ATMOSPH ND ER A I C C I A N D A M E I C N Nearshore-Offshore Differences in Mesozooplankton Community O I S

L T

A R

N A

T

O

I

I

O

T

N A N

U E Structure during the Recurrent Coastal Plume in Southern Lake Michigan .S C .D R E E PA M RT OM 1 2 1 MENT OF C Megan Agy , Henry Vanderploeg , Thomas Johengen 1GLERL/University of Michigan, Cooperative Institute for Limnology and Ecosystems Research, Ann Arbor, MI 2NOAA, Great Lakes Environmental Research Laboratory, Ann Arbor, MI INTRODUCTION The most significant change between March 1998 and 1999 was the dramatic decrease in the overall number of diaptomids (Figure 6). As part of the Episodic Event: Great Lakes Experiment (EEGLE), we Figure 2. Retrieving Copepoda Abundance for Muskegon Copepoda Abundance for St. Joseph Specifically, the density of adult diaptomids in 1999 at St. Joseph zooplankton net tow 5000 March 1998 was three to four times less than the density in 1998. Another are investigating the effects of the Lake Michigan Recurrent Coastal during the March 1999 5000 March 1998 interesting trend from these data is that D. ashlandi decreased in Plume (LMRCP) on mesozooplankton community structure. The cruise. 4000 4000 LMRCP is a nearshore phenomenon that may affect nearshore cyclopoids relative abundance from 1998 to 1999 whereas the relative 3000 3000 cyclopoids calanoids abundance of D. minutus, the smallest of the diaptomid species, zooplankton communities differently from offshore communities. calanoids Therefore, the objective of this research is to compare the near- 2000 calanoids 2000 increased. The largest diaptomid, D. sicilis, decreased in relative ) ) 3 3 shore-offshore differences in zooplankton species composition at 1000 1000 abundance as well (Figure 7). cyclopoids cyclopoids 0 five different regions around the southern basin of the lake (Figure 0 10 30 80 10 45 110 10 30 80 March 1998 Adult Diaptomus species March 1999 Adult Diaptomus species 1). Results presented here describe zooplankton community 4000 1000 5000 March 1999 Muskegon Muskegon 5000 March 1999 D. oregonensis structure during the plume in March from both 1998 and 1999. 3000 750 D. oregonensis D. sicilis Mean Density (#/m

Mean Denisty (#/m D. sicilis 4000 2000 500 4000 D. minutus D. minutus 1000 D. ashlandi 250 D. ashlandi 3000 In addition to investigating the impact that the LMRCP has on the 3000 0 0 cyclopoids 10 45 110 ) 15 45 110 3 4000 ecosystem,this research will also provide valuable information on calanoids 2000 1000 2000 St. Joseph St. Joseph 3000 750

the zooplankton community structure during the winter. Preliminary ) 1000 2000 3 1000 cyclopoids calanoids 500 evidence shows that when the LMRCP occurs, there is an increase 1000 250 0 0 15 45 80 0 0 in nutrient supply, bacterial abundance and production, and a shift in 15 45 110 10 30 80 15 45 80 microbial food web and phytoplankton community structure. It is Station Depth (m) Station Depth (m) Mean Density (#/m 4000 1000 Gary Gary important to understand how zooplankton populations respond to 3000 750 Mean Density (#/m 2000 500 Copepoda Abundance for Chicago Copepoda Abundance for Gary the physical and biotic effects of this large-scale episodic event. A 1000 250 comparison of zooplankton species composition, biomass and 0 0 5000 5000 10 30 15 45 ) March 1999 March 1998 3 production from nearshore to offshore regions will improve our Station Depth (m) 1000 calanoids Chicago 4000 4000 750 understanding of the spatial dynamics of zooplankton in the In the laboratory, two or more subsamples of a well-mixed sample calanoids Figure 6. Distribution of Diaptomus calanoids 500 3000 3000 species identified from March 1998 and southern basin of Lake Michigan. were taken with a Stempel pipette until at least 600 animals were 250 cyclopoids 1999.

cyclopoids ) 2000 0 identified. Another subsample, equal in volume to the sum of the 2000 cyclopoids 3 15 45 80

subsamples, was examined for the rare taxa whose total count in Mean Density (#/m 1000 Station Depth (m) 1000 cyclopoids the first set of subsamples was less than forty. All adult copepods 0 0 10 30 and cladocerans in each subsample were identified to sex and 15 45 80 Station Depth (m) species (Figure 3). Immature copepodites were identified to genus, 5000 Figure 7. Female Mean Density (#/m March 1999 while nauplii were combined into one group. Zooplankton were calanoids Diaptomus sicilis. 4000 enumerated in a circular counting dish using an Olympus dissecting calanoids Figure 4. Copepod abundance from 3000 microscope at a magnification of 20 to 57X (Table 1). four nearshore-offshore transects during March 1998 and 1999. 2000 cyclopoids

1000 cyclopoids 0 15 45

Figure 3. Adult Station Depth (m) copepods from Lake Michigan. transect was the exception to this trend however the deepest station sampled was only 30 m. Due to inclement weather, the Chicago SUMMARY/FUTURE RESEARCH transect was not sampled during March 1998. The next phase of this research is to analyze and compare the net Since these collections were made at the same time of the year, tow data from cruises before and after the LMRCP in 1998 and using the same techniques, we expected that the results would not 1999. These data will enable us to better understand changes in exhibit dramatic variations in species abundance and distribution. zooplankton community structure during the winter-spring transition. However, there was a substantial shift in the ratio of calanoids to In addition to looking at the distribution of zooplankton populations, cyclopoids between 1998 and 1999. Calanoids dominated the we will also measure zooplankton biomass and egg production composition of zooplankton at all stations in March 1998, whereas in before, during and after the winter-spring transition in areas within March 1999, cyclopoids dominated at every station. Specifically, the and outside the influence of the LMRCP. Future research will cyclopoids sampled from the St. Joseph transect increased four-fold address the drastic changes in copepod abundance and composition at the nearshore station (10-15 m), and increased eight-fold at the observed between March 1998 and 1999 during the LMRCP offshore station (80 m) (Figure 5). Results reflect a change in the Table 1. Copepod species (in order of increasing length) found during the (Figures 4 and 5). Factors that influence this dramatic shift in Figure 1. Location of EEGLE sampling stations and transects in community size structure since cyclopoids are smaller in length than southern Lake Michigan. March cruises for 1998 and 1999. zooplankton composition may include the introduction of exotic calanoids. The community has also become more predacious with species, such as Bythotrephes cederstroemi and Dreissena Cyclopoids Calanoids the switch over from calanoids to cyclopoids. This dramatic shift in polymorpha; the occurrence of tumors on zooplankton (Omair et al. the ratio of calanoids In press); the tremendous decline in the macroinvertebrate Diporeia Tropocyclops prasinus Diaptomus minutus to copepods is not (Nalepa et al. 1998); and a potential shift in fish predation pressure METHODS Cyclops bicuspidatus Diaptomus ashlandi unprecedented in (Evans 1990); all of which may alter zooplankton community Diaptomus oregonensis Lake Michigan. structure. Survey cruises for EEGLE took place aboard the R/V LAKE Diaptomus sicilis Calanoids are GUARDIAN in January, March and May of 1998 and in February, Epischura lacustris typically greater in March, April and June of 1999. On most cruises, samples were Senecella calanoides abundance com- REFERENCES collected at a shallow site (10-15 m), a mid-depth site (45 m) and a Limnocalanus macrurus pared to cyclopoids deep site (80-110 m). Replicate samples for determining zooplank- however there was a Evans, M. S. 1990. Large-lake responses to declines in the abundance of a major fish ton community structure were collected by vertical hauls of a reversal of domi- planktivore – the Lake Michigan example. Can. J. Fish. Aquat. Sci. 47:1738-1754. metered 153 mm mesh zooplankton net (0.5 m diameter) from 2 Haney, J. and Hall, D. 1973. Sugar-coated Daphnia: A preservation technique for RESULTS nance in 1966 as Cladocera. Limnol. Oceanogr. 18: 331-333. meters above the bottom to the surface. For each net towed, flow cyclopoids were three Makarewicz, J., Beertram, P., Lewis, T., and Brown, E. 1995. A decade of predatory control meter readings were recorded and the nets were hosed down to of zooplankton species composition of Lake Michigan. J. Great Lakes Res. 21: 620- Copepods are the dominant members of the mesozooplankton times greater in collect zooplankton in a screened collection bottle at the cod-end of 640. community in Lake Michigan during the winter-spring transition number over Nalepa, T., Hartson, D., Fanslow, D., Lang, G., and Lozano, S. 1998. Declines in benthic the net (Figure 2). The zooplankton were narcotized and then (>95%). For most of the transects sampled, copepod abundance calanoids macroinvertebrate populations in southern Lake Michigan, 1980-1993. Can. J. Fish. preserved with sugar formalin (Haney and Hall 1973). (Makarewicz et al. Aquat. Sci. 55: 2402-2413. decreased as station depth increased (Figure 4). The 1998 Gary Figure 5. Female and male Cyclops bicuspidatus. Omair, M., Vanderploeg, H., Jude, D., and Fahnenstiel, G. 1999. First observation of tumor- 1995). Dominant cyclopoid species in Lake Michigan. like abnormalities (exophytic lesions) on Lake Michigan. Can. J. Fish. Aquat. Sci. in press