Long-term trends in Lake Ontario's benthic macroinvertebrate community from 1994–2008

The benthic macroinvertebrate community of Lake Ontario was assessed through a lakewide survey in 2008. <i>Diporeia</i> was very rare throughout the lake at all depths in 2008, and only four of 52 locations had densities &gt;100 m⁻², all of them at depths &gt;90 m. The maximum density of <i>Diporeia</i> found at any location was at 257 m⁻² in 2008, which can be compared to maximum densities of 13,280 m⁻² observed in 1994. Lakewide <i>Diporeia</i> abundance declined with an additional order of magnitude from an average of 342 m⁻² in 2003 to 21 m⁻² in 2008. The Quagga Mussel (<i>D. rostriformis bugensis</i>) dominated the benthic macroinvertebrate community in 2008, comprising over 70% of the density and 98% of the biomass. No Zebra Mussels were identified in the 2008 samples. Quagga Mussels, Oligochaetes and Chironomids were most abundant between 31 and 90 m. Sphaeriids were rare at all depths, but were more abundant at sites deeper than 90 m. Between 2003 and 2008, lakewide <i>Dreissena</i> abundance declined by 43% primarily due to significant declines in the 10–30 m depth region (from 6500 m⁻² to 900 m⁻²). <i>Dreissena</i> did not decline significantly in the 30–90 m or over 90 m depth regions. The 2008 survey revealed a continued decline in <i>Diporeia</i> and Sphaeriid Clams, a replacement of Zebra Mussels by Quagga Mussels, and a decline in Quagga Mussels at depths shallower than 30 m. Oligochaetes and chironomids showed no significant changes since the 1990s.