Mapping the spatial distribution of the biomass and filter-feeding effect of invasive dreissenid mussels on the winter-spring phytoplankton bloom in Lake Michigan

1. The effects of the invasive bivalves Dreissena polymorpha (zebra mussel) and Dreissena rostriformis bugensis (quagga mussel) on aquatic ecosystems, including Lake Michigan, are a topic of current interest to scientists and resource managers. We hypothesised that the winter-spring phytoplankton bloom in Lake Michigan is reduced at locations where the fraction of the water column cleared per day by Dreissena filter feeding approached the net growth rate of phytoplankton, when the water column was not stratified. To test this hypothesis, we compared the spatial distribution of Dreissena filter-feeding intensity (determined from geostatistical modelling) to the spatial distribution of chlorophyll (determined from satellite remote sensing). 2. To map the spatial distribution of Dreissena biomass and filter-feeding intensity, we developed a geostatistical model based on point observations of mussel biomass measured in Lake Michigan in 1994/1995, 2000, 2005 and 2010. The model provided fine-scale estimates of the spatial distribution of biomass for the survey years and provided estimates, with their uncertainty, of total biomass lakewide and within subregions. The approach outlined could be applied more generally to map the distribution of benthic biota in lakes from point observations. 3. Total biomass of Dreissena in Lake Michigan, estimated from the geostatistical model, increased significantly over each five-year period. The total biomass in units of 10(6)kg ash-free dry mass (AFDM) (with 90% confidence interval) was 6 (4-8) in 1994/1995, 18 (14-23) in 2000, 408 (338-485) in 2005 and 610 (547-680) in 2010. From 1994/1995 to 2005, increases were observed in all regions of the lake (northern, central and southern) and in all depth zones (<30, 30-50, 50-90 and >90). However, from 2005 to 2010, for depths of <50m, biomass declined in the northern region, remained constant in the central region and increased in the southern region; biomass continued to increase in all three lake regions for depths >50m. 4. The filter-feeding intensity of Dreissena exceeded the benchmark spring phytoplankton growth rate of 0.06day(-1) in 2005 for depths <50m (lakewide). In 2010, the filter-feeding impact exceeded 0.06day(-1) within depths <90m (lakewide), which greatly increased the spatial area affected relative to 2005. A regression analysis indicated a significant relationship between the reduction in satellite-derived chlorophyll concentration (pre-D.r.bugensis period to post-D.r.bugensis period) and spatially co-located filter-feeding intensity (fraction of water column cleared per day) during periods when the water column was not stratified (December to April).