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.

To predict the spatial responses of biodiversity to climate change, studies typically rely on species-specific approaches, such as species distribution models. In this study, we propose an alternative methodology that investigates the collective response of species groups by modelling biogeographical regions. Biogeographical regions are areas defined by homogeneous species compositions and separated by barriers to dispersal. When climate acts as such a barrier, species within the same region are expected to respond similar to changing climatic conditions, enabling the prediction of entire region shifts in response to future climate scenarios. We applied this approach to the Southern Ocean, which exhibits sharp climatic transitions known as oceanic fronts, focusing on the mesopelagic lanternfishes (family Myctophidae). We compiled occurrence data for 115 lanternfish species from 1950 onwards and employed a network-based analysis to identify two major biogeographical regions: a southern and a subtropical region. These regions were found to be distinct, with minimal overlap in species distributions along the temperature gradient and a separation around 8°C, indicating that temperature likely acts as a climatic barrier. Using an ensemble modelling approach, we projected the response of these regions to future temperature changes under various climate scenarios. Our results suggest a circumpolar expansion of the subtropical region and a contraction of the southern region, with the Southern Ocean becoming a cul-de-sac for southern species. Ultimately, our results suggest that when support is found for the climatic barrier hypothesis, community-level models from a ‘group first, then predict’ strategy may effectively predict future shifts in species assemblages.

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.

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.

Models that estimate rates of energy flow in complex food webs often fail to account for species-specific prey selectivity of diverse consumer guilds. While DNA metabarcoding is increasingly used for dietary studies, methodological biases have limited its application for food web modeling. Here, we used data from dietary metabarcoding studies of zooplankton to calculate prey selectivity indices and assess energy fluxes in a pelagic resource-consumer network. We show that food web dynamics are influenced by prey selectivity and temporal match-mismatch in growth cycles and that cyanobacteria are the main source of primary production in the investigated coastal pelagic food web. The latter challenges the common assumption that cyanobacteria are not supporting food web productivity, a result that is increasingly relevant as global warming promotes cyanobacteria dominance. While this study provides a method for how DNA metabarcoding can be used to quantify energy fluxes in a marine food web, the approach presented here can easily be extended to other ecosystems. 

Abiotic conditions shape biological communities around the globe. Through spatial and temporal heterogeneity, environments impact plankton physiology, phenology and distribution. Understanding the dynamic relationships between biotic and abiotic components is essential to assess the resistance and resilience of biological communities to environmental changes. In an intricate interaction network, plankton organisms are channeling energy to higher trophic levels. However, the relative importance of the different energy sources and trophic pathways within the pelagic food web remains to be quantified. A better comprehension of the ecological niches of the main plankton species, and the influence of abiotic conditions on their interaction network is needed to enhance our understanding of fluxes and predicting the response of oceans to environmental change. 

In this thesis, we employed diverse approaches to explore the influence of environmental conditions on feeding interactions and spatial distribution of Baltic Sea plankton species. Using metabarcoding tools, we show the broad trophic niche of mesozooplankton species and shed light on the diet variability across species belonging to a similar size class. Additionally, we observed for some species, the ability to change trophic behavior on a spatial and temporal scales. These new insights were incorporated in flux models to quantify energy pathways, which revealed the essential role of cyanobacteria in supporting the pelagic food web. However, biotic interactions are sensitive to abiotic conditions, therefore, expected environmental changes could lead to modifications in the marine network. We assessed the influence of changes in abiotic parameters on the spatial and seasonal distribution of key Baltic Sea species by projecting their suitable habitat areas in both current and future conditions. In this doctoral project, we unveil the future loss in habitat suitability of several important zooplankton species of the Baltic Sea food web, potentially leading to cascading effects. In addition, we mapped the distribution of suitable habitats of the newly introduced Cnidarian species Blackfordia virginica to the Baltic Sea. We show that a notable proportion of coastal areas present favorable levels of environmental parameters for the growth of this species, which could alter the pelagic communities in these regions.

Overall, this thesis refines our comprehension of the trophic interactions, illustrates the role of cyanobacteria in the Baltic Sea and projects potential modifications in zooplanktonic communities, through changes in habitat suitability levels and invasion, due to expected changes in abiotic conditions.

The plankton community consists of diverse interacting species. The estimation of species interactions in nature is challenging. There is limited knowledge on how plankton interactions are influenced by environmental conditions because of limited understanding of zooplankton feeding strategies and factors affecting trophic interactions. In this study, we used DNA-metabarcoding to investigate trophic interactions in mesozooplankton predators and the influence of prey availability on their feeding behavior. We found that mesozooplankton feeding strategies vary within species across an environmental gradient. Some species, such as Temora longicornis consistently used a selective strategy, while diets of Centropages hamatus and Acartia spp. varied between stations, showing a trophic plasticity with the prey community. We found a dominance of Synechococcales reads in Temora’s gut content and a high prey diversity for the cladoceran Evadne nordmanni. Our study shows the wide range of prey species that supports mesozooplankton community and helps to understand the spatial and temporal complexity of plankton species interactions and discriminate the selectivity ability of four zooplankton key species. Due to the central role of plankton in marine waters, a better comprehension of the spatiotemporal variability in species interactions helps to estimate fluxes to benthic and pelagic predators. 

 Zooplankton and bacteria are two critical groups of aquatic organisms and their associations play important roles in contributing to ecological processes. However, the taxonomy patterns and dynamics of zooplankton-associated bacterial communities across different hosts over temporal and spatial gradients are seldom described in nature[KJ1] . Here, 16s rRNA sequencing and functional annotation were implemented on the bacterial communities of 12 zooplankton genera sampled across the Baltic Sea salinity gradient in two seasons. Our results suggest that functional grouping of the zooplankton-associated bacteria captures host and environment specific patterns better than bacteria taxonomic composition. The distribution of functional clusters of bacteria identified by K-medoid did not strictly follow host taxonomy, temporal and spatial gradients. But certain clusters, such as clusters of higher potential for unsaturated fatty acid synthesis showed host and temporal specificity. These specificities were further analyzed by random forest, suggesting that the dynamics of zooplankton-associated bacteria were related to environmental parameters such as temperature and phosphorus, and host diet composition. These results implied the co-effects of abiotic factors and biotic host lifestyles shaping the dynamics of zooplankton bacterial communities.

Estimating energy fluxes, or weight of trophic interactions, in food webs allows linking food web structure to properties of ecosystem functioning. Due to limitations in traditional methods, resolved food-web models are often binary or based on body size. Species-specific feeding traits and weights of interactions are typically not included, particularly at the base of the pelagic food web. Consequently, models are prone to underestimate the trophic diversity of species interactions.Here, we focus on trophic pathways of primary production in the Baltic Sea using a bioenergetic model that includes several trophic levels from primary producers to fish. For the first time, dietary DNA metabarcoding data of zooplankton are combined with accumulated biomass data from long-term pelagic monitoring and metabolic theory in a network model to describe energy pathways, including the diversity of planktonic organisms.We show that picocyanobacteria and filamentous cyanobacteria are the main contributors to secondary production in the Baltic Sea and that the latter experience high predation pressure from zooplankton. In contrast, the combination of high biomasses and low predation pressure on dinoflagellates and diatoms suggests that a significant fraction of the spring bloom is not utilized in the pelagic food web, explaining the high export of biological material to the sea seafloor.This study can be used to address ecosystem management objectives in the Baltic Sea under changing environmental conditions. Furthermore, the novel framework presented in this study, integrating DNA metabarcoding and biomonitoring data to assess energy fluxes, can be extended to dynamic food web modeling in other ecosystems.

 The marine alveolates consist of a group of protist parasitic dinoflagellates belonging to the order Syndiniales, which infect other planktonic taxa, including other dinoflagellates, copepods, and cladocerans. The ecological roles of Syndiniales linked to zooplankton remain poorly understood, though some Syndiniales Groups (SGs) are known to consistently kill their hosts. We identified the associated Syndiniales-zooplankton interactions of selected taxa using DNA metabarcoding across the environmental gradient of the Baltic Sea. We determined the abiotic drivers of these interactions and the infection pathways through the prey consumed by the hosts. We found variations in Amplicon Sequence Variants (ASVs) of different Syndiniales groups associated with zooplankton. The Syndiniales associations with zooplankton were driven highly by low-oxygen water conditions. The environmental variables create unique niches for both the hosts and the parasites, driving niche-specific associations. These interactions demonstrate species specificity shaped by host feeding behaviour and the surrounding abiotic environments. These findings indicate that niche-specific interactions occur along the environmental gradients of the Baltic Sea.