Interactive effects of omnivorous fish and nutrient loading on net productivity regulation of phytoplankton and periphyton

Top predators and nutrient loading are key components in the regulation of community structure and biomass in aquatic ecosystems. However, few studies have addressed how these 2 com- ponents interact to determine metabolic variables of the community, such as oxygen production and respiration rates. The pelagic zone has been the focus of most research on trophic cascades. Never- theless, recent data increasingly show the importance of periphyton to the functioning of lakes. We conducted a fully factorial field experiment in enclosures within a humic coastal lagoon to analyze the interacting effects of nutrient addition (N and P) and the presence/absence of omnivorous fish on phytoplankton and periphyton net primary productivity. Fish presence positively affected net plank- tonic productivity, especially when nutrients were added. However, fish presence negatively affected net periphytic productivity, probably due to 2 non-exclusive mechanisms: the increase in competition with phytoplankton, which was released from zooplankton grazing, and the direct periphytic biomass consumption by fish. We conclude that fish can alter fundamental ecosystem processes in lakes, such as internal carbon dynamics, by increasing the relative importance of pelagic to periphyton productivity.