Trends in Early Spring Lake Superior Zooplankton Communities, 1989-2000

1. Trends in Early Spring Lake Superior Zooplankton Communities, 1989-2000 Owen Gorman and Lori Evrard U.S. Geological Survey Lake Superior Biological Station

2. Introduction • Multi-year, inshore zooplankton assessments in Lake Superior are rare. • Published studies on foodweb/trophic dynamics in lower Great Lakes demonstrate tight linkage between zooplankton and prey fish populations • Linkage between Lake Superior inshore zooplankton community dynamics and prey fish populations has not been previously demonstrated.

3. Objectives • Assess long-term trends in Lake Superior inshore zooplankton communities. • Compare zooplankton communities from different regions of Lake Superior. • Identify potential impacts of planktivore fish populations on zooplankton communities. • Utilize large set of zooplankton samples taken by USGS over a 12 year period (1989-2000) during spring prey fish assessment cruises.

4. Study Areas • Criteria for selection of ecoregions for analysis: • region with distinct geographic and environmental features • more than 5 continuous years of data • 5 or more sample locations per region • Compare inshore zooplankton communities from four ecoregions of Lake Superior • Ecoregions sampled: MNNS – Minnesota North Shore (8 sites, 11 yrs) APIS – Apostle Islands (9 sites, 12 yrs,) EKEW – Eastern Keweenaw Bay (5 sites, 10 yrs) WFBY – Whitefish Bay (6 sites, 6 yrs)

6. Methods • Sampling conducted in Spring, typically within 30 days following ice-out. • Zooplankton sampled with 50 cm, 63 F mesh conical net, towed vertically from bottom to top of water column. • Three, 1-ml subsamples from each sample were enumerated and results averaged. • Abundance was expressed as # zooplankters per m3 for the entire water column. • Abundance values for sites within an ecoregion were averaged for each year sampled.

7. Plankton sampling in Lake Superior

8. Dominant zooplankton species in Lake Superior spring samples • Calanoid copepods • Limnocalanus macrurus • Diaptomus sicilis • Cyclopoid copepods • Diacyclops thomasi • Other copepods and cladocerans are rare at this time of year.

9. Limnocalanus macrurus 2.0-2.5 mm length, overwinters as adults, spawns February-March; 1 generation/year. Adults and nauplii present in spring.

10. Diaptomus sicilis 1.5-2.0 mm length, overwinters as adults, reproduces March-May; 1 +? Generation/year. Adults and nauplii present in spring.

11. Diacyclops thomasi 1.0-1.5 mm length, overwinters as copopodites, reproduces June-September; 2 +? generations/year. Adults present in spring.

12. Total Density of Zooplankton by Ecoregion and Year

13. Relative Abundance of Zooplankton Taxa by Ecoregion and Year

14. Structure of Zooplankton Communities by Ecoregion and Year

15. Mean Size of Zooplankton by Ecoregion and Year

16. Sizes of dominant zooplankton and trophic index by region

17. Summary of zooplankton communities by ecoregion

18. Prey fish-zooplankton interaction

19. Conclusions - 1 • Spring zooplankton communities in Lake Superior were dominated by large calanoid copepods, indicating an oligotrophic condition. • There was strong year-to-year variation in zooplankter density and composition of zooplankton communities. • Inter-regional variation in plankton communities was evident; eastern regions were more consistently dominated by larger calanoids.

20. Conclusions - 2 • The Apostle Islands ecoregion, with the greatest densities of fish, had the lowest average zooplankton density, smallest average zooplanton size and a greater proportion of the smaller cyclopoid copepods. • A strong trophic interaction between fish and zooplankton was evident in the Apostle Islands region.

21. Time for questions