Chiara D'Agata: Perspectives on coastal vegetated habitats: Integrating ecological dynamics, historical use, and fishers’ ecological knowledge

Abstract

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.