Trends in nutrients, phytoplankton, and chloride in nearshore waters of Lake Ontario: Synchrony and relationships with physical conditions

Consistent routine monitoring at four municipal water treatment plant intakes revealed dramatic long-term changes in water quality in the nearshore waters of Lake Ontario over the last three decades. Total P concentrations decreased significantly, but concentrations leveled off at around 0.01 mg/L after 1996. Silica (Si) and nitrate + nitrite-N (NO2 + 3) concentrations increased and chloride concentrations decreased until the mid-1990s after which they began to increase. There were significant decreasing trends in chlorophyll a and algal cell densities, the greatest rate of decrease being observed in diatom abundance. Interannual and seasonal variations in all parameters were synchronous between stations, indicating the influence of drivers such as climate and hydrology operating at a broad scale. The interannual variations in NO2 + 3 and Si concentrations were significantly related to snow depth, and in Si were also related to annual climate indices, the North Atlantic Oscillation Index (NAOI) and Southern Oscillation Index (SOI), the two most dominant large-scale atmospheric circulation models controlling climate in large parts of the northern hemisphere. The relationships indicated that Si concentrations were lower in the milder, drier years and higher when winters were colder and precipitation and snow depth were greater.

► The water quality in the nearshore waters of Lake Ontario has changed since 1980.
► Phosphorus concentrations decreased while silica and nitrate increased.
► Chloride concentrations decreased until the mid-1990s then increased.
► Chlorophyll and phytoplankton densities decreased with large reductions in diatoms.
► The variations in parameters were influenced by drivers like climate and hydrology.