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OBJECTIVE: PROTECT, CONSERVE, AND RESTORE COASTAL RESOURCES, HABITATS, AND THEIR BIODIVERSITY
GOAL: SUSTAIN HEALTHY COASTS
PM: Number of watersheds examined for multiple stressors and their effects, using selected species and regions.
Milestone: Prepare final report on 1993-94 EMAP benthic environmental
indicators of Lake Michigan and make it available to Lake Michigan management
agencies and interest groups.
Scientist: T. Nalepa
(GLERL; 734-741-2285)
Since EMAP is a multi-institutional program, it was decided that a final
report on just the benthic environmental indicators would be of minimal
use. In lieu of a final report, all the benthic macroinvertebrate data
collected as part of the EMAP Program was provided to the EPA for synthesis
by the EPA into an overall Great Lakes EMAP report. It was agreed that
widespread dissemination of the benthic data and the related interpretation
will be provided by GLERL via a series of papers published in the scientific
literature. The first paper, which is in press, documents trends in macroinvertebrate
populations in the southern basin of the lake through 1993. A second paper,
which is in preparation, focuses on the distribution and physiological
condition of the amphipod Diporeia throughout the entire lake.
Diporeia is a surface-feeding detritivore which ingests organic
material freshly-settled from the pelagic region. This species, in turn,
is fed upon by most species of fish during at least one stage in their
life cycle. Consequently, it is an important pathway by which energy produced
in the pelagic region is made available to upper trophic levels.
Synopsis of Observations:
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Densities were higher on the west side of the lake than the east side. Upwelling events are more frequent on the west side during the period of stratification (June to September). Subsequently, this area is characterized by cooler water temperatures and presumably greater productivity, conditions which tend to favor higher abundances. In contrast, low densities were found in the southeast and northeast portions of lake, and in Green Bay. As found previously, zebra mussels likely caused numbers to decline in the southeast, and low densities in Green Bay are likely a result of warm temperatures and enriched conditions, but reasons for low densities in the northeast are not clear. Lipid content and weight per unit length were significantly lower in the southern portion of the lake than in the northern portion. We suspect that higher values in the north are related to the greater occurrence of diatoms, a rich food source readily fed upon by Diporeia. However, the evidence also suggests that both lipid content and weight per unit length have recently declined in the south. The low lipid content of mature Diporeia in the south (mean = 20 % of dry weight) is near the minimum considered for successful reproduction. Estimates of both densities and physiological condition will provide a baseline to assess future changes in the Diporeia populations resulting from environmental stressors such as zebra mussels. |
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Figure 1. Diporeia densities were highly variable throughout the lake, with areas of high and low densities clearly defined. |