The Microbial Food Web Composition and dynamics During the Winter-Spring Transition in Southern Lake Michigan

Peter Lavrentyev, Paul Kovalcik, Deborah Hersha (The University of Akron), and Wayne S. Gardner (University of Texas). In collaboration with: Henry Vanderploeg and Joann Cavaletto (NOAA-GLERL), Marie Bundy (Academy of Natural Sciences).

Objectives:

  1. Determine the composition, abundance, spatial distribution, and growth rates of microzooplankton and nanoplankton.
  2. Examine their trophic relationships with bacteria, phytoplankton, and planktonic copepods.

Approach:

  1. Microzooplankton and nanoplankton samples were collected from the standard EEGLE stations in southern Lake Michigan from February 1998 through April 2000 and examined using a digital microphotography system and cytological techniques.
  2. A series of the Landry-Hasset type dilution experiments were conducted at selected stations June 1999 and February-April 2000 to examine the grazing impact of planktonic protists on bacteria and phytoplankton. Some of these incubations were conducted in conjunction with the fluoresently labeled bacteria (FLB) experiments.
  3. The trophic interactions between planktonic calanoid copepods (Diaptomus sicilis and Limnocalanus macrurus) and microzooplankton were examined in a series of 24-h shipboard bottle experiments.

Results:
The microzooplankton biomass (1998-1999 data) in offshore waters was due primarily to mixotrophic oligotrichs and the dinoflagellate Gymnodinium helveticum. Following storm resuspension events, the nanoplanktonic suspension-feeding choreotrichs Rimostrombidium spp. became abundant, whereas tintinnids and colonial chrysophytes dominated the river-influenced waters. The highest and lowest concentrations of microzooplankton were found in the turbidity zones near the St. Joseph River and Chicago (Fig. 1)

The growth rates of specific protist taxa exceeded 1.0 day-1 and the microzooplankton community growth rates varied between 0.1 and 0.5 day-1, increasing from February through April in 1999. The community herbivory and bacterivory rates (0 to 0.35 day-1) were close to or even exceeded the growth rates of phytoplankton and bacteria.

The copepods L. macrurus and D. sicilis consumed maxima of 630 and 118 ng C of microzooplankton animal-1 day-1, respectively. These rates were similar to or higher than their herbivory rates (Fig. 2). Both copepods demonstrated selective predation on certain microzooplankton taxa.

Conclusions:

  1. The abundance microzooplankton during the winter-spring transition period is comparable to the summer stratification period;
  2. The offshore microzooplankton community has a uniform distribution, while there is a significant spatial heterogeneity in resuspension-influenced waters;
  3. The predominant microzooplankton taxa posses high growth rates despite low water temperature and form important trophic links between phyto- and bacterioplankton and planktonic copepods.