Temporal changes in the Bothnian Sea food web reveal a deterioration linked to fishing pressure and recent eutrophication

Abstract Changes in food web dynamics can have large and irreversible effects for many species, due to natural, climate-related, or anthropogenic factors. In the Bothnian Sea, changes in key drivers, including climate change and fisheries, raise concerns that food web dynamics are deteriorating. However, no method exists for evaluating food web status for the region. We examine how the Bothnian Sea open sea food web has changed over time (1979–2021) using the EU Marine Strategy Framework Directive criteria for food web status. To analyse food web components and potential key drivers, we apply integrated multivariate analysis and generalized additive models. Results indicate changes in the MSFD criterion ‘balance between trophic guilds’, mainly associated with changes in herring fishing mortality, dissolved inorganic phosphorus, and salinity. Changes in the ‘balance of species within trophic guilds’ mainly reflected increased cyanobacteria in the phytoplankton biomass, increased stickleback abundance within the planktivorous fish guild, and declining Monoporeia affinis among benthic deposit feeders. The results corroborate a worsening food web status of the Bothnian Sea food web and that measures such as lowering phosphorus loading and fishing mortality would prevent further depreciation. This study contributes to development of management objectives for food webs in the region.