Diverse taxa of zooplankton inhabit hypoxic waters during both day and night in a temperate eutrophic lake

Abstract  As the frequency and intensity of hypoxic events increase in both fresh and marine waters, understanding the ecological effects of hypoxia becomes more important. The extant literature reports varying effects of hypolimnetic hypoxia on the vertical distribution and diel vertical migration(DVM)of zooplankton, with some but not all taxa reported to avoid hypoxic waters. We studied the vertical distribution and DVM of diverse zooplankton taxa throughout three seasons over 2 years (2014 and 2015) in Lacamas Lake, WA, USA. We observed hypoxia (<2 mg L −1 dissolved oxygen) in the hypolimnion of Lacamas Lake during five of six sampling periods,with zooplankton populations often exhibiting ‘h-metric’ values (defined as the proportion of a zooplankton population residing within hypoxic waters) ranged from 0.14 to 1.00, with an overall mean of h=0.66. Moreover, we observed a lack of DVM in most zooplankton taxa on most occasions. Our findings indicate both community-level and taxon-specific zooplankton tolerances to hypoxia, although the exact mechanisms at play remain to be fully elucidated. Nevertheless, the common residency in hypoxic waters and the lack of DVM by diverse zooplankton taxa that we observed likely have implications for food web dynamics in Lacamas Lake and other water bodies.