Recent changes in the phytoplankton of the Bay of Quinte: The relative importance of fish and phosphorus



Nicholls, K. H.
Hurley, D. A.

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Ontario Ministry of the Environment


The effects of eutrophication control in the Bay of Quinte of Lake Ontario, resulting from a 50% reduction in phosphorus loading from municipal wastewater treatment plants in late 1977, have been examined. A major decline in phytoplankton biomass was observed in the highly eutrophic upper bay in 1978. However, in recent years (1982-1985), average May-October phytoplankton biomass increased to levels approaching those of the pre-phosphorus control period, but decreased again during 1986. No major shifts in phytoplankton composition occurred; domination by the diatoms Melosira and Stephanodiscus spp. and the blue-green algae Anabaena and Aphanizomenon spp. has continued. Although N-to-P ratios increased significantly from an average of 11.0:1 during the pre-P control period to 17-20:1 during 1985-1986 in the upper bay, no decrease was found in the proportion of the total phytoplankton contributed by N-fixing blue-green algae. Phosphorus loading and phosphorus concentrations in the upper bay remained low, relative to the pre-1978 values. No significant correlations existed between total P and total phytoplankton biomass at the upper and middle bay sampling stations, either before or after phosphorus loading reductions; in part, this is because the phytoplankton was often N-limited before 1978. Highly significant positive correlation coefficients (r=0.90-0.96) were found for phytoplankton-planktivorous fish relationships during both pre- and post phosphorus removal periods. A die-off of white perch (Morone americana) in the Bay of Quinte occurred during the winter of 1977-78. This was followed by several strong walleye (Stizostedion vitreum) year classes, which have apparently since kept white perch numbers low. Between 1981 and 1985, alewife (Alosa pseudoharengus) numbers increased dramatically, but declined again in 1986. It is hypothesized that the combined densities of small sized white perch and alewife have contributed to altered grazing rates on phytoplankton by zooplankton and/or zoobenthos, and have been an important influence on phytoplankton biomass in the upper Bay of Quinte during the period 1972-1986.


Ontario Ministry of the Environment
Biological Surveys/Investigations Reports


euthrophication, phosphorus loading, municial wastewater treatment plants, phytoplankton, phytoplankton biomass, phosphorus, fish