Biomass, Condition of Western Lake Erie Dreissenids

Tom Nalepa

Collaborators
Steve Lozano, Dave Fanslow, Greg Lang, Great Lakes Environmental Research Laboratory (GLERL home)
Don Schloesser, US Geological Survey (USGS web site)

The introduction and expansion of dreissenid mussels (zebra and quagga mussel) has led to large-scale changes in nutrient dynamic and food webs in the Great Lakes. A fundamental need in assessing present and potential impacts is knowledge of dressenid abundance and biomass, and rates of population change. The latter can be predicted by examining trends over time, and by assessing the relative physiological condition of the population. This project will estimate dreissenid biomass and assess relative physiological condition of populations in western Lake Erie. Comparisons will be made to the condition of mussel populations in other regions of the Great Lakes.

Scientific Rationale Recent and unexpected increases in water-column phosphorus levels and the spatial extent of the anoxic zone in the central basin of Lake Erie have prompted theories concerning potential causes. One theory is related to expanding populations of dreissenids in the western basin and their ability to alter nutrient dynamics through excretion and filtering activities. These activities promote blooms of blue-green algae and excessive growth of benthic algae and plants, which eventually settle into the central basin and decompose, utilizing oxygen in the process. Assessing the potential for mussels to cause such large-scale changes is dependent upon accurate estimates of standing stocks. In 2003, triplicate grab samples were collected at 60 sites throughout the western basin by the USGS (contact: D. W. Schloesser). While mussels in these samples can be used to obtain density estimates and spatial distributions, length-weight regressions are needed to obtain dry-weight biomass. Because of large differences in mussel sizes and weights, accurate projections of mussel impacts must be based on estimates of biomass. We propose to determine length-weight relationships for both zebra and quagga mussels from around the basin, and use these relationships to calculate overall biomass. Measures of physiological condition will also be determined. These measures will provide insights into the state of populations within the basin. For comparison, the physiological condition of zebra and quagga mussels will also be determined in zebra and quagga mussels at several depths in Lake Michigan.

2005 Plans

In 2005, we propose to analyze all the samples collected in 2004. We collected mussels (zebra and quagga mussels) at five sites in western Lake Erie in spring (May), summer (July), and fall (October). We also collected monthly at four sites in Lake Michigan. Lengths and weights (total wet and dry tissue) of about 25 mussels were obtained immediately after collection at each site. Variables to be measured in 2005 include total lipid, C:N ratio, and glycogen. To determine mussel abundances in the western basin, samples (Ponar grabs) were collected at 30 sites in May, 2004. The processing of these samples (i.e. counting mussels) will be completed in mid-2005.

2004 Progress

Models to determine the impact of dreissenids on the western Lake Erie ecosystem are dependent upon accurate estimates of dreissenid biomass, and presently, there are no accurate estimates of biomass in this portion of the lake. Biomass is calculated from abundances, size-frequencies, and length-weights. The goal of this project is to determine these three variables for the population in the western basin.

We established 60 sites throughout the western basin to sample for dreissenids. These were the same sites sampled by GLSC-USGS in 2003. In spring 2004, 30 sites were sampled by GLSC and 30 sites were sampled by GLERL. These samples will provide estimates of dreissenid abundances and size frequencies. In addition, dreissenids (zebra and quagga mussels) were collected at five sites in May, July, and October. The sites were located in different regions of the basin with varying environmental conditions: Detroit River plume, Maumee River plume, deep mid-lake, shallow eastern islands, and south mixed plume. For both zebra and quagga mussels, length-weight, lipid content, and C:N ratio will be measured at each of the sites. Length-weight will be used to determine biomass, and all three variables will be used to assess physiological condition. For comparison, zebra and quagga mussels were also collected on a monthly basis (May to October) at a 25-m site and a 45-m site in Lake Michigan off Muskegon. The same three physiological variables will be measured on these Lake Michigan mussels.

Last updated 2005-06-09 ahc