PLANKTIVORES AND PLANKTON DYNAMICS - EFFECTS OF FISH BIOMASS AND PLANKTIVORE TYPE

We quantified the effects of planktivore biomass and planktivore type in an experimental mesocosm study of factorial design in which five levels of fish biomass (0-75 g/m3) were cross-classified with two plantivore types: filter-feeding gizzard shad (Dorosoma cepedianum) and visual-feeding bluegill (Lepomis macrochirus). As fish biomass increased, cladocerans, cyclopoids, particulate phosphorus (PP) > 200-mu-m, and chironomids declined; conversely, rotifers, primary productivity, chlorophyll a, turbidity, unicellular flagellates, colonial and unicellular green algae, pennate diatoms, total phosphorus, and 20-200 and 12-20-mu-m PP were enhanced. In the presence of gizzard shad, as compared with bluegill, cyclopoids, turbidity, unicellular green algae, pennate diatoms, > 200-mu-m PP, and chironomid tubes were higher whereas colonial green algae and < 0.2-mu-m PP were lower. Fish biomass operated independently of planktivore type for most variables, except copepods, colonial green algae, turbidity, and 20-200-mu-m PP. Although gizzard shad and bluegill have different trophic cascade pathways, fish biomass was more important than planktivore type as a regulator of plankton communities and water quality.