Aquatic vegetation in coastal ecosystems : The role of biotic interactions and environmental change for ecosystem functions and resilience in the Baltic Sea

Coastal ecosystems are among the most productive on Earth but subjected to many human pressures. In shallow coastal areas, aquatic vegetation constitutes foundation species that sustain secondary production and act as a nutrient filter, which may buffer human impacts. But little is known about how anthropogenic factors alter biotic interactions in aquatic vegetation, and how these changes affect ecosystem functions and resilience. The aim of this thesis was to investigate how natural and anthropogenic factors alter aquatic vegetation communities and biotic interactions, and how these in turn affect ecosystem functions and resilience to common stressors. Shallow coastal bays in the Baltic Sea were used as model system. A large field survey was conducted to investigate effects of natural and anthropogenic gradients, including bay topographic openness and nutrient runoff, on vegetation communities and ecosystem functions. Results suggest that high vegetation cover can improve water clarity, whereas sediment-driven turbidity can negatively affect vegetation by decreasing the light penetration of the water (Paper I). This dual relationship indicates the potential for two alternative, self-sustaining states in shallow bays; with or without vegetation. Using data from the same survey I investigated the influence of species richness and cover of rooted aquatic vegetation and drift wrack (Fucus vesiculosus), for ecosystem multifunctionality (MF) (Paper II). MF was estimated as the mean of four variables used as proxies for key functions; large predatory fish recruitment, grazer biomass, inverted ‘nuisance’ algal biomass and water clarity. MF was highest when the two functionally different vegetation types (rooted and drifting) co-occurred at high covers, and high species richness increased multifunctionality by increasing rooted vegetation cover. To understand in greater detail if and how interactions within and between vegetation species mediate the effects of environmental change, I conducted two experiments. First, a cage experiment to test if intraspecific plant facilitation may buffer effects of altered top-down and bottom-up control (Paper III), then a mesocosm experiment to test if shading alters interspecific interactions between three common plant species (Paper IV). The cage experiment showed that high shoot density of a common plant (Myriophyllum spicatum) increased individual shoot performance, but only when subjected to both fertilization and large predatory fish exclusion (Paper III). The mesocosm experiment showed that individual species’ traits had stronger effect than shading on interspecific competition and community yield (Paper IV). In conclusion, my thesis shows that single and multiple ecosystem functions benefit from high vegetation cover, with direct and indirect effects of diversity, but are sensitive to anthropogenic stressors (Papers I, II). Further, shading alters biotic interactions among vegetation species in a eutrophic coastal ecosystem by increasing the competitive advantage of dominant species (Paper IV), while intraspecific facilitation increases resilience to interacting stressors (Paper III). Together, the results highlight the need for ecosystem-based management where efforts to reduce anthropogenic influence (e.g. by nutrient reduction and fishing restrictions) are combined with improved protection and restoration of the ecologically and economically valuable aquatic vegetation communities