Monika Winder

Marine alveolate parasites, particularly early-branching dinoflagellates of the order Syndiniales, play critical yet often overlooked roles in shaping marine microbial communities. These parasitoids infect diverse hosts, including dinoflagellates, copepods, and fish eggs, and rely on short-lived, free-living dinospores for transmission. Despite their ecological significance, the distributions of Syndiniales dinospores and host-associated life stages across environmental gradients remain poorly understood. We used 18S rRNA gene region DNA metabarcoding combined with size-fractionated water filtration across vertical and horizontal gradients of salinity, oxygen, and nutrients in the Baltic Sea-Skagerrak system to characterize Syndiniales life-stage distributions and identify clades producing dinospores. This approach enables the differentiation of free-living dinospores and host-associated life stages. Most reads were assigned to Syndiniales groups I and II, indicating the presence of both host associations and dinospore presence. Dinospore communities were more diverse than host-associated life stages and showed variation in spatial distribution and community composition. The spatial variation was related to salinity, oxygen, and nitrogen concentrations, emphasizing the role of environmental conditions in shaping niches suitable for host-parasite associations and infection transmission. Our findings highlight the prevalence of Syndiniales communities across an environmental gradient and the influence of environmental conditions on their distribution patterns.

Syndiniales are a group of parasitic dinoflagellates belonging to marine alveolates that infect a wide range of planktonic taxa, including other protists and metazoans. Sequence-based correlation networks have revealed that although Syndiniales account for the majority of links associated with mesozooplankton. However, these studies have not determined whether these parasite–zooplankton interactions result from overlapping environmental niches, direct infection, or indirect transmission via intake of infected prey items. To address these open questions, we identified associations between Syndiniales and zooplankton from selected copepod and cladoceran taxa using DNA metabarcoding. We further identified the role of prey consumption and environmental factors shaping Syndiniales–zooplankton associations across a brackish-to-marine environmental gradient. Our results show that Syndiniales are highly associated with zooplankton, but their association varies and is shaped by diet and environmental conditions. Syndiniales groups I and II were associated with all zooplankton species studied. The transmission to zooplankton likely occurs through secondary intake of infected prey, although direct ingestion of dinospores cannot be excluded. In contrast, Syndiniales group IV showed more specific associations, with copepods being the most likely final hosts given the high read abundance detected in copepods and previously described parasitic associations with crustaceans. Low oxygen concentration enhanced associations between Syndiniales group IV and copepods, whereas groups I and II showed the opposite pattern, indicating that environmental variables create distinct ecological niches for both the hosts and the parasites, driving niche-specific associations. These findings suggest that parasitic Syndiniales play a key role in planktonic food webs by redirecting carbon cycling.

Small pelagic fish have a central position in pelagic food webs, linking plankton production to higher trophic levels. They often favour crustacean mesozooplankton and are thought to compete for the same resource, an assumption that relies on microscopy diet identification that neglects digested and soft-bodied prey. Here, we aimed to systematically identify the entire resource use and overlap among the dominant small pelagic fish in the central Baltic Sea at high taxonomy resolution. The diet composition of two clupeid species, herring (Clupea harengus) and sprat (Sprattus sprattus), and the three-spined stickleback (Gasterosteus aculeatus) was assessed in May and October using DNA metabarcoding, stable isotopes, and microscopy. All three methods gave consistent results. The clupeids shared a similar diet in May when prey diversity was low, composed mainly of the copepods Pseudocalanus and Acartia, whereas three-spined stickleback favoured different copepod species and the rotifer Synchaeta, which was confirmed by a different isotopic value as compared to the two clupeids. In October, all forage fish preyed on diverse zooplankton species, mainly composed of the copepods Acartia, Eurytemora, and Temora, while Pseudocalanus was only important for herring. The observed resource use partitioning between sprat and herring was confirmed by the stable isotope values from October, suggesting that different prey species were targeted during the summer period. Our study highlights that resource use overlaps among small pelagic fish were limited and varied with prey availability. This suggests that shifts in zooplankton dynamics, rather than competition for resources, have the potential to drive small pelagic fish population fluctuations.

Environmental changes reshape biological communities, inducing cascading effects throughout the food webs. These changes pressure species either to adapt or to track favorable habitats. Estuaries represent an interesting case study to investigate such responses as species will rapidly reach physical boundaries if they cannot adapt fast enough and need to track suitable conditions. One such estuary is the Baltic Sea, characterized by a salinity and temperature gradient that shapes species distribution and imposes physiological stress on organisms. The Baltic Sea is projected to be affected by substantial modifications in environmental conditions by the end of the 21^st century, which could have major consequences for species distribution and community composition. However, despite the impending changes and their potential impact, there is a gap in understanding the potential consequences on pelagic species of the Baltic Sea. This study employs long‐term observations of primary zooplankton species in the pelagic food web to model changes in their distribution under future climate projections. We found that the parameters having the largest influence on habitat suitability varied across species, although maximal temperature was the most important for six out of seven species. In addition, there was a shrinkage of suitable area for several key species driven by a decrease in salinity and a rise in water temperature. We discuss the complex interplay between environmental changes and the spatial distribution of pelagic species in the Baltic Sea, highlighting the need for proactive management strategies to mitigate potential ecological impacts in the face of future climate scenarios.

Seagrass habitats play a major role in fisheries productivity through nursery functions and feeding grounds for diverse fish species. However, little is known about the seasonal distribution of fish larvae at large spatial scales in coastal East Africa. We investigated drivers of the seasonal fish larvae abundance and composition in seagrass habitats in Kenya and Tanzania. We found a high diversity of fish larvae (54 families) inhabiting seagrass habitats that differed between sites and seasons. Fish larvae abundance were highest in Kenya, particularly during the northeast monsoon season. Overall, total larval abundances per site were low, reaching less than 190 individuals/100 m³ in Kenya and less than 40 individuals/100 m³ in Tanzania, likely related to the low productivity and strong hydrodynamic processes in this region. Our data suggests that most of the fish spawn year-round in these tropical waters as we did not find strong seasonal patterns. All sites had a high relative abundance of larvae from demersal spawning fishes, indicating that many fish species move to coastal sites for spawning. Primary productivity and dissolved oxygen, driven by hydrodynamics conditions are positively related to fish larvae productivity both in Kenya and Tanzania. These findings indicate that the occurrence of both resident and transient fish larvae in seagrass meadows is driven by strong hydrodynamic and tidal processes that transport fish larvae across adjacent habitats.

Understanding species phenology and temporal co‐occurrence across trophic levels is essential to assess anthropogenic impacts on ecological interactions. We analyzed 15 yr of monitoring data to identify trends and drivers of timing and magnitude of bloom‐forming phytoplankton and diverse zooplankton taxa in the central Baltic Sea. We show that the timings of phytoplankton blooms advance, whereas crustacean zooplankton seasonal timings remain constant. This increasing offset with the spring bloom is linked to the decline of Pseudocalanus, a key copepod sustaining pelagic fish production. The majority of copepod and cladoceran taxa, however, are co‐occurring with summer blooms. We also find new developing fall blooms, fueling secondary production later in the season. Our study highlights that response to climate change differs within and between functional groups, stressing the importance of investigating plankton phenologies over the entire annual cycle in pelagic systems.

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.

In highly seasonal systems, the emergence of planktonic resting stages from the sediment is a key driver for bloom timing and plankton community composition. The termination of the resting phase is often linked to environmental cues, but the extent to which recruitment of resting stages is affected by climate change remains largely unknown for coastal environments. Here we investigate phyto- and zooplankton recruitment from oxic sediments in the Baltic Sea in a controlled experiment under proposed temperature and light increase during the spring and summer. We find that emergence of resting stage differs between seasons and the abiotic environment. Phytoplankton recruitment from resting stages were high in spring with significantly higher emergence rates at increased temperature and light levels for dinoflagellate and cyanobacteria than for diatoms, which had highest emergence under cold and dark conditions. In comparison, hatching of copepod nauplii was not affected by increased temperature and light levels. These results show that activation of plankton resting stages are affected to different degrees by increasing temperature and light levels, indicating that climate change affects plankton dynamics through processes related to resting stage termination with potential consequences for bloom timing, community composition and trophic mismatch.