Status of the amphipod Diporeia spp. in Lake Superior, 2006-2016

The amphipod Diporeia spp. has historically been an important component of the benthic food web of the Laurentian Great Lakes. The Great Lakes Water Quality Agreement included its population density as an indicator of ecological condition for Lake Superior, with target values of 220-320 m^-2 in nearshore areas (≤100 m depth) and 30-160 m^-2 in offshore areas (>100 m). To assess the status of Diporeia in Lake Superior, we used a probability-based lake-wide survey design to obtain estimates of Diporeia density and biomass in 2006, 2011 and 2016. A PONAR grab sampler was used to collect Diporeia at 50-53 sites each year, with approximately half in the nearshore (<100 m depth) region of the lake and half in the offshore. The mean area-weighted lake-wide density was 395 ± 56 (SE) m^-2 in 2006, 756 ± 129 m^-2 in 2011, and 502 ± 60 m^-2 in 2016. For all years, both density and biomass were greater in the nearshore than in the offshore stratum. The densities for 2006-2016 were 3-5 times higher than those reported from a lake-wide survey conducted in 1973 by the Canada Centre for Inland Waters. The severe declines in Diporeia populations observed in the other Great Lakes during recent decades have apparently not occurred in Lake Superior. Further research is needed to understand spatial and temporal variability of Diporeia populations in Lake Superior to enhance the utility of Diporeia density as an indicator of benthic condition.