Coastal inputs of nutrients and contaminants impact phytoplankton community composition and health in localized and large areas of coastal Lake Ontario. Advanced fluorimetric instrumentation (Fast Repetition Rate Fluorimeter [FRRF], FluoroProbe) was used during June 2006 to assess the in situ phytoplankton community of eight tributaries of Lake Ontario (Chaumont Bay, Sacketts Harbor/Black River, Oswego River, Port Bay, Sodus Bay, Irondequoit Bay, Genesee River, and Eighteen Mile Creek) representing a range of geomorphological features (rivers, protected embayments, open embayments). The instrumentation enabled high resolution (160 m) continuous monitoring of phytoplankton community composition (FluoroProbe) and photosynthetic efficiency (FRRF) along 1 m deep transect from nearshore (coastal Lake Ontario, > 7 m depth) to inshore (within the tributary, < 7 m depth). Limnological parameters such as water temperature, colored dissolved organic matter content (CDOM), total phosporus, nitrate, chloride, light extinction coefficient, and extracted size fractionated (0.2–2 μ m, 2–20 μ m, > 20 μ m) chlorophyll a concentrations were also determined. Results from this study illustrate the strong environmental gradients that exist along tributary transition zones into Lake Ontario. Offshore water quality was homogeneous and reflected nutrients levels (e.g. total phosphorus) that were below the Great Lakes Water Quality Agreement target value (10 μ g per litre), whereas most tributary inputs had elevated nutrient and chloride levels. Phytoplankton community composition differed between inshore and offshore sites and the transition coincided with changing water quality (followed using water temperature and CDOM concentrations) at the interface of the surface water masses. The greatest observed photosynthetic efficiencies occurred in the inshore environment; this is attributed to the greater nutrient availability rather than lower light levels. Fluorimetric applications provide useful techniques for monitoring water quality. Given the observed diversity of phytoplankton community and health among these tributary inputs, it follows that monitoring capabilities using the instrumentation applied here should be enhanced to observe these systems.

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