Ecological niche dynamic, lessons from plankton

Abstract: 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.