A HORIZONTAL GRADIENT IN ZOOPLANKTON COMMUNITY STRUCTURE, AND ITS IMPLICATIONS FOR THE RELATIONSHIPS AMONG MESOZOOPLANKTON, MICROZOOPLANKTON, AND PHYTOPLANKTON

Twelve Mile Bay is a long (about 16 km), narrow bay within Georgian Ba y, Lake Huron, without a significant tributary and not strongly affect ed by cultural eutrophication. The Moon River Basin, immediately to th e north, is also lightly inhabited, but has a significant input of sof t water from a tributary draining the Canadian Shield. A strong gradie nt in zooplankton community structure exists at Twelve-Mile Bay. At th e nearshore end of the bay, the zooplankton community is dominated by Rotifera, small Cladocera, and Cyclopoida. Rotifera decline in importa nce toward the entrance of the bay, and larger Cladocera, Daphnia and Holapedium, increase in relative importance. The highest biomass of Da phnia and Holopedium is attained midway along the length of the bay. T owards the entrance to the bay, Calanoida dominate the community. Cili ophora are a relatively stable, but smaller, component of zooplankton biomass. This gradient is accompanied by an increase in secchi depth, and a decline in total phosphorus, chlorophyll, and chlorophyll to TP ratio, toward the offshore end of the bay. Phytoplankton biomass and s pecies composition also change. Flagellated nanoplankton were a relati vely constant component of the community, but other groups, including Cyanophyta and non-motile Chlorophyta, decreased along the length of t he bay. These changes are consistent with the hypothesis of a greater abundance of grazing-resistant nanoplankton in the microzooplankton-do minated upper bay. Close to the outlet of the Moon River, the zooplank ton community is quite different from that at the upper end of Twelve Mile Bay, suggesting an important role of the river in modifying the z ooplankton community.