Zooplankton Distribution and Species Composition Along an Oxygen Gradient in Puget Sound, WA
Julie E Keister1, Tim Essington2, Lingbo Li1, John K Horne2, Mei Sato3, Sandra L Parker-Stetter4 and Pamela Moriarty2, (1)University of Washington, School of Oceanography, Seattle, WA, United States, (2)University of Washington, School of Aquatic and Fishery Sciences, Seattle, WA, United States, (3)Oregon State University, College of Earth, Ocean, and Atmospheric Sciences, Corvallis, OR, United States, (4)Northwest Fisheries Science Center, NOAA, Seattle, WA, United States
Abstract:
Low dissolved oxygen (hypoxia) is one of the most pronounced, pervasive, and significant disturbances in marine ecosystems, yet our understanding of its effects is incomplete, particularly with respect to impacts on lower trophic levels. As part of a study of how hypoxia affects predator-prey relationships and energy flow through marine food webs, we are studying relationships between ocean chemistry and zooplankton in Puget Sound, Washington—a deep, seasonally hypoxic fjord in the Pacific Northwest that supports a productive and diverse pelagic community. From summer through fall in two years that differed in the timing and intensity of hypoxia, we conducted multi-frequency bioacoustic surveys, CTD casts, and depth-stratified zooplankton sampling to examine changes in distribution and species composition of animals in relation to oxygen concentrations. We exploited a natural gradient in oxygen along the axis of the fjord by sampling at moderately hypoxic and normoxic sites with otherwise similar hydrography and species composition to disentangle the effects of oxygen from changes in other environmental factors.
Our results support the hypothesis that zooplankton species composition and vertical distributions are altered by hypoxia, but only when examined at the species and life-stage level. Relatively few taxa showed clear responses to hypoxia, and bioacoustic backscatter data (which was dominated by adult euphausiids and amphipods) indicated that those taxa were not affected by the levels of hypoxia we observed. Examination of net tow data revealed more subtle changes, including behavioral avoidance of low oxygen by some copepods and young euphausiid life stages. Overall, the high species diversity and relatively low susceptibility of many zooplankton to hypoxia in Puget Sound may confer ecosystem resilience to near-future projected changes in this region.
