Trends in Early Spring Lake Superior Zooplankton Communities, 1989-2000 Owen Gorman and Lori Evrard...
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Transcript of Trends in Early Spring Lake Superior Zooplankton Communities, 1989-2000 Owen Gorman and Lori Evrard...
![Page 1: Trends in Early Spring Lake Superior Zooplankton Communities, 1989-2000 Owen Gorman and Lori Evrard U.S. Geological Survey Lake Superior Biological Station.](https://reader031.fdocuments.us/reader031/viewer/2022012919/56649cd65503460f9499cfe3/html5/thumbnails/1.jpg)
Trends in Early Spring Trends in Early Spring Lake Superior Lake Superior Zooplankton Zooplankton
Communities, 1989-Communities, 1989-20002000
Owen Gorman and Lori Evrard
U.S. Geological Survey
Lake Superior Biological Station
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IntroductionIntroduction• 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.
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ObjectivesObjectives
• 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.
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Study AreasStudy 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)
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MethodsMethods
• Sampling conducted in Spring, typically within 30 days following ice-out.
• Zooplankton sampled with 50 cm, 63 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.
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Plankton sampling in Lake Superior
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Dominant zooplankton species in Dominant zooplankton species in Lake Superior spring samplesLake Superior spring samples
• Calanoid copepods– Limnocalanus macrurus– Diaptomus sicilis
• Cyclopoid copepods– Diacyclops thomasi
• Other copepods and cladocerans are rare at this time of year.
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Limnocalanus macrurus 2.0-2.5 mm length, overwinters as adults, spawns February-March; 1 generation/year. Adults and nauplii present in spring.
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Diaptomus sicilis 1.5-2.0 mm length, overwinters as adults, reproduces March-May; 1 +? Generation/year. Adults and nauplii present in spring.
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Diacyclops thomasi 1.0-1.5 mm length, overwinters as copopodites, reproduces June-September; 2 +? generations/year. Adults present in spring.
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Total Density of Zooplankton by Ecoregion and YearTotal Density of Zooplankton by Ecoregion and Year
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Relative Abundance of Zooplankton Taxa by Ecoregion and YearRelative Abundance of Zooplankton Taxa by Ecoregion and Year
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Structure of Zooplankton Communities by Ecoregion and YearStructure of Zooplankton Communities by Ecoregion and Year
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Mean Size of Zooplankton by Ecoregion and YearMean Size of Zooplankton by Ecoregion and Year
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Sizes of dominant zooplankton and Sizes of dominant zooplankton and trophic index by regiontrophic index by region
Ecoregion Mean total lengths Trophic index
L. macrurus D. sicilis D. thomasi
MNNS 1.98 1.60 1.18 3.17
APIS 1.85 1.57 1.07 1.94
EKEW 2.32 2.06 1.44 3.16
WFBY 2.41 1.71 1.34 3.40
Overall 2.14 1.73 1.26
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Summary of zooplankton communities Summary of zooplankton communities by ecoregionby ecoregion
Relative density
Dominant species
Relative size and biomass
Relative trophic state
Planktivore fish population
MNNS High D. sicilis Moderate >oligotrophic Smaller
APIS Moderate D. sicilis-
D. thomasi
Smallest oligotrophic- mesotrophic
Larger
EKEW High D. sicilis Large >oligotrophic Smaller
WFBY High D. sicilis Largest >oligotrophic Smaller
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Prey fish-zooplankton interactionPrey fish-zooplankton interaction
0
2
4
6
8
10
12
1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000year
tro
ph
ic s
tate
in
de
x/g
m b
iom
ass APIS Fish-WI
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Conclusions - 1Conclusions - 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.
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Conclusions - 2Conclusions - 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.
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Time for questionsTime for questions
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