Short-term Changes in “Caged” Phytoplankton at Three Locations in Severn Sound, Southern Georgian Bay, Lake Huron

Abstract

A short-term in situ phytoplankton incubation technique was used in Severn Sound of Georgian Bay to determine the relative importance of municipal sewage treatment plant and river sources of nutrients for phytoplankton. A phytoplankton assemblage collected from a nearby reference area of Georgian Bay was incubated in translucent polyethylene cages, with 10-μm mesh (1985) and 1-μm mesh (1987) “windows.” The “windows” facilitated exchange of dissolved substances between the caged phytoplankton and the exterior water. Incubations were for 5-day periods in April, June, and August of 1985 for 4 days in June and August of 1987. Each incubation included three replicates. Although the phytoplankton species composition of the inoculum differed between years and between April, June, and August within a year, the response of caged phytoplankton was highly reproducible. In all cases, the biovolume of caged phytoplankton appeared to respond to the quantity and availability of nutrients in the ambient water (outside the cage) and highly significant positive correlations between phytoplankton biovolume and phosphorus were observed. Yields of caged phytoplankton per unit total phosphorus at the Penetang Bay incubation site were relatively constant seasonally and reflect the importance of a constant source of available phosphorus in treated sewage effluent. In contrast, phytoplankton yield per unit total phosphorus ranged over nearly two orders of magnitude at the Severn River site and was apparently related to seasonal changes in hydrology and associated changes in availability of phosphorus in land drainage.