MEPS 541:15-30 (2015)  -  DOI: https://doi.org/10.3354/meps11548

ABSTRACT: The great cormorant (Phalacrocorax carbo sinensis) population in the Baltic Sea has increased rapidly since the 1990s. As cormorants nest in high densities on islands, they can affect benthic communities through nutrient runoff from colonies and may also induce trophic cascades through fish predation. Here, we describe fish, invertebrate, and algal communities, as well as algal growth and biomass accumulation, around colony and control islands along the Finnish coast to determine how cormorants affect communities through top-down and bottom-up processes. While cormorants did not affect the species richness or biodiversity of any trophic levels, there were differences in the community composition of pelagic fish between colony and control islands, and mesopredators perch Perca fluviatilis and ruffe Gymnocephalus cernua were less abundant near colonies. The abundance of several species correlated with cormorant abundance, density and colony age, indicating that these are essential factors in determining the strength of colony impacts. Filamentous algae tended to be more abundant and primary production was higher near older, denser colonies, likely due to nutrient enrichment from the colonies, which overwhelms top-down control. Consequent increased competition by filamentous algae probably explains the decrease in biomass of the foundation algal species Fucus vesiculosus near denser, more populated colonies. Cormorants can thus affect benthic communities through both top-down and bottom-up processes, with bottom-up impacts being locally concentrated near colony islands. The impacts can become more pronounced over time and with increasing cormorant populations, potentially leading to shifts in community composition and ecosystem functioning around cormorant colonies.

KEY WORDS: Top-down · Trophic cascade · Bottom-up · Nutrient enrichment · Benthic communities · Cormorants · Baltic Sea