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Trends in Lake Superior benthos with particular emphasis on the amphipod Diporeia spp.
Citation:
Mehler, K., L. Burlakova, A. Karatayev, AND J. Scharold. Trends in Lake Superior benthos with particular emphasis on the amphipod Diporeia spp. State of Lake Superior Conference, Houghton, MI, October 09 - 12, 2018.
Impact/Purpose:
Benthic macroinvertebrates play an important role in the Great Lakes ecosystem, and they are useful as indicators of ecological condition. During the last few decades, benthic macroinvertebrate assemblages have undergone substantial changes in Lakes Ontario, Erie, Michigan and Huron, following introduction of invasive dreissenid mussels. Although dreissenids are present in Lake Superior, their populations have not expanded as in the other lakes, and they are restricted to limited areas of the lake. Still, their presence raises concerns about their possible spread and effects on other benthic invertebrates. To assess the status of benthic macroinvertebrate assemblages in Lake Superior, we conducted a lake-wide nearshore survey in 2016. The survey included 25 sites in the US nearshore zone (depths to approximately 100 m) that had been previously sampled in 1994, 2000 and 2003, plus 34 new, randomly selected stations in the US and Canadian nearshore. The nearshore zone was targeted because it is the region with highest densities of organisms, and the highest variability. Sample selection used a probability-based design to provide statistically-valid, unbiased estimates. Our results showed that the assemblage in 2016 was dominated by amphipods, oligochaetes, fingernail clams and midge larvae, similar to 1994-2003. Density of the amphipod Diporeia was significantly lower than in 1994, whereas densities of the other major groups were not significantly different from 1994. The study provides information on the variability in benthic invertebrate populations in the nearshore zone, and underlines the importance of frequent monitoring to detect trends in benthic macroinvertebrate assemblages and thus the health of the Great Lakes. The survey design developed and demonstrated in this study can be further refined for use by monitoring and management agencies, including the Great Lakes National Program Office and States, to obtain unbiased estimates of benthos and other ecological endpoints to inform management decisions.
Description:
A lake-wide nearshore benthic survey was conducted in Lake Superior in 2016 as part of the Cooperative Science and Monitoring Initiative to assess the current status of the macroinvertebrate community with particular focus on the amphipod Diporeia. Benthic samples were collected at 59 nearshore stations, 25 of which were previously sampled in 1994, 2000, and 2003. The most common benthic taxon by density in 2016 was Diporeia (48%), followed by Oligochaeta (32%), Sphaeriidae (16%), and Chironomidae (2%). However, at 22 of the 25 previously sampled stations we found a significant decline of Diporeia densities in 2016 compared to 1994 (paired t-test, P < 0.001), and at three of these 25 stations Diporeia completely disappeared. Oligochaeta, Sphaeriidae and Chironomidae declined at some of the stations in 2016 compared to 1994, but the changes were not significant. In contrast to other Great Lakes, Dreissena spp. have not established in Lake Superior and therefore cannot be a reason for Diporeia decline, suggesting that benthivorous fish predation or other environmental factors might have caused the changes. This decline in nearshore Diporeia density should be checked against inter-annual variation, and stresses the importance of annual monitoring of permanent stations to detect significant temporal trends.