Small-bodied zooplankton communities yet strong top-down effects on phytoplankton in the absence of fish

This investigation reports the community composition and biomass size-distribution of crustacean zooplankton communities in five small fishless lakes of the Precambrian Shield in Algonquin Provincial Park across one or two growing seasons. Contrary to predictions based on the size efficiency hypothesis and invertebrate predation models, zooplankton in these lakes were predominantly small. In most of the lakes surveyed, zooplankton communities were dominated by small calanoid copepods (mean adult body length 1.5 mm) make up more than 25 % of the zooplankton biomass. Chaoborus abundance among these fishless lakes was an order of magnitude greater than in other lakes in the region with fish. We suggest the relatively small biomass of large bodied zooplankton compared with other fishless lakes is due to heavy Chaoborus predation combined with slowed zooplankton productivity in low pH waters. Despite the absence of large Cladocera, the relationship between zooplankton and phytoplankton biomass indicates strong "top-down" effects on phytoplankton. Grazing experiments in one of the lakes, and in a nearby lake with fish, indicate high grazing on some phytoplankton species, especially small dinoflagellates, and on protozoa.