Multi‐Decadal Warming in Sweden Drove Fish Community Thermophilization Through Species Turnover in Marine Coastal Waters, but Was Limited in Brackish Coastal Waters and Lakes

ABSTRACT

Large‐scale biodiversity patterns are increasingly reshaped by climate change, yet long‐term empirical comparisons across aquatic systems remain scarce. Using 3–4 decades of incidence data from fish monitoring in Sweden we analyzed long‐term trends in species richness (SR), turnover, beta diversity, functional characteristics and individual species trajectories, for lakes of different sizes and coastal areas representing different salinity conditions. Differences in inventory completeness were accounted for by rarefaction‐extrapolation, enabling comparisons across systems and time using standardized SR (SR _std ). Inventory completeness was high (69%–100%) and generally declined from systems with lower to higher SR. Fish SR remained largely unchanged over time in lakes, which had low connectivity with other water bodies. Fish SR _std increased by 11%–32% in the more marine sub‐basins, a trend mainly driven by increasing SR of small/medium‐sized, demersal/demersal‐pelagic and warm‐affiliated marine species, whereas SR of cold‐affiliated and pelagic species decreased. Notably, while species appearances slowed down in recent years, local extirpations became more frequent. The community turnover led to biotic homogenization across the more marine sub‐basins over the years 2000–2017, when colonization‐resultant changes in beta diversity dominated, whereas biotic differentiation occurred during recent years, when extinction‐resultant changes dominated. In all, the observed patterns suggest that warming drove progressive thermophilization of the fish communities in the highly connected, more marine sub‐basins, while thermophilization was limited by the brackish conditions for the inner Baltic Sea, and by low hydrological connectivity for the lakes. With continued warming, further shifts in fish communities and associated ecosystem functions are expected, underscoring the importance of monitoring to support ecosystem‐based management and climate change adaptation. While constraining climate change is crucial to address the key driver of the ongoing thermophilization, targeted management actions such as habitat restoration and implementing climate refugia can help to strengthen local resilience of species and ecosystems under pressure.