U.S. Environmental Protection Agency
EPA Science Advisory Board
Background Information

Document NameDistribution of zooplankton on a seasonally hypoxic continental shelf.
Document AuthorQureshi, N.A.
Rabalais, N.N.
Short DescriptionPp 61-76 in N. N. Rabalais and R. E. Turner (eds.), Coastal Hypoxia: Consequences for Living Resources and Ecosystems. Coastal and Estuarine Studies 58, American Geophysical Union, Washington, D.C.
CategorySubgroup 1: Characterization of the Cause(s) of Hypoxia
Publication Year2001

Abstract: The vertical distribution of zooplankton was documented for a station in 20-m water depth through a seasonal decline of bottom-water dissolved oxygen concentration, and across a broad area of hypoxic bottom-water in mid-summer of two years.There was a seasonal progresson of zooplankton abundance with a spring peak and summer decline and a change in the relative proportion of taxa through the year. Copepods (adults and copepedites) were more abundant in the lower water column than in the upper water column (daytime samples) across all monthly samples at the 20-m station that experienced severe hypoxia for extended parts of the summer. Copepods were present at normal or negligible densities for the two sampling dates when the oxygen concentration was below 1 mg 1-1. Copepod nauplii, on the other hand, were reduced in abundance in the bottom water when the oxygen was less than 1 mg 1-1. Across the broad area of hypoxia in the two summer surveys, copepods and copepod nauplii were concentrated below the pycno-oxycline but above the bottom water where they were reduced when the bottom-water oxygen concentration was less than 1 mg 1-1. Meroplankton were concentrated above oxygen-deficient bottom waters in summer and were either delaying metamorphisis or were unable to recruit to the seabed. Bottom-water oxygen concentrations less than 1 mg 1-1 may have disrupted the daytime migration of copepods and copepod nauplii into that layer. The potential for indirect effects of altered zooplankton distributions and behavior on zooplankton food webs, energy transfer, trophic interactions, and secondary production, both pelagic and benthic, exist but are not known.