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Three-spined sticklebacks (Gasterosteus aculeatus) have become dominant in many Baltic Sea coastal fish assemblages, yet life-stage-specific habitat use remains poorly understood. We surveyed shallow water habitats along the coast of Gotland, Sweden, to examine how juvenile and adult stickleback abundances relate to variables such as submersed aquatic vegetation (SAV) biomass and vertical structure, macrophyte species richness, biomass of drift filamentous algae and potential invertebrate prey abundance. Juvenile stickleback abundance increased with SAV variables, macrophyte species richness, and drift filamentous algae, whereas, in contrast, adult abundance was only significantly associated with potential prey (invertebrate abundance). Our findings reveal life-stage-specific habitat associations of three-spined sticklebacks and highlights the importance of both long-living vegetation and ephemeral drift algae in supporting juvenile sticklebacks. This study contributes to a better understanding of sticklebacks' coastal ecology in the central Baltic Sea.

Coastal vegetated habitats, including submersed aquatic vegetation (SAV), provide three-dimensional structures essential to marine organisms and supporting key ecosystem services. In the Baltic Sea, eutrophication favours filamentous algae that can outcompete SAV, shifting habitats from long-living canopies to minute, ephemeral assemblages, affecting vegetation-associated fauna. Rising densities of three-spined stickleback (Gasterosteus aculeatus) in the central Baltic Sea have been linked to reinforcing filamentous algae dominance through predation on invertebrate grazers.

Ecological dynamics unfold within broader social-ecological contexts, where people and ecosystems influence one another, changing over time. This thesis asks how variation in coastal vegetation shapes faunal communities, and what historical human habitat use and fishers’ ecological knowledge can reveal about these dynamics. To address this, I used an interdisciplinary approach centred on Gotland, Sweden, (central Baltic Sea), with complementary insights from Salina, Italy (Tyrrhenian Sea). Five questions were examined: how SAV and filamentous algae shape epifaunal communities; how they affect juvenile and adult sticklebacks; which habitats were historically used on Gotland to capture euryhaline fish; how can engagement with fishers’ ecological knowledge be improved; and how integration of scientific disciplines advances understanding of coastal habitat dynamics? Methods included ecological surveys, isotope analysis of archaeological fish remains, and semi-structured interviews with fishers using photo elicitation. Results showed that SAV vertical structure increased overall epifauna abundance, whereas filamentous algae favoured only gastropod grazers (Paper I). Juvenile sticklebacks increased with all vegetation variables, while adults showed no associations (Paper II). These results suggest that shifts to filamentous algae could alter epifaunal communities and grazing dynamics, while providing nursery habitat for sticklebacks. Paper III reconstructed millennia of coastal and inland vegetated fishing habitats use, and demonstrated long-standing importance of these habitats for supporting humans. Paper IV showed that photo elicitation accesses explicit and tacit dimensions of fishers’ ecological knowledge, revealing rich understandings of habitat dynamics. A final reflection highlighted epistemological agility, effective communication, role clarity, and time allocation as key to interdisciplinary work. By linking present-day ecological patterns with historical human use and methods to engage with fishers’ knowledge, the thesis provides an integrated perspective to inform management and conservation of coastal vegetated habitats.

Submersed aquatic vegetation (SAV) provides essential habitat and food to numerous coastal invertebrate species. In the eutrophic Baltic Sea, fast-growing drifting algae form extensive mats that can negatively impact SAV. However, these mats also offer additional habitat and food to epifauna. The aim of this study was to assess the effects of SAV and filamentous mats on epifaunal communities in shallow soft-bottom habitats around Gotland, Sweden, in the central Baltic Sea. We used generalised linear models (GLMs) to evaluate the influence of SAV vertical structure, biomass and macrophyte species richness (including macroalgae) and filamentous mat biomass on epifaunal community properties as well as on those of key grazer species. Diversity, vertical structure and biomass of SAV were positively associated with higher total epifaunal abundance and greater abundance gastropod grazers. In contrast, filamentous mats only increased gastropod abundance and biomass. In addition to introducing a rapid tool for quantifying vegetation structural complexity, this study highlights the selective effects of different habitat types on invertebrate communities in a relatively understudied region of the Baltic Sea. As warming temperatures and eutrophication promote filamentous mat growth, reducing nutrient pollution and protecting SAV will be crucial for sustaining abundant and diverse epifaunal communities.

Multi-decadal monitoring data and archived biological samples make the rapidly changing Baltic Sea an ideal system for quantifying alterations in ecosystem function. The Baltic Sea blue mussel (Mytilus edulis trossulus) acts as a keystone species by providing food and habitat for other species, and through its filtering activity clearing the water and promoting organic matter and nutrient cycling between benthic and pelagic ecosystems. A decrease in biomass of blue mussels along with altered environmental conditions has been observed in the northern Baltic Sea over a 24-year period (1993-2016), but the size distribution of the mussels was unknown. The present study focuses on retrospective measurements of the individual shell lengths of these archived blue mussels. The mean shell length was found to be significantly lower at the end of the time series than in the beginning (11.1-11.5 mm for the years 1993-1996 compared to 10.3-10.9 mm during 2012-2015), with the proportion of larger mussels decreasing in the population. This 6% decrease in shell length translated into a significantly lower (16%) mean filtration capacity at the end of the time series compared to the beginning, influencing the ecosystem services blue mussels provide.