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GLERL 2000 Milestone Reports
GOAL: SUSTAIN HEALTHY COASTS
OBJECTIVE 1: Protect, Conserve, and Restore Coastal Habitats and Their Biodiversity
PM: Number of U.S. coastal regions with reduced introductions and impacts of non-indigenous species.
Milestone: Determine the role of zebra mussels on the decline of the amphipod Diporeia spp.
Scientist: Thomas F. Nalepa
Purpose: The purpose of this milestone was to determine the role of the zebra mussel, Dreissena polymorpha, in the dramatic decline of the amphipod Diporeia in the Great Lakes. Diporeia is the dominant benthic macroinvertebrate, and a keystone component of the food web in cold, deep regions of the lakes. It lives in the upper few centimeters of sediment and feeds on material settled to the bottom from the overlying waters. In turn, it is fed upon by most species of fish. In particular, Diporeia is a major diet item of many forage fish species that serve as prey for the salmonids; sport fishing for salmonids is a multi-million dollar industry in the Great Lakes. Also, some commercial species feed heavily on Diporeia (i. e., whitefish). Since earlier studies in southern Lake Michigan indicated Diporeia was declining coincidentally with the arrival of the zebra mussel in this portion of the lake, efforts were focused on establishing a temporal and spatial link between increasing mussel densities and decreasing densities of Diporeia.
Efforts: Intensive sampling (monthly) was initiated in the southeastern portion of the lake at four sites that represented locations where Diporeia populations had declined, were declining, or were present at healthy, high levels. Numbers of both Diporeia and zebra mussels were documented over a 3-year period. In addition, sedimentation traps were deployed to estimate the amount of food material settling to the bottom. Several of the sites have been sampled since the 1980s so data was available prior to the introduction of mussels into the lake in 1989. Also, laboratory experiments were conducted to determine if there were other potential reasons for the decline in Diporeia, such as an acute response to a contaminant.
Customers: Since Diporeia is an important food item for many fish species, declines in this organism will likely have an impact on the abundance, relative condition, spatial distribution, and feeding patterns of fish. Thus, managers involved with maintaining healthy, sustainable fish populations will be interested in the extent of the decline and the rate at which the decline is occurring. Also, the presence of Diporeia is an indicator of a healthy ecosystem as defined by the Great Lakes Water Quality Agreement.
Significance: Diporeia is a glacial-relict that has thrived in offshore habitats of the Great Lakes since the glaciers receded. Its great abundance and high lipid content provide an easily attainable and energy-rich food source for upper trophic levels. A clear link between the decline in Diporeia and zebra mussels would provide further evidence of the severe and negative ecosystem changes resulting from the introduction of nonindigenous species.
Success: At some of the sites, data was collected that showed a direct impact of zebra mussels on the abundance of Diporeia. Numbers of Diporeia started to decline the first year mussels were found, and the rate of decrease over subsequent years was directly related to mussel abundance. Potentially, mussels were outcompeting Diporeia for available food, yet sedimentation traps and sediment analysis indicated that food, as measured by organic carbon and chlorophyll, was still present. At other sites, Diporeia declined even though no mussels were found in the immediate vicinity. These sites were located in an area of high sediment deposition, which suggests other causative factors besides food limitation. Acute bioassay tests using sediments from all the sites did not show any acute response; that is, there was no mortality in Diporeia when exposed to these sediments for 28 days. Further short-term experiments (5 days) indicated that Diporeia actively avoided sediments from the site with the highest deposition rate relative to sediments from the other sites. This avoidance behavior was no longer evident when food material (diatoms) was placed as a layer over the sediment. All these results suggest that the decline in Diporeia is related to the activities of the zebra mussel, but the exact cause of the decline is not clear, and may be different depending upon the specific habitat conditions in a given area.
Next Steps: Research activities are planned to further investigate the negative relationship between Diporeia and zebra mussels. Also, surveys are planned in Lake Michigan to document the areal extent of the decline on a lake-wide basis.
Last updated: July 9, 2002 mbl