Annual productivity and phytoplankton changes between 1969 and 1974 in Gull Lake, Michigan

SUMMARY. The seasonal and depth distributions of dominant phytoplankters, in situ primary productivity, organic carbon, and major chemical variables are described for Gull Lake, Michigan for the years 1971–74. Despite efforts by residents surrounding this moderately large dimictic lake to reduce phosphorus loading, all indications are of progressive eutrophication over the period. Previously dominant Cyclotella michiganiana has almost completely disappeared while araphidinean diatoms have increased greatly; certain blue-green algae that were absent previously are becoming characteristic. The ratio of araphidinean to centric diatoms in recent sediments has also increased. Primary productivity and particulate and dissolved organic carbon concentrations have also increased. Voluntary attempts to reduce nutrient loading have apparently been inadequate to arrest increasing rates of eutrophication.