Results from the U.S. EPA's Biological Open Water Surveillance Program of the Laurentian Great Lakes: II. Deep Chlorophyll Maxima

Abstract

Deep chlorophyll maxima (DCM) were found in all five Laurentian Great Lakes during August, 1998. Chlorophyll profiles were consistent over large areas in Lakes Superior and Michigan, while distinct inter-site differences were apparent in the other three lakes. Shade adaptation appeared to be primarily responsible for increases in chlorophyll at depth in Lakes Huron and Ontario, while in Lake Superior increases in phytoplankton biovolume were also noted. Deep living phytoplankton populations in the latter lake exhibited improved nutrient status at depth, where concentrations of both soluble phosphorus and silica were higher. Phytoplankton community composition in the DCM differed from that previously reported for the lakes, most notably in the reduced populations of Cyclotella, relative to the epilimnion, seen at most sites. Filamentous chlorophytes were often more abundant at depth, as were certain species of Dinobryon.