The Recurrent Coastal Plume in Lake Michigan: A Critical Event for Copepod Reproduction and Recruitment

H. Vanderploeg and M. Bundy

Preliminary results of grazing experiments conducted in 1998, 1999, and 2000 reveal that the dominant calanoid copepods (Limnocalanus macrurus and Diaptomus sicilis) exhibit size selective feeding, with lower grazing rates on < 10 Ám phytoplankton and elevated grazing rates on larger cells. In particular, D. sicilis preyed heavily on the 10-53 Ám and > 53 Ám component of the phytoplankton community, while L. macrurus preyed heavily on the 10-53 Ám component, but did not graze on the > 53 Ám size fraction.

When these data are coupled with those of Lavrentyev and Kovalcik, it appears that production of zooplankton should be elevated when a more heterotrophic food web dominates, compared to when phytoplankton dominate. Both calanoids had elevated grazing rates on protozoan prey, with clearance rates of microzooplankton reaching 10 times that of phytoplankton. If in fact the plume's sediment load promotes a more heterotrophic food web, then overwintering copepods may get a significant boost from increased concentrations of microzooplankton. This is important because the late winter/early spring is a critical time for reproduction of these animals. Grazing studies with nauplii suggest that new copepod production will probably also be elevated when there is increased productivity of the microbial food web, although we have not comprehensively looked at feeding of juveniles.

Scheduled monthly "monitoring" cruises starting in September 1998 on Muskegon and St. Joseph transects were carried out to characterize the the plankton community. Discrete samples of nutrients, size-fractionated chlorophyll, and suspended solids were sampled at 15-m, 45-m, and offshore sampling sites on each transect (Fig. 1). We also did a major cruise in June that covered all transects (Racine, Chicago, Gary, St. Joseph, and Muskegon) to evaluate offshore-onshore patterns at a variety sites in the southern basin (Fig. 1) outside of the plume time frame. There were tremendous differences in the nearshore areas among some of the transects. For example, chlorophyll concentration was lower and zooplankton concentration was very much lower in the nearshore area near Chicago as compared to Muskegon (Figs. 2a, 2b, 2c, 2d). These data are the first to fully span the inshore to offshore and show that inshore regions of different parts of the southern basin are very different. Note we did also sample at the deepest portion of the southern basin, the station labeled DWS in Fig. 1. Major differences were also seen in nutrients in the different regions. For example, diatom production (as measured by the accumulation of biogenic silica) in the St. Joseph region was significantly depressed relative to that near Muskegon for 3 month during 1999.

The PSS showed that zooplankton counts were similar in March 1998 and March 1999 transects; however, PSS-measured biomass was considerably lower in 1999. This correlated to a shift from Diaptomus spp. to cyclopoids, which are smaller and more predacious than Diaptomus, and a shift from large to small species of Diaptomus. This was probably due to a stong year class of alewives preying on larger zooplankton. Over 100 zooplankton samples have been counted from the St. Joseph transects and we are beginning to determine their biomass from length weight regressions so we can calibrate biomass seen with the PSS. Likewise fluorescence will be correlated to chlorphyll measurements. The fish predation driven changes outweigh any impact of plume-driven changes. The shift in zooplankton composition to cyclopoids and small diaptomids implies a shift away from predation on large diatoms to micozooplankton. Examination of data from 2000 revealed a return to the usual commuinty dominated by medium and large-sized diaptomids.

Figure 1. Location of EEGLE sampling stations and transects in southern Lake Michigan. Survey area for Evans et al. (1980) study represented by box surrounding symbol ME. Stations for Makarewicz et al. (1995) study represented by circles.

Figure 2a. Chicago transect in early June 1999: Chlorophyll fluorescence, water temperature, and light attenuance. White sinusoidal lines on panels are PSS traces.

Figure 2b. Chicago transect in early June 1999: Zooplankton number concentration and biomass.

Figure 2c. Muskegon transect in early June 1999: Nominal chlorophyll concentration, water temperature, and light attenuance.

Figure 2d. Muskegon transect in early June: Zooplankton number concentration and biomass.