Trophic status may influence top–down effects of anadromous alewife Alosa pseudoharengus (Actinopterygii, Clupeidae) in lakes

We investigated the seasonal influence of an effective size-selective planktivore, the anadromous alewife (Alosa pseudoharengus Wilson), on summer zooplankton and phytoplankton communities in a series of lakes in Maine, USA (4 with and 4 without alewife) that ranged from oligotrophic to eutrophic to determine the role of lake trophic state in influencing the relative strength of top–down forces on phytoplankton biomass. Predation by young of the year alewife reduced the mean body length of cladoceran and copepod biomass from spring to summer in alewife lakes, while mean body length remained unchanged in nonalewife lakes. Cladoceran biomass decreased substantially from spring to summer in 3 of 4 alewife lakes, increasing only in the most eutrophic lake. Conversely, cladoceran biomass increased from spring to summer in 3 of 4 nonalewife lakes. Predation by alewife on zooplankton did not have consistent cascading effects on phytoplankton biomass. Analysis of planktonic biomass ratios (phytoplankton biomass: zooplankton biomass) suggests that cascading effects were stronger in oligotrophic systems and weakened with increasing trophic status, as ratios in alewife and nonalewife lakes converged at higher total phosphorous levels. Our results suggest that lake trophic status may influence the relative importance of top–down control of both zooplankton and phytoplankton biomass. The mechanisms that explain this pattern remain elusive and likely require additional efforts to estimate alewife densities, rates of zooplankton and phytoplankton production relative to consumption, and the influence of other physical lake features.