The ecological role, bloom extent and long-term dynamics of jellyfishes are mostly overlooked due to sampling limitations, leading to the lack of continuous long-term datasets. A rise in frequency and magnitude of jellyfish invasion around the world is shedding new light on these organisms. In this study, we estimate the current and future distribution of the introduced jellyfish Blackfordia virginica in the Baltic Sea. We determine the combination of favorable levels of temperature and salinity for this species by analyzing presence/absence data from areas outside the Baltic Sea and project the distribution of suitable habitat in the Baltic Sea across different scenarios with variable climate forcing and eutrophication levels. Our results show that suitability increases with rising temperature and optimal salinity range from 13 to 20 for this species. In addition, a relatively large area of the Baltic Sea represents favorable abiotic conditions for B. virginica, enhancing the concerns on its potential range expansion. Spatial analysis illustrates that the coastal areas of the southern Baltic Sea are particularly at risk for the invasion of the species. The observation of the projection of habitat suitability across time highlights that future Baltic Sea environmental conditions increase suitability levels for B. virginica and suggest a potential expansion of its distribution in the future.

The introduction of non-native fish species into new environments has raised global concerns due to potential ecological impacts on recipient ecosystems. A previous study focusing on the introduced fish species Arapaima gigas in Bolivian Amazon waters showed that its isotopic niche significantly overlapped with most co-occurring native fish species, suggesting potential competition. To evaluate this hypothesis, we extended here the investigation by comparing the trophic position and isotopic niche width of eleven abundant native fish species inhabiting both colonized and non-colonized floodplain lakes. We found lower trophic positions in colonized versus non-colonized lakes only for native piscivores, mostly driven by a shift towards increased dietary proportion of detritivorous fishes. Conversely, results showed that the isotopic niche width of most fish species analyzed (i.e. 10 over 11 species) did not significantly decrease in colonized compared to non-colonized lakes. Our overall results suggest potentially low competitive interactions between A. gigas and native fishes, with the notable exception of piscivorous species. We attribute our findings to the high abundance of available resources in Amazon oxbow lakes.

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.

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.

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.

Havsförsurningen väntas de kommande decennierna bli påtaglig även i Östersjön. För det redan hårt pressade ekosystemet innebär det ytterligare en belastning. Den samlade effekten av försurningen och annan miljöpåverkan kan stressa arter och minska den biologiska mångfalden. För att skydda den unika miljön och den framtida matproduktionen krävs både kraftigt minskade koldioxidutsläpp och åtgärder mot övergödning, överfiske och utsläpp av farliga ämnen.

In the coming decades, ocean acidification is expected to become significant also in the Baltic Sea. For an already stressed ecosystem, it represents an additional pressure, and the cumulative effect of this and other environmental impacts can stress species and reduce biodiversity. Protecting the unique environment and future food production requires both significant reductions in carbon dioxide emissions and measures against eutrophication, overfishing and emissions of hazardous substances.

Havsförsurningen väntas de kommande decennierna bli påtaglig även i Östersjön. För det redan hårt pressade ekosystemet innebär det ytterligare en belastning. Den samlade effekten av försurningen och annan miljöpåverkan kan stressa arter och minska den biologiska mångfalden. För att skydda den unika miljön och den framtida matproduktionen krävs både kraftigt minskade koldioxidutsläpp och åtgärder mot övergödning, överfiske och utsläpp av farliga ämnen.

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.

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 21st 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.