Fluorometric analysis of chlorophyll a as a representation of phytoplankton in phosphorus - limited media

Trends in data from recent years show movement toward eutrophication in Otsego Lake; these include increasing total phosphorus and resultant increasing chlorophyll a concentrations, decreasing Secchi transparency readings and decreasing hypolimnetic oxygen concentrations(Harman et al., 1997). The increase in phosphorus has triggered the changes in chlorophyll a, Secchi transparency, and dissolved oxygen levels. Of the total phosphorus entering Otsego Lake more than 80% is retained (Albright, 1996) and that which is available is consumed immediately by primary producers, particularly phytoplankton. Most phosphorus is associated with particulate material which sediments to the lake bottom. It has been suggested that this fraction may subsequently become available to algae following sediment resuspension (France and Albright, 1994; Bostrom ef al., 1982). According to studies that compare phosphorus levels to transparency, a quantity of phosphorus that doubles in size will be equivalent to a transparency reduction of 50% due to phytoplankton growth (Godfrey, 1977; Carlson, 1977; Albright, 1996). Oxygen-consuming bacteria decompose phytoplankton in the hypolimnion, thereby decreasing overall oxygen levels. The decrease in oxygen concentrations affects the entire food web in the lake by decreasing the habitat for cold water fish. These fish prey on alewives, so without them the already-significant alewife population would increase. This would further lead to a decrease in the zooplankton population and ultimately to higher standing crops of phytoplankton. More phytoplankton will cause a greater deficiency of oxygen by their decomposition (Harman et al., 1997).