The Phytoplankton Zooplankton Link in the Lake Ontario Food Web

Abstract

Monitoring in Lake Ontario in 1970 and 1982 demonstrated that the zooplankton community was dominated by microzooplankton, which suggested a longer, perhaps inefficient food chain. In this study, annual monitoring of the offshore region of Lake Ontario between 1986 and 1992 was used to determine if microzooplankton were still dominant despite recent changes in nutrient loading and species introductions. Microzooplankton accounted for 49.7% of the total summer zooplankton biomass while small edible phytoplankton accounted for 67.0% of the biomass during the summer. By direct in situ measurement using a Haney grazing chamber, rather than size grazing relationships, the relative impact of micro- and mesozooplankton grazers on phytoplankton production during the summer of 1995 was evaluated. Microzooplankton filtration rates (%/d) for 1995 were significantly higher than mesozooplankton filtration rates. Zooplankton consumed only 17.5% /d of the primary production with microzooplankton grazing representing 69.8% to 93.2% of this amount. Microzooplankton are clearly still dominant and their consumption of primary production in Lake Ontario is low. The major pathway of energy transfer can not be through the classical phytoplankton > large zooplankton > planktivore > piscivore food chain but rather through the phytoplankton > microzooplankton and presumably predacious zooplankton and fish. The longer food chain is a result of the introduction of a size-selective planktivore, the alewive, which has decreased the length and presumably lowered the consumption rate of the entire zooplankton community. This structural impact, a longer food chain, theoretically creates a higher factor of biomagnification of organic chemicals for top-level predators along with lower rates of energy transfer within the food web and suggests lower fish production than in a shorter food web.