Fluxes and transformations of aquatic pigments in Lake Mendota, Wisconsin
Concentrations and fluxes of chlorophyll and carotenoid pigments were measured in suspended and settling particulate matter and in surface sediments in Lake Mendota. Flux comparisons were used to calculate the extent of alteration or degradation within the water column and at the sediment surface. Losses within the water column for specific time intervals ranged from almost negligible for diatoxanthin to 96\% for peridinin. The extent of loss was influenced by pigment type and transport process. Surface sediment diagenesis in Lake Mendota resulted in differential degradation rates among chlorophyll and carotenoid constituents. Deposition of pheophorbide a-a grazing indicator-was most important during late spring, as zooplankton populations increased and chlorophyll levels in the water column dropped below 1 nmol liter-‘. The ability to trace the magnitude of this annual event through the sedimentary record was obscured by selective degradation of pheophorbide relative to chlorophyll and pheophytin. Similarly, carotenoid composition was dramatically altered both within the water column and at the sediment surface. Fucoxanthin and peridinin, major water- column carotenoids, were degraded extensively before incorporation in the sediments.