Batch culture experiments indicated that increased phosphorus availability during the spring bloom contributed to the seasonal increase in A. Chlorophyll-a concentrations indicated that other phytoplankton were also more abundant dur-ing the spring bloom. ambigua colonies were generally larger and more abundant in spring prior to the termination of the bloom in June. Results of the field-based observations were complimented with controlled laboratory experiments to evaluate the effects of phosphorus availability on A. In addition, several chemical and biological variables thought to influ-ence A. Further, comparisons with paleoecological analyses suggest that some inferred increases in production during lake ontogeny may arise from changes in regulation of pigment fluxes rather than from increased algal production.Īulacoseira ambigua abundance and filament length were measured weekly during spring and autumn bloom periods in Trout Lake, Wisconsin, USA. The model suggests that algal production may be underestimated in sedimentation studies that do not consider variability in water column depth. Digestion by zooplankton caused the most pigment degradation, but grazing increased pigment deposition when digestive losses were less than those due to oxidation of pigments in ungrazed cells. Unexpectedly, pigment sedimentation rate was not sensitive to variation in photooxidation rates, even though bleaching accounted for the second greatest amount of pigment loss. Pigment deposition increased with production, sinking rate, and phytoplankton depth and declined as lake depth and the depth of oxygen penetration increased. We used an individual-based model of pigment flux to quantify the relative importance of production and degradation as controls of pigment sedimentation.
However, significant pigment degradation can occur during algal sedimentation. Vertical fluxes of pigments are used in limnology to monitor phytoplankton abundance, herbivore grazing, ecosystem efficiency, and historical changes in production. 1999), it has proven difficult to establish the importance of herbivores and metalimnetic blooms in regulating pigment deposition (e.g., Poister et al. Although direct correspondence between euphotic zone algal standing crop and pigment deposition has been recently demonstrated (Leavitt and Findlay 1994 Leavitt et al. 1999), or grazing by large-bodied herbivores (Welschmeyer and Lorenzen 1985b Carpenter et al. 1983), the abundance of metalimnetic phytoplankton (Hurley and Garrison 1993 Baines and Pace 1994), sinking of large cells ( Poister et al. For example, observational studies suggest that pigment deposition is correlated with total algal biomass (Gorham et al. Mass balance budgets and mesocosm experiments have been used to identify the important factors that influence pigment sedimentation, but regulation of material flux is difficult to quantify with experiments, and empirical studies often support apparently contradictory hypotheses. In addition, ungrazed diatoms apparently were the main source of pheophorbide in trap material during the clear-water phase, indicating that pheophorbide should not be used as a surrogate for fecal pellet sedimentation in lakes influenced by the sedimentation of large diatoms. Fucoxanthin concentrations and microscopic examination indicated that the sedimentation of large, dense diatoms increased the sedimentation rate of chlorophyll during the clear-water phase.
Chlorophyll a, pheophorbide, and pheophytin showed temporal trends in fluxes to sediment traps that were similar for all pigments but did not correspond to trends in water column pigment concentration. The lack of a corresponding decrease in carotenoid pigment concentrations indicated that carotenoid pigments were degraded to a lesser extent than chlorophyll a during this period of heavy grazing pressure.
A decrease in water column concentrations of chlorophyll a coincident with an increase in pheophorbide and pheophytin concentrations occurred during a "clear-water" phase in June. The concentrations of phytoplankton pigments in suspended and settling particles were measured in Trout Lake, Wisconsin, U.S.A., during the 1993 ice-free season.