Zooplankton and profundal macroinvertebrate responses to hypolimnetic oxygenation in North Twin Lake, Washington

Abstract Cross BK, Taylor TN, Moore BC, Lanouette BP. 2021. Zooplankton and profundal macroinvertebrate responses to hypolimnetic oxygenation in North Twin Lake, Washington. Lake Reserv Manage. 37:275–288. Hypolimnetic oxygenation (HO) has been used to address lake eutrophication and fish habitat limitations, but details on zooplankton and profundal macroinvertebrate community responses are required to fully understand the trophic linkages and impacts. Compared to adjacent South Twin Lake (an unoxygenated reference), zooplankton and profundal macroinvertebrate densities and distributions were altered by HO in North Twin Lake. Overall, temporal trends in zooplankton density were similar between lakes despite oxygenation, suggesting annual and seasonal influences remained a primary factor affecting their density trends. During the day, Chaoborus density was significantly greater in the water column of unoxygenated South Twin Lake compared to oxygenated North Twin Lake (P < 0.0001) as the oxygenated hypolimnion no longer provided refuge from fish predation. Daphnia and copepod densities were greater in North Twin Lake during oxygenation compared to South Twin Lake (P = 0.0011 and P = 0.0005), which may reflect increases in zooplankton habitat availability and reduced predation from Chaoborus in response to HO. With higher levels of dissolved oxygen near the sediment–water interface in North Twin Lake, larval Chaoborus and chironomid densities in the benthic sediments were significantly higher compared to South Twin Lake (P < 0.0001 in both cases). Vertical distribution of zooplankton likely reflected some balance between habitat availability, energetic costs, and predation risk. Lakes with different fish species assemblages may not respond similarly, so managers should reflect on lake similarities between North Twin Lake and systems they manage to understand potential effects of HO and how those effects align with management objectives.