The deep chlorophyll maximum in Lake Superior

Summer surveys conducted on Lake Superior from 1996-2001 indicated that a deep chlorophyll maximum (DCM) is a common feature of the offshore waters. The DCM was usually observed in the upper hypolimnion between 23 and 35 m, a region lacking a pronounced density gradient. Chlorophyll a concentrations in the DCM were typically 1.5-2.5 (median = 2.0) times epilinmetic concentrations, although these were associated with minimal or no increases in particulate organic carbon concentrations. Seston carbon:phosphorus ratios were consistently lower in the DCM than in the epilimnion, indicating increased phosphorus content of DCM phytoplankton. This could have resulted from either improved nutrient conditions or light limitation at depth. A phosphorus-rich phytoplankton community at depth could serve as a resource for the large, deep-living calanoid copepods that constitute the majority of summer zooplankton biomass. The phytoplankton communities at the level of the DCM were taxonomically distinguishable from those in the epilimnion, with the most notable difference being a relative reduction in the abundance of Cyclotella species in the DCM.