A new open-access study in Biological Diversity challenges a comforting assumption in Baltic coastal monitoring: that mesozooplankton communities are broadly uniform across space. Instead, researchers report sharp fine-scale differences among three brackish bays in the Central and Eastern Baltic Sea—differences so pronounced they emerge over sub-kilometre distances.
Led by Neele Schmidt and Amanda Adam Jansson at Uppsala University, the work links community turnover to local hydrography. Salinity and temperature, the authors argue, act as primary environmental filters that reshape which plankton taxa dominate at a given site, both between separate bays and within individual embayments.
Field sampling took place in June 2022 across Baggensfjärden, Kappelshamnsviken, and Tvären. At each bay, scientists deployed a 100 μm WP2 closing net to collect vertically integrated mesozooplankton samples (>200 μm), using three replicate stations to capture within-bay variability.
To anchor biological patterns to physical drivers, hydrographic profiles of temperature and salinity were recorded with multiparameter probes at the deepest point of each bay. In the laboratory, preserved samples were identified taxonomically and standardized as abundance per cubic metre, enabling quantitative comparisons across stations.
Statistical analyses used nonmetric multidimensional scaling (NMDS) with Bray–Curtis dissimilarity, alongside diversity metrics calculated in the vegan framework. This approach revealed community separation consistent with environmental gradients rather than simple spatial averaging.
Between-bay contrasts were especially clear. Kappelshamnsviken, the more marine bay with deep-water salinity reaching about 9, carried high densities of calanoid copepods such as Pseudocalanus sp. and Temora sp., and showed the highest Shannon–Wiener diversity and evenness.
By contrast, lower-salinity systems—Tvären and Baggensfjärden—were rotifer-dominated. Tvären recorded the highest total zooplankton abundance (46,241 ± 25,941 ind/m³) but also the lowest evenness, indicating that one taxon disproportionately structured the community.
Within individual bays, assemblages changed dramatically over less than 1 km. Deeper stations favored Pseudocalanus sp., while shallow zones were more likely to host cyclopoid copepods and branchiopods, suggesting that stratification and local mixing regimes create ecological microhabitats.
The results carry practical consequences for fisheries and conservation. If zooplankton communities are patchy at fine scales, then sparse sampling can miss key ecological niches and misrepresent ecosystem health signals, including responses to eutrophication and climate change.
To generalize across seasons, the authors call for multi-season sampling and broader tracking of environmental variables. Their findings align with the Essential Ocean Variable (EOV) perspective, emphasizing that coastal monitoring designs must be dense enough to resolve real biological heterogeneity.
Article Title: Between and Within Bay Differences in Mesozooplankton Assemblages in the Central and Eastern Baltic Sea: An Exploratory Study
News Publication Date: 17-Jul-2026
Web References: http://dx.doi.org/10.1002/bod2.70032
References: 10.1002/bod2.70032
Image Credits: Biological Diversity Editorial Office
Keywords: zooplankton, coastal zones, salinity, niche differentiation, environmental monitoring, community ecology, marine ecology, habitat diversity, plankton biodiversity
