Spatial and long-term temporal assessment of Lake Ontario water clarity, nutrients, chlorophyll a, and zooplankton

Abstract

Secchi depth, total phosphorus, soluble reactive phosphorus, silica, chlorophyll a, and zooplankton (density, biomass, and average size) were sampled as part of a lake-wide, seasonal (spring, summer, and fall) assessment of Lake Ontario in 2003 to characterize the status of the lower aquatic food web. For each parameter, spatial comparisons were performed to test for differences between habitats (nearshore and offshore) and between regions (east and west) during each season. Significant differences between habitats were found only for silica and chlorophyll a; silica was higher nearshore in fall, and chlorophyll a was higher offshore in fall. Significant differences between regions were detected in Secchi depth, epilimnetic zooplankton mean length, total phosphorus and Cercopagis pengoi density and biomass; Secchi depth and zooplankton mean length were higher in the east in spring, total phosphorus and Cercopagis pengoi biomass were higher in the west in summer, and Cercopagis pengoi biomass was higher in the east in fall. Cercopagis pengoi was present lake-wide in summer and fall, but Bythotrephes longimanus was present only in fall in the Kingston basin. Mean spring total phosphorus, soluble reactive phosphorus, chlorophyll a, and epilimnetic zooplankton density and biomass are at or near record low levels. As we move into the future, persistent low levels of these lower food web elements will continue to stress alewife populations both through reduced food resources and food quality for zooplankton, and may force these fish to seek alternative food such as Mysis.

Keywords:

• phosphorus
• silica
• Cercopagis
• Bythotrephes

Acknowledgements

We thank the crews of the USEPA R/V Lake Guardian and CCGS Limnos. We acknowledge the leadership of Fred Luckey and Jack Kelly of the USEPA and Vi Richardson of Environment Canada in the LOLA program. This work was funded by EPA Grant CR-83209001 to Cornell University and by a Canada Ontario Agreement (COA) Grant from Ontario Ministry of Natural Resources. This is contribution # 253 of the Cornell Biological Field Station.

Notes

^a Significant difference (t-test; p < 0.05) detected between nearshore and offshore habitats (higher value in superscript parentheses).

^b Significant difference (t-test; p < 0.05) detected between eastern and western regions (higher value in superscript parentheses).

^{a a} Significant difference (t-test; p < 0.05) detected between nearshore and offshore habitats (higher value in parentheses).

^b Significant difference (t-test; p < 0.05) detected between eastern and western regions (higher value in parentheses).