Dynamics Of The Lake Michigan Deep Chlorophyll Layer (phytoplankton, Sedimentation, Primary Production, Zooplankton, Growth).

Abstract

An investigation of the Lake Michigan deep chlorophyll layer (DCL) was conducted in 1982-1984 using a variety of field, laboratory, and computer simulation procedures. The DCL was a seasonal feature, developing after the onset of thermal stratification and continuing into mid-stratification (July-August). The layer initially formed in the 15 to 30 m region and deepened to the 25 to 50 m region in July and the 40 to 70 m region in August. Chlorophyll concentrations in the DCL averaged 1.75x and 5.70x epilimnetic concentrations in June and July-August, respectively. These higher DCL chlorophyll concentrations were attributable to two factors; decreased epilimnetic chlorophyll concentrations and shade adaptation within the DCL. In contrast to the epilimnetic community, phytoplankton carbon concentrations in the DCL remained relatively constant. The roles of various growth and loss processes in maintaining phytoplankton concentrations within the DCL were evaluated. In situ growth, controlled by the interaction of light and nutrients, was the primary process maintaining phytoplankton concentrations within the DCL. An average of 30% of water column production occurred within the DCL and growth rates were as high as 0.3 day('-1). During early stratification, diatoms, sinking from the surface waters, accumulated in the DCL. However, the importance of sedimentation as a net source of phytoplankton to the DCL was small in comparison to in situ growth. By the period of mid-stratification, sedimentation resulted in a loss of phytoplankton from the layer. Phytoplankton sinking was more important as a factor regulating the depth of the DCL. In July 1984, in situ growth was found to balance plytoplankton losses due to sinking and crustacean grazing. Methods of measuring in situ production in the DCL were compared and evaluated. Generally, ('14)C primary production estimates that considered the vertical movements caused by internal waves were not significantly different than standard, fixed-depth production estimates. Results from short (1-2 hr) and long (24 hr) ('14)C incubations were compared to independent estimates of phytoplankton growth obtained from changes in phytoplankton carbon. Short-term experiments extrapolated to daily rates provided a measure between net and gross production and long-term ('14)C experiments provided a measure close to net production.