Abstract

Zooplankton, nutrients, and chlorophyll a were sampled as part of a lake-wide assessment of Lake Ontario in 2003 to characterize the status of the lower food web and to compare results to long-term trends. Spring total phosphorus (TP) has been gradually decreasing since 1981; mean spring TP concentration was 7.0 ug/L in 2003. In contrast to TP, both silica and nitrate/nitrite nitrogen concentrations have been increasing. Compared to historical findings, record high water clarity (mean Secchi disc=9.5m) in offshore and record low summer chlorophyll in nearshore waters (~1.3 ug/L) were observed in 2003. Mean summer zooplankton density and biomass was among the lowest observed in Lake Ontario since 1981. Cyclopoid copepods were dominant (84% of the total biomass) in spring 2003, while daphnids (49%) and bosminids (19%) were dominant offshore in the stratified summer period. Mean zooplankton length significantly decreased from 753 μm in spring to 591 μm in summer and 635 μm in fall. Two invasive cladocerans, Cercopagis pengoi and Bythotrephes longimanus , appeared during stratified thermal conditions in August and September and exhibited strong regional patterns.