Trends in the western Lake Erie zooplankton community:Evidence for top-down control by age-0 percids

摘要

Several factors interact to determine zooplankton community structure, including phytoplankton abundance and composi-tion, physical variability in temperature, light, mixing, and predation effects by higher trophic levels, making it difficult to predict and understand zooplankton variation and community shifts (SOMMER et al. 1986). Furthermore; variability is a char-acteristic of plankton communities, making it more difficult to separate natural fluctuation from long-term trends and uncover the forcers of variability (PINEL-ALLOUL 1995). In Lake Erie, the situation is further complicated because many ecological changes have occurred over the past several years, including the reduction of phosphorus loading in the 1970s, the invasions of several non-native species, and the recent recurrence of cyanobacterial blooms. In addition, climate warming has recently become a concern. Many of these changes have either direct or indirect effects on the zooplankton community because it is sensitive to ecosystem changes and often responds rapidly (SCHINDLER 1987). Because zooplankton are food for many first-feeding sport and forage fish species, it is extremely important to describe patterns in the zooplankton community and understand the mechanisms driving any changes. Both top-down and bottom-up factors may be influencing the zooplankton community in western Lake Erie, and it is important to examine the relative importance of each.We describe zooplankton community changes in the west-ern basin of Lake Erie from 1995 through 2005 using results from a long-term plankton monitoring study and identify the important factors influencing these changes. We asked three questions: (1) Is there evidence of long-term trends in the zoo-plankton community as a result of ecosystem changes? (2) Is there evidence of top-down, predatory control over the zoo-plankton community? (3) Is there evidence of bottom-up con-trol over the zooplankton community as a result of variation in edible phytoplankton abundance?