Influences of grazing on temporal patterns of algal pigments in suspended and sedimenting algae in a north temperate lake

The concentrations of phytoplankton pigments in suspended and settling part icles were measured in Trout Lake, Wisconsin, U.S.A., during the 1993 ice-f ree season. A decrease in water column concentrations of chlorophyll a coin cident with an increase in pheophorbide and pheophytin concentrations occur red during a "clear-water" phase in June. The lack of a corresponding decre ase in carotenoid pigment concentrations indicated that carotenoid pigments were degraded to a lesser extent than chlorophyll a during this period of heavy grazing pressure. Chlorophyll a, pheophorbide, and pheophytin showed temporal trends in fluxes to sediment traps that were similar for ail pigme nts but did not correspond to trends in water column pigment concentration. Fucoxanthin concentrations and microscopic examination indicated that the sedimentation of large, dense diatoms increased the sedimentation rate of c hlorophyll during the clear-water phase. In addition, ungrazed diatoms appa rently were the main source of pheophorbide in trap material during the cle ar-water phase, indicating that pheophorbide should not be used as a surrog ate for fecal pellet sedimentation in lakes influenced by the sedimentation of large diatoms.