Long-term trends in major ions and nutrients in Lake Ontario

The Great Lakes Surveillance Program has been monitoring water quality for almost 40 years in Lake Ontario. The program provides some of the most comprehensive, systematic and detailed information that is available in the world for such a large lake. The water quality in Lake Ontario has shown dramatic changes over the last 40 years, with the early measurements indicating high phosphorus concentrations that were subsequently reduced by management responses to the Great Lakes Water Quality Agreement. Other water quality parameters, such as some of the major ions, showed reductions during the 1970s and 1980s as well. Nitrate plus nitrite nitrogen has increased in the lake throughout the period of record, likely driven by increasing watershed and atmospheric sources. A major driver of more recent trends in water quality appears to be the invasion and subsequent expansion of invasive Dreissena mussel populations that first appeared in Lake Ontario in 1989. Total phosphorus concentrations have further declined, and the proportion of total phosphorus that is soluble is increasing, possibly due to the filtering action of these mussels. Concentrations of major ions that are incorporated in mussel shells such as calcium have declined, while those that do not, such as magnesium, have increased. Spring silica concentrations are increasing; an ominous signal of declining diatom populations, which may also be a symptom of the proliferation of invasive mussels in the lake.