Comparison of 5 benthic samplers to collect burrowing mayfly nymphs (Hexagenia spp.:Ephemeroptera:Ephemeridae) in sediments of the Laurentian Great Lakes

The recent return of burrowing mayfly nymphs (Hexagenia spp.) to western Lake Erie of the Laurentian Great Lakes has prompted a need to find a sampler to obtain the most accurate (i.e., highest mean density) and precise (i.e., lowest mean variance) abundance estimates of nymphs. The abundance of burrowing nymphs is important because it is being used as a measure of ecosystem health to determine management goals for fisheries and pollution abatement programs for waters in both North America and Europe. We compared efficiencies of 5 benthic grab samplers (Ponar, Ekman, petite Ponar, Petersen, and orange-peel) to collect nymphs from sediments of western Lake Erie and Lake St. Clair. Samplers were used at one site with soft substrates in both lakes in 1997 (Ponar, Ekman, petite Ponar, and Petersen) and 1998 (Ponar and Ekman), and at one site with soft and one site with hard substrates in Lake St. Clair in 1999 (Ponar and orange-peel). In addition, the Ponar, Ekman, and Petersen samplers were used at one site with soft substrates of western Lake Erie in 2000 to examine the causes of differences among samplers. The Ponar was more accurate than the other samplers; it collected the highest densities of nymphs for 31 of 32 date and site comparisons. In soft substrates, the order of decreasing overall densities was: Ponar>Petersen>petite Ponar>Ekman in western Lake Erie and Ponar>Petersen> Ekman>petite Ponar in Lake St. Clair in 1997, Ponar>Ekman in both lakes in 1998, and Ponar>orange-peel in Lake St. Clair in 1999. In hard substrates, the Ponar was more accurate than the orange-peel in Lake St. Clair in 1999. Precision of the Ponar was generally greater than the Ekman, petite Ponar, and Petersen but similar to the orange-peel. Higher densities of nymphs obtained with the Ponar than other grabs are attributed to its relatively heavy weight, which allows it to sample deeper in sediments than the Ekman and petite Ponar. Also, the Ponar has a screened top, which allows it to minimize hydraulic shock waves more than the Petersen, and uniform sides, which allow it to sample nymphs more uniformly through sediments than the orange-peel. We recommend that future estimates of burrowing mayfly densities be obtained with a standard Ponar sampler similar to the one used in our study because it will yield the most accurate and precise measurements of burrowing mayfly nymphs such as Hexagenia spp.