtropical bay, the Maowei Sea, South China Sea. The phytoplankton assemblage was numerically dominated by colonial P. globosa, with its abundance ranging from 1.23 × 10(8) to 11.12 × 10(8) cells m(-3) and contributing nearly 90 % to the total abundance. Totally 66 mesozooplankton (>169 µm) and 19 ciliates species were recorded, with the densities ranged from 169 to 1633 ind m(-3) and 74 to 1118 cells L(-1), respectively. The dominant species for mesozooplankton were Copepoda (larvae), Bestiola sinicus, B. amoyensis, Macrura (larvae) and Acartia spinicauda, respectively. The ciliate assemblage was numerically dominated by Codonella rapa, Strombidium globosaneum and Mesodinium rubrum. During the bloom, P. globosa seemed to be negatively affected by the nutrient phosphate significantly (p < 0.05). However, no correlation between P. globosa and ciliate assemblage was detected, but P. globosa was negatively correlated with total biomass of mesozooplankton and abundance of B. sinicus (p < 0.05), suggesting that P. globosa was uncoupled from the grazing by both ciliates and mesozooplankton when appearing as colonies form. On the other hand, both positive and negative correlations among the dominant groups of mesozooplankton and ciliates were observed (p < 0.05) which possibly indicated that the predation of mesozooplankton upon ciliates might be strengthened during the Phaeocystis bloom and the complex effect also varied from species to species.

Carotenoid metabolic profiling and transcriptome-genome mining reveal functional equivalence among blue-pigmented copepods and appendicularia.

The tropical oligotrophic oceanic areas are characterized by high water transparency and annual solar radiation. Under these conditions, a large number of phylogenetically diverse mesozooplankton species living in the surface waters (neuston) are found to be blue pigmented. In the present study, we focused on understanding the metabolic and genetic basis of the observed blue phenotype functional equivalence between the blue-pigmented organisms from the phylum Arthropoda, subclass Copepoda (Acartia fossae) and the phylum Chordata, class Appendicularia (Oikopleura dioica) in the Red Sea. Previous studies have shown that carotenoid-protein complexes are responsible for blue coloration in crustaceans. Therefore, we performed carotenoid metabolic profiling using both targeted and nontargeted (high-resolution mass spectrometry) approaches in four different blue-pigmented genera of copepods and one blue-pigmented species of appendicularia. Astaxanthin was found to be the principal carotenoid in all the species. The pathway analysis showed that all the species can synthesize astaxanthin from ß-carotene, ingested from dietary sources, via 3-hydroxyechinenone, canthaxanthin, zeaxanthin, adonirubin or adonixanthin. Further, using de novo assembled transcriptome of blue A. fossae (subclass Copepoda), we identified highly expressed homologous ß-carotene hydroxylase enzymes and putative carotenoid-binding proteins responsible for astaxanthin formation and the blue phenotype. In blue O. dioica (class Appendicularia), corresponding putative genes were identified from the reference genome. Collectively, our data provide molecular evidences for the bioconversion and accumulation of blue astaxanthin-protein complexes underpinning the observed ecological functional equivalence and adaptive convergence among neustonic mesozooplankton.

Multi-decadal range changes vs. thermal adaptation for north east Atlantic oceanic copepods in the face of climate change.

Populations may potentially respond to climate change in various ways including moving to new areas or alternatively staying where they are and adapting as conditions shift. Traditional laboratory and mesocosm experiments last days to weeks and thus only give a limited picture of thermal adaptation, whereas ocean warming occurring over decades allows the potential for selection of new strains better adapted to warmer conditions. Evidence for adaptation in natural systems is equivocal. We used a 50-year time series comprising of 117 056 samples in the NE Atlantic, to quantify the abundance and distribution of two particularly important and abundant members of the ocean plankton (copepods of the genus Calanus) that play a key trophic role for fisheries. Abundance of C. finmarchicus, a cold-water species, and C. helgolandicus, a warm-water species, were negatively and positively related to sea surface temperature (SST) respectively. However, the abundance vs. SST relationships for neither species changed over time in a manner consistent with thermal adaptation. Accompanying the lack of evidence for thermal adaptation there has been an unabated range contraction for C. finmarchicus and range expansion for C. helgolandicus. Our evidence suggests that thermal adaptation has not mitigated the impacts of ocean warming for dramatic range changes of these key species and points to continued dramatic climate induced changes in the biology of the oceans.

Mesozooplankton production and respiration across a trophic gradient in a coastal Eastern Mediterranean ecosystem.

The Inner Saronikos Gulf has served as an area of continuous environmental monitoring for many decades, primarily due to the operation of primary and secondary sewage treatment facilities of the Psyttalia Waste Water Treatment Plant (WWTP). In this groundbreaking study within the region, we conducted measurements of biochemical indices related to respiration (referred to as spETS) and growth (referred to as spAARS) of mesozooplankton. These measurements were then correlated with both biological and environmental data. The ecological quality of the inner Saronikos Gulf was ameliorated moving away from the sewage outfall and thus mesozooplankton sampling encompassed various sites located across a trophic gradient from the WWTP during different seasons (May, September, November 2017, and January 2018). Our observations revealed seasonal variations in both zooplankton abundance and composition, alongside spatial disparities in environmental data. In terms of biochemical data, we observed significant seasonal fluctuations in spETS and R respiration values. Notably, the lowest respiration R value (0.22 mg C m⁻³ d⁻1) was recorded in May, while the highest (4.74 mg C m⁻³ d⁻1) was observed in September. Nevertheless, the variations in spETS and spAARS values among different sites did not demonstrate statistically significant differences. Furthermore, the values of these indices (spETS and spAARS) exhibit a positive correlation with temperature and mesozooplankton biomass at a significance level of 0.01. Lastly, it is noteworthy that phytoplankton production, measured in terms of carbon content (mg C m⁻2), appeared to adequately meet the carbon requirements of the zooplankton throughout all the examined months.

Genotoxic effects of marine pollutants on coastal meso-zooplankton populations - A mini-review.

Meso-zooplankton plays a vital role in maintaining healthy marine ecosystems, and some of the taxa provide biological indications for the monitoring of environmental and climate change. Recently, several newly emerging stressors were shown to impact marine and coastal meso-zooplankton in some ways. Marine organisms' genomic core, tightly packed with high-level integrity, can be damaged by anthropogenic activities in coastal zones worldwide and impact their integrity. Genomic integrity loss leads to a cascade of effects on the destruction of the food chain sequences, from primary producers to higher invertebrates. Therefore, monitoring genomic integrity loss using ecotoxicological approaches that focus on genetic changes appears to be a suitable approach. A literature review shows that different stressors severely impact genomic integrity through DNA damage at different concentrations and exposure times. Contaminated sediments also strongly impact the genomic integrity of estuaries and adjacent coastal meso-zooplankton communities.

Mesozooplankton assemblage in the gulf of cádiz estuaries: Taxonomic and trait-based approaches.

In this study, two different approaches based on taxonomic assemblages and on copepod functional groups were used to investigate the mesozooplankton assemblage structure and its relationship with environmental variables in the main estuaries of the Gulf of Cádiz (Guadalquivir, Guadiana and Tinto-Odiel) during the dry-warm season. In general, the mesozooplankton assemblages were dominated by copepods, especially the calanoid Acartia tonsa, which reached its highest abundance in the inner zones while the adjacent coastal zones were characterized by a mixture of copepods and cladocerans, especially Penilia avirostris. Regarding the trait-based approach, three copepod functional groups were identified, principally sorted by their feeding strategy. Group 1 (composed of omnivorous copepods displaying a mixed feeding strategy and broadcast-spawners) was found mainly in the inner areas, while Groups 2 (omnivorous cyclopoids, sac-spawners that feed via active ambush) and 3 (herbivores-omnivores employing a filter feeding strategy and mostly broadcast-spawners) were predominant in the adjacent coastal zones. The relative abundance of copepod functional groups suggested that Group 1 could be considered the most important contributor to secondary production in the estuarine systems of the Gulf of Cádiz. In relation to environmental factors, salinity was the most influential variable on mesozooplankton assemblages in both approaches. Our results suggest that the studied estuaries, although taxonomically different, have mesozooplankton assemblages that perform similar ecological functions. Both methods provide valuable and complementary information about mesozooplankton assemblage dynamics in the main estuaries of the Gulf of Cádiz.

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.

Annual cycle of mesozooplankton at the coastal waters of Cyprus (Eastern Levantine basin).

This study is the first to explore monthly and seasonal succession of the zooplankton community in coastal waters of Cyprus using a 12-month period time series. A total of 192 taxa of mesozooplankton (MZ), 145 of which were copepods, were identified at three sites at the southern and one site at the northern coasts of the island. Zooplankton distribution and community structure were influenced mostly by stratification, temperature and Chl-a. The combination of upwelling and advection from the Rhodes Gyre during summer, causing cooler waters in the southern coast of Cyprus, seems to control the food supply and offered favorable feeding conditions to zooplankton, enhancing their numbers. The proximity to a fish farm also positively affected MZ abundance and biomass. This study also revealed the importance of smaller species (e.g. Clausocalanus paululus) and juvenile stages (e.g. Clausocalanus, Oithona and Corycaeus spp.) in composition, structure and functionality of the copepod community. These species seems to be more important in low Chl-a environments, where the relative size of primary consumers is expected to be smaller and the microbial components dominant. This baseline study paves the way for further investigation of the elements of marine food webs in the ultra-oligotrophic environment of the Eastern Mediterranean.

Metal and metallothionein concentrations in mesozooplankton from an oligotrophic offshore area in the eastern Mediterranean Sea (Cretan Passage/Levantine Sea).

Spatial variability of Cd, Cu, Cr, Ni, Zn, Fe, Mn, Pb and metallothionein (MT) concentrations were determined in mesozooplankton samples along the west-east axis of the Cretan Passage in the western Levantine Sea (Eastern Mediterranean). Metal and MT values from the present study are proposed as background levels, due to the lack of substantial anthropogenic activities in the area, where only maritime traffic and atmospheric deposition could be potential sources. Higher concentrations, of both metals and MTs, were recorded mainly at the western stations indicating higher metal bioavailability than in the eastern part. An inverse relationship of the metal zooplankton levels with zooplankton biomass, abundance and vital rates (production, respiration and ingestion rates), as well as salinity, was evident. We discuss the hypothesis that physical and biological characteristics of the marine environment, affecting growth dynamics of phytoplankton and zooplankton communities, may also act on metal uptake in oligotrophic marine systems.

Latitudinal transition of epipelagic mesozooplankton in the northwestern Pacific in winter.

The northwestern Pacific (NWP) is a hotspot of marine biodiversity study, and zooplankton is a crucial secondary producer in the marine ecosystem. It is of utmost importance to do extensive study on the distribution of zooplankton community in the NWP. The distribution of epipelagic zooplankton community in the 143-146°E section between the equator and 36°N in winter was examined in this study. The findings indicated that the Kuroshio extension, the North Pacific Subtropical Gyre, the North Equatorial Current and the North Equatorial Countercurrent were the four main ocean currents in the NWP that regulated the latitudinal transition of epipelagic mesozooplankton and split the sample section into four station groups. The key factors influencing zooplankton's geographic distribution were temperature, primary productivity, and current movement. In general, as latitude increased, zooplankton abundance and biomass first decreased and subsequently flourished in the NWP. Diversity indexes and indicator species also revealed the difference across the communities in four station groups. The body length of zooplankton declined gradually from north to south under the combined influence of temperature and primary production, with the minimum in the oligotrophic subtropical zone and a sub-peak in the high primary production area near the equator. Additionally, the abundance of zooplankton was higher in the winter than in the summer due to seasonal fluctuations in the properties of the ocean currents. This study elucidated the control effects of ocean currents on the latitudinal distribution of zooplankton, supplemented records of the geographic distribution and body length characteristics of zooplankton communities in the NWP, and provided the basis for further research on the ecological role of zooplankton communities and the global changes of marine ecosystems.

Mesozooplankton communities related to water masses in the Persian Gulf and the Gulf of Oman.

The spatial and temporal variations of mesozooplankton in relation to environmental parameters were investigated in the Persian Gulf and the Gulf of Oman. For this purpose, the physicochemical parameters were measured using a CTD probe and mesozooplankton were sampled vertically using a closing net aboard the RV Persian Gulf Explorer during two expeditions. Results showed a relationship between mesozooplankton assemblages and water mass distributions. Four mesozooplankton cluster station groups divided the sampled stations into masses, which was also confirmed by the PERMANOVA test. Overall, ninety taxa were identified that were dominated by copepods and characterized by small cyclopoids. Based on the results the mesozooplankton communities of the Persian Gulf and the Gulf of Oman are mostly shaped by the combined effect of temperature, salinity, and dissolved oxygen that significantly influences mesozooplankton occurrence during summer and spring.

Interannual changes in winter-spring zooplankton estuarine community forced by hydroclimatic variability - With special reference to bioindicator species Eurytemora americana.

Climatic variability and anthropogenic pressures impact the structure and dynamics of pelagic ecosystems and copepods are good indicators of such changes. This investigation aims to explore the interannual pattern of the mesozooplankton community, in relation to environmental variables in the Bahía Blanca Estuary during winter-spring from last two decades focusing on the dominant species Eurytemora americana. Main changes recorded include increased temperature, alteration of the nutrient balance, a decrease in chlorophyll-a, modifications in the abundance-structure of the phytoplankton assemblages, and changes in the abundance-structure of the mesozooplankton community. A significant decrease was observed in species richness of the mesozooplankton over time. Alterations in abundance and reproductive traits of E. americana, were also found. The population of E. americana dropped from mean relative abundance of 47% in first years to 20-12% in lasts one, accompanied with an increase of copepod species characterized by higher trophic plasticity in eutrophic conditions, like Acartia tonsa and Euterpina acutifrons.

Mesozooplankton size structure in the Canary Current System.

Changes in plankton composition influences the dynamics of marine food webs and carbon sinking rates. Understanding the core structure and function of the plankton distribution is of paramount importance to know their role in trophic transfer and efficiency. Here, we studied the zooplankton distribution, abundance, composition, and size spectra for the characterization of the community under different oceanographic conditions in the Canaries-African Transition Zone (C-ATZ). This region is a transition zone between the coastal upwelling and the open ocean showing a high variability because of the physical, chemical, and biological changes between eutrophic and oligotrophic conditions through the annual cycle. During the late winter bloom (LWB), chlorophyll a and primary production were higher compared to that of the stratified season (SS), especially in the upwelling influenced area. Abundance distribution analysis clustered stations into two main groups according to the season (productive versus stratified season), and one group sampled in the upwelling influenced area. Size-spectra analysis showed steeper slopes during daytime in the SS, suggesting a less structured community and a higher trophic efficiency during the LWB due to the favorable oceanographic conditions. We also observed a significant difference between day and nighttime size spectra due to community change during diel vertical migration. Cladocera were the key taxa differentiating an Upwelling-group, from a LWB- and SS-group. These two latter groups were differentiated by Salpidae and Appendicularia mainly. Data obtained in this study suggested that abundance composition might be useful when describing community taxonomic changes, while size-spectra gives an idea of the ecosystem structure, predatory interactions with higher trophic levels and shifts in size structure.

Chemical markers in marine food web: A simple workflow based on methyl tert-butyl ether extraction for fatty acids and stable isotopes assessment in plankton samples.

Fatty acids (FAs) are used, often in combination with stable isotopes (SIs), as chemical biomarkers to assess the contribution of different prey to the diet of consumers and define food web structure and dynamics. Extraction of lipids is traditionally carried out using methanol (MeOH) combined with chloroform or dichloromethane, these latter being well-known environmental pollutant and potential carcinogenic agents. Recently, extraction protocols based on methyl tert-butyl ether (MTBE) and MeOH have been proposed as an alternative to halogenated solvents in lipidomic studies. However, no specific investigation has been performed to assess MTBE suitability in marine ecological studies including FA analysis together with SI measurements. We used an analytical workflow for qualitative and quantitative analysis of FAs and SIs in field samples of phytoplankton, zooplankton and the scyphomedusa Pelagia noctiluca, applying MTBE in comparison with chloroform- and dichloromethane-based protocols for total lipid extraction. Our analysis suggested that MTBE is a reliable substitute for lipid extraction in trophic ecology studies in marine planktonic organisms.

Microplastics in copepods reflects the manmade flow restrictions in the Kochi backwaters, along the southwest coast of India.

This baseline study on microplastics (MPs) in calanoid copepods in the Kochi backwaters (KBW), India's largest estuary system on the west coast, focuses on (a) the spatiotemporal variations of MPs with the seasonal hydrography setting, and (b) how man-made flow restrictions of a large saltwater barrage contribute to MPs in copepods and their potential to transfer to higher trophic levels. This study found that MPs in copepods in the KBW ranged from av. 0.01 ± 0.014 to 0.11 ± 0.03 no./ind. seasonally. When the saltwater barrage shutters were fully/partially closed during the Pre-monsoon/Northeast Monsoon, MPs in copepods were considerably larger (av. 0.11 ± 0.03 no./ind., and av. 0.075 ± 0.02 no./ind., respectively) as compared to the Southwest Monsoon (av. 0.03 ± 0.01 no./ind.), when the barrage shutters were fully open. This shows the potential of man-made flow restrictions to increase the bioconcentration of MPs in copepods and their possible transfer to higher trophic levels through the food chain, adding to the region's previous discovery that much higher trophic level resources are polluted with a high concentration of MPs.

Mesozooplankton community in near-hypoxic and hypoxic layers of the Persian Gulf and the Gulf of Oman.

To assess the role of near-hypoxic and hypoxic conditions on the mesozooplankton community, we studied twenty stations in the Persian Gulf and the Gulf of Oman. The physicochemical parameters were measured using a CTD probe and mesozooplankton were sampled vertically using a closing net. Results showed the hypoxic conditions of 0.52 ± 0.11, 1.40 ± 0.48, and 0.84 ± 0.08 in the lower layers in stations T11S3, T0S8B, and T9S5 in the Gulf of Oman and near-hypoxic conditions in station T6S3 (2.99 ± 0.05) in the Persian Gulf. The marginal test revealed that stratification during summer and dissolved oxygen and salinity during spring in the Persian Gulf and temperature, stratification, and dissolved oxygen during spring in the Gulf of Oman significantly explained the variation of mesozooplankton composition in the study areas (p < 0.05). The present study reveals some general patterns of mesozooplankton assemblage in near-hypoxic and hypoxic conditions, but a future detailed investigation will be essential.

Year-round variation in the isotopic niche of Scopoli's shearwater (Calonectris diomedea) breeding in contrasting sea regions of the Mediterranean Sea.

Top marine predators are key components of marine food webs. Among them, long-distance migratory seabirds, which travel across different marine ecosystems over the year, may experience important year-round changes in terms of oceanographic conditions and availability of trophic resources. We tested whether this was the case in the Scopoli's shearwater (Calonectris diomedea), a trans-equatorial migrant and top predator, by sampling birds breeding in three environmentally different regions of the Mediterranean Sea. The analysis of positional data and stable isotopes (δ1³C and δ15N) of target feathers revealed that birds from the three regions were spatially segregated during the breeding period while they shared non-breeding areas in the Atlantic Ocean. Isotopic baseline levels of N and C (meso-zooplankton) were significantly different among marine regions during breeding. Such variation was reflected at the higher trophic levels of pelagic and demersal fish muscles as well as in shearwater feathers grown in the Mediterranean. δ15N- and δ13C-adjusted values of shearwaters were significantly different among populations suggesting that birds from different breeding areas relied on prey species from different trophic levels. Conversely, the non-breeding spatial and isotopic niches overlapped greatly among the three populations. Shearwater trophic niches during breeding were narrower and segregated compared to the non-breeding period, revealing a high plasticity in trophic resource use. Overall, this study highlights seasonal and region-specific use of trophic resources by Scopoli's shearwater, suggesting a broad trophic plasticity and possibly a high adaptability to environmental changes.