Cryptic Trophic Cascade along a Gradient of Lake Size

Lake size and depth mediate the strength of interaction between fish and zooplankton. We test whether this variation in zooplanktivory indirectly affects the phytoplankton by comparing 19 lakes that represent two food webs resulting from the absence of piscivores in small, partial-winterkill lakes. Lakes with piscivorous fish are further distinguished by thermal stratification, which provides a refuge for zooplankton to avoid fish predation. We contrasted phytoplankton abundance in these three categories of lakes over six years, using both direct measures of concentration and a growth bioassay that measures phytoplankton from the perspective of a standard grazer (Daphnia). Contrary to expectations, phytoplankton abundance was largely unaffected by trophic structure or the presence of a deep-water refuge. However, grazer growth differed dramatically among the three categories of lakes. Consistent with trophic cascade, increased fish planktivory resulted in more phytoplankton food as measured from the grazer's perspective. This effect was independent of lake productivity or total abundance and size of phytoplankton. Instead, variation in food quality for grazers was associated with compositional differences in phytoplankton. These results indicate that persistent trophic cascades are more dramatic in the plankton than previously realized but primarily influence composition, rather than biomass. Although cryptic, such top-down effects create functional variation in grazer-resource coupling.