Abstract

The Lake Ontario ecosystem has undergone significant alterations during the past three decades due to eutrophication, toxic chemicals, phosphorus abatement, invasion of exotics, stocking of salmonids and the control of sea lampreys. Our understanding has considerably increased during the past years due to the publication of both a monograph and a synthesis paper. This presentation is based on a comparative structural and functional assessment of the planktonic food web during the Lake Ontario Trophic Transfer (LOTT) study of 1990 and the recently conducted Lake Ontario Lower Aquatic Food Web Assessment (LOLA) of 2003. Microbial food web parameters such as bacteria, picoplankton, heterotrophic nanoflagellates, ciliates, phytoplankton and size fractionated primary productivity were evaluated during both studies using the standard techniques of Fisheries & Oceans Canada. For example, phytoplankton has shown dramatic changes between spring 1990 and 2003, with biomass declining from 5.9 g m^-3 to 0.14 g m^-3. The compositional changes indicated that Diatomeae were being replaced by Chlorophyta. The mean cell size (ESD) also declined from 26 mm to 12 mm. This presentation will discuss the significant changes observed in the planktonic food web during a very short period of time.