Opportunistic vs selective feeding strategies of zooplankton under changing environmental conditions

Mercury dynamics at the base of the pelagic food web of the Gulf of Gdańsk, southern Baltic Sea

Planktonic organisms, which have direct contact with water, serve as the entry point for mercury (Hg), into the marine food web, impacting its levels in higher organisms, including fish, mammals, and humans who consume seafood. This study provides insights into the distribution and behavior of Hg within the Baltic Sea, specifically the Gulf of Gdańsk, focusing on pelagic primary producers and consumers. Phytoplankton Hg levels were primarily influenced by its concentrations in water, while Hg concentrations in zooplankton resulted from dietary exposure through suspended particulate matter and phytoplankton consumption. Hg uptake by planktonic organisms, particularly phytoplankton, was highly efficient, with Hg concentrations four orders of magnitude higher than those in the surrounding water. However, unlike biomagnification of Hg between SPM and zooplankton, biomagnification between zooplankton and phytoplankton was not apparent, likely due to the low trophic position and small size of primary consumers, high Hg elimination rates, and limited absorption.

Dynamics of Gut Bacteria Across Different Zooplankton Genera in the Baltic Sea

In aquatic ecosystems, zooplankton-associated bacteria potentially have a great impact on the structure of ecosystems and trophic networks by providing various metabolic pathways and altering the ecological niche of host species. To understand the composition and drivers of zooplankton gut microbiota, we investigated the associated microbial communities of four zooplankton genera from different seasons in the Baltic Sea using the 16S rRNA gene. Among the 143 ASVs (amplified sequence variants) observed belonging to heterotrophic bacteria, 28 ASVs were shared across all zooplankton hosts over the season, and these shared core ASVs represented more than 25% and up to 60% of relative abundance in zooplankton hosts but were present at low relative abundance in the filtered water. Zooplankton host identity had stronger effects on bacterial composition than seasonal variation, with the composition of gut bacterial communities showing host-specific clustering patterns. Although bacterial compositions and dominating core bacteria were different between zooplankton hosts, higher gut bacteria diversity and more bacteria contributing to the temporal variation were found in Temora and Pseudocalanus, compared to Acartia and Synchaeta. Diet diatom and filamentous cyanobacteria negatively correlated with gut bacteria diversity, but the difference in diet composition did not explain the dissimilarity of gut bacteria composition, suggesting a general effect of diet on the inner conditions in the zooplankton gut. Synchaeta maintained high stability of gut bacterial communities with unexpectedly low bacteria-bacteria interactions as compared to the copepods, indicating host-specific regulation traits. Our results suggest that the patterns of gut bacteria dynamics are host-specific and the variability of gut bacteria is not only related to host taxonomy but also related to host behavior and life history traits.

Isotopic and biochemical trophic markers reveal the complexity of interactions at the base of pelagic food webs (Mediterranean sea)

To gain insight into the impact of bottom-up changes in the plankton community on planktivorous fish in the context of the decline of small pelagic fisheries in the Northwestern Mediterranean Sea, we have conducted an extensive year-long study. The investigation combined biochemical analyses (proteins, carbohydrates, and lipids) with C and N stable isotope analyses (SIA) to simultaneously study phytoplankton, zooplankton, and eight planktivorous fish species (Engraulis encrasicolus, Sardina pilchardus, Sardinella aurita, Sprattus sprattus, Cepola macrophthalma, Chromis chromis, Boops boops, and Spicara maena). This study is the first to analyze both stable isotope and biochemical compositions in coastal particulate organic matter (POM) size classes (0.7-2.7 μm, 2.7-20 μm, and 20-200 μm), zooplankton size classes (200-300 μm, 300-500 μm, 500-1000 μm, 1000-2000 μm, and >2000 μm), and taxonomic groups. We demonstrated that: (1) POM stable isotope compositions varied based on its spatial origin, the taxonomic composition of its biota, and its biochemical content; (2) δ15N values increased with zooplankton size classes and groups, indicating different trophic levels; (3) Phytoplankton exhibited a lipid-rich composition (∼55%), while zooplankton and fish muscles were protein-rich (∼61% and ∼66%, respectively). Bayesian stable isotope mixing models revealed that, on average: (1) POM from oceanic waters contributed the most to the POM in the bay (>51%), with a dominance of pico-POM (∼43%); (2) The 200-1000 μm zooplankton primarily consumed nano-POM, the 1000-2000 μm zooplankton mostly consumed micro-POM (∼64%), and the >2000 μm zooplankton also mostly consumed micro-POM; (3) Mesozooplankton (200-2000 μm) constituted the main portion (∼42%) of the diet for planktivorous fish species, while macrozooplankton organisms (>2000 μm) were the primary food resource (∼43%) for both B. boops and S. sprattus. Our study underscores the complexity of the pelagic food web and highlights the bottom-up transfer of organic matter from the smallest phytoplankton size fractions to planktivorous fish.