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GLERL 2001 Milestone Reports
GOAL: Sustain Healthy Coasts
OBJECTIVE 1: Protect, Conserve, and Restore Coastal Habitats and their Biodiversity
PM:Number of coastal regions with reduced introductions and impacts of nonindigenous species
Milestone: Distribution of the amphipod, Diporeia spp., throughout Lake Michigan and changes over the past 5 years since the introduction of the zebra mussel.
Scientist: T. F. Nalepa, GLERL
Purpose: The purpose of this milestone was to determine distributions of the important amphipod Diporeia throughout Lake Michigan and assess changes that had occurred over the past 5 years. Diporeia is the dominant benthic macroinvertebrate and a keystone component of the food web in the offshore waters of most of the Great Lakes. From earlier studies, we knew that this species was declining in the southern basin of the lake, and our efforts in 2000 focused on determining whether it was also declining in other lake regions.
Efforts: In July/August 2000, 160 sites were sampled throughout Lake Michigan. Most of the 85 sites that were sampled in 1994/95 were re-sampled. Additional sites established along transects between 20 and 80 m water depth were also sampled. These depths correspond to the depths at which most fish species that feed on Diporeia are found. A large number of sites established in northern Green Bay and Grand Traverse Bay were also sampled. Both bays are commercially fished for lake whitefish, a species that relies heavily on Diporeia as a food source.
Customers: Managers involved with maintaining healthy, sustainable fish populations will be interested in knowing the lakewide status of Diporeia. Besides the above-mentioned lake whitefish, other species such as sculpin, bloater chub, smelt, and alewife also feed on Diporeia. These species provide the forage base for the salmonines which form the basis of a large sport fishery. In addition, the presence of Diporeia is an indicator of a healthy ecosystem as defined by the Great Lakes Water Quality Agreement.
Significance: A lakewide decline of Diporeia would have profound implications for the Lake Michigan web. Diporeia is an easily attainable, high-caloric food source. With the loss of this species, fish would need to feed on alternate food items, thereby creating competitive interactions and changing trophic transfer efficiencies within the Lake Michigan ecosystem.
Success: We successfully sampled at all 160 sites, and collected individuals have been picked, counted, and tabulated. Between 1994/95 and 2000, the Diporeia population in Lake Michigan declined by 68 %. Overall declines within various water depth intervals (meters) were as follows: 71 % at < 30 m, 84 % at 31-50 m, 60 % at 51-90 m, and 44 % at >90 m. Decreases were noted in most all portions of the lake, but the greatest decline occurred in the north (Figure 1). Diporeia was rare or completely gone from Green Bay, Grand Traverse Bay, and shallower regions (< 50 m) of the northern basin. As noted, these areas have a large commercial lake whitefish fishery, and the decline of Diporeia is already having an impact on lake whitefish distribution, condition, and growth.
Figure 1: The left hand image shows the density of Diporeia in Lake Michigan in 1994&1995 while the right hand image shows the density in 2000.
Next Steps: Further research activities will focus on examining causes for the decline in Diporeia. The spatial decline of Diporeia is coincidental with the expansion of zebra mussels in the lake, but the exact reason for the negative response of Diporeia to zebra mussels has yet to be determined.
Last updated: August 8, 2002 mbl