Shifts in coastal fish communities: Is eutrophication always beneficial for sticklebacks?

Following declines of predatory fish, mesopredators such as sticklebacks have been linked to shifts incoastal trophic networks through both top-down (preying on mesograzers and facilitating algal blooms)and bottom-up (benefitting from eutrophicated conditions) processes. Here, we tested whether theassociation between eutrophication effects (filamentous algae and turbidity) and sticklebacks held truein the Finnish Archipelago Sea where predatory fish populations have remained stable. If so, sticklebacksshould be more abundant in the middle archipelago, where eutrophic conditions have led to increasedturbidity, higher filamentous algal loads, and decreased cover of submerged aquatic vegetation (SAV),than in the outer archipelago, where environmental conditions are better. We measured the spatial andseasonal variation of sticklebacks (three-spined Gasterosteus aculeatus and nine-spined Pungitius pungitius) in middle and outer archipelago sites, as well as environmental variables potentially affecting theirabundance.

Adults and juveniles of both species were more abundant in the outer than middle archipelago. Theouter archipelago was characterized by greater Secchi depth throughout the summer and higher SAVcover in late summer. Secchi depth was positively correlated with stickleback abundance of both species,while SAV cover was also positively correlated in late summer. Filamentous algal cover was high in boththe middle and outer archipelago, but not consistently associated with stickleback abundancethroughout the summer. While sticklebacks have been thought to both contribute to, and benefit from,eutrophication, our results instead suggest that the resulting environmental changes may have adverseeffects on sticklebacks, especially if predators are present. This may lead them to shift their breedinggrounds and spatial distribution to less eutrophicated areas where lower turbidity and the resultingincreased availability of SAV provide refuge from predators for juveniles, and higher quality breeding andfeeding grounds for adults.