Contributions of Lower Trophic Level
Dynamics, Dreissenid Mussels, and
Physical Processes to Lake Erie Changes
Joseph D. Conroy1, William J. Edwards2,
and David A. Culver1
1 – Dept. of EEOB, The Ohio State University
2 – Dept. of Biology, Niagara University
Lake Erie Basin Forcers
Geologic
Processes
Human
Population
Ecosystem
Changes
Result:
¾
¾
Modified, interdependent system
Current changes mediated through each of these boxes
How Do We Observe Ecosystem
Change?
• Lake Erie continually undergoes change
¾
¾
¾
¾
Nutrient loading
Plankton dynamics
Exotic species
Land use
• We are particularly interested in lower trophic level
changes
¾
¾
Basis of fish production
Allows quantification of many individual processes
• Developed LEPAS (1995)
Nutrient Loading Changes
External Phosphorus Loading (kilotonnes)
30
28
Dolan 1993
personal comm. 2002
Estimated from Sly 1976
26
Gopalan et al. 1998
24
22
20
18
16
14
12
Target Loading (11 kilotonnes)
10
8
6
4
2
0
1900
1910
1920
1930
1940
1950
Year
1960
1970
1980
1990
2000
How are these changes in nutrient loading
reflected in the lower trophic levels?
Phytoplankton – Western Basin
6
Munawar & Munawar (1976)
Culver
5
Biomass (mg/L)
Devault & Rockwell (1986)
4
3
Makarewicz (1993)
2
1
0
1965
1970
1975
1980
1985
Year
1990
1995
2000
2005
Phytoplankton – Central Basin
6
5
Biomass (mg/L)
Culver
4
Munawar & Munawar (1976)
3
Devault & Rockwell (1986)
2
Makarewicz (1993)
1
0
1965
1970
1975
1980
1985
Year
1990
1995
2000
2005
Phytoplankton – Eastern Basin
6
Biomass (mg/L)
5
Culver
**NOTE: Sample analysis from
1998-2002 are incomplete.
4
3
Munawar & Munawar (1976)
2
Makarewicz (1993)
Devault & Rockwell (1986)
1
0
1965
1970
1975
1980
1985
Year
1990
1995
2000
2005
Zooplankton – Western Basin
0.35
Bean (1970)
CLEAR
0.30
Biomass (mg/L)
0.25
0.20
Culver
Makarewicz (1993)
0.15
0.10
0.05
0.00
1965
1970
1975
1980
1985
Year
1990
1995
2000
2005
Zooplankton – Central Basin
0.35
0.30
Biomass (mg/L)
0.25
Culver
0.20
Bean (1970)
Makarewicz (1993)
0.15
0.10
0.05
0.00
1965
1970
1975
1980
1985
Year
1990
1995
2000
2005
Zooplankton – Eastern Basin
0.35
0.30
Biomass (mg/L)
0.25
0.20
0.15
Bean (1970)
Makarewicz (1993)
0.10
Culver
0.05
0.00
1965
1970
1975
1980
1985
Year
1990
1995
2000
2005
Hypotheses for Change
• External loading does not completely explain
changes
• Other possibilities:
¾ Increased
internal loading from dreissenids
¾ Changes in dreissenid species composition has
altered internal loading
¾ Interaction of weather and physics drives the
system
¾ Combination or interaction of all these
Ecosystem modeling: investigating the
causes of change
WESTERN BASIN
CENTRAL BASIN
ALGAL
BLOOMS
MORE
ALGAL
BLOOMS
NUTRIENT ENRICHED
WATER
TRANSPORT
MIXING
NITROGEN PHOSPHORUS
SINK AND
DIE
CONSUME
BBL
MATERIAL
Photo from OSU Dept. of Agriculture
DECOMPOSITION BY
BACTERIA Æ “Dead Zone”
FORMATION
Calibrating Model
•
•
•
•
Plankton biomass data
Dreissenid distribution, size-frequency data
Information on “Dead Zone” extent and duration
Dreissenid excretion data
¾
Differences between zebra and quagga mussels
PO4-P Excretion (ug/mg dry weight/mussel/day)
• Physical transport and mixing data
0.7
Zebra Mussel
Quagga Mussel
0.6
*
Indicates Significant Difference
at the α=0.01 level.
*
0.5
*
0.4
*
0.3
*
0.2
*
0.1
0.0
<10mm
10-15mm
15-20mm
Size Class
20-25mm
25-30mm
11
Zebra Mussel
Quagga Mussel
10
9
*
*
p-value = 0.01
8
7
6
5
4
3
2
1
0
<10mm
10-15mm
15-20mm
Size Class
20-25mm
25-30mm
PO4-P Excretion (ug/mg dry weight/mussel/day)
NH3-N Excretion (ug/mg dry weight/mussel/day)
Dreissenid Excretion –
Quagga vs. Zebra Mussel
0.7
Zebra Mussel
Quagga Mussel
0.6
*
Indicates Significant Difference
at the α=0.01 level.
*
0.5
*
0.4
*
0.3
*
0.2
*
0.1
0.0
<10mm
10-15mm
15-20mm
Size Class
20-25mm
25-30mm
Physical Mixing Data – SCAMP
Implications for “Dead Zone”
• Ecosystem model allows testing of“Dead
Zone” formation hypotheses
• But, also must take into account:
– Water column mixing in Spring and Autumn
– Storm frequency
– Wind speed/direction
– Precipitation (nutrient loading)
– Temperature
Conclusions
• Greater appreciation for
inherent complexity of
large lake ecosystems
• Complexity depends on:
¾
¾
¾
¾
¾
Humans
Exotics
Economic demands
Ecosystem modification
Climate Change
Lake Erie is anything but
homogeneous!
14 APRIL 2003
Image from LandSat 7 Image Server: http://dmc.ohiolink.edu/GEO/ls7/
Acknowledgements
Ohio Department of Natural Resources
United States EPA
Ohio SeaGrant
Lake Erie Protection Fund
Canada National Water Research Institute
and Fisheries and Oceans Canada
Many colleagues for their collaboration and helpfulness
on associated projects