Phytoplankton Distribution in Lake Erie as Assessed by a New in situ Spectrofluorometric Technique

In recent years, there has been concern about the trophic status of Lake Erie as phytoplankton and nutrients in some parts of the lake seem to have increased and toxic cyanobacterial species such as Microcystis aeruginosa have been reported. In the summer of 2002, a new in situ spectrofluorometric technique was used to survey phytoplankton on the lake-wide scale and determine the patterns of phytoplankton abundance and major taxonomic composition between regions of differing nutrient levels and dreissenid influence. Pigment fluorescence suggested substantial vertical and temporal variations of biomass and taxonomic composition, even within the epilimnion, on scales of meters and hours that conventional sampling would not resolve. The inferred phytoplankton biomass, expressed as total chlorophyll-a (chl-a), correlated well with chl-a estimated by traditional ethanol extraction and spectrophotometry. Average epilimnetic pigment biomass was highest in the west-central and west basins but appeared overall to be at or near historic lows. Fluorescence-inferred chl-a of diatoms and their relatives was generally dominant, followed by that of greens and cryptophytes. Estimates of average major taxonomic group composition were consistent with the most recent comparative data from traditional microscopic analysis. Two main incidents of toxic cyanobacterial occurrence were observed, one in the west basin and the other in the northern coastal zone of the east basin. Although very different in nutrient levels, both are areas in which substantial impacts of dreissenids on total phytoplankton abundance have been inferred previously.