Living Assemblages from the “Dead Zone” and Naturally Occurring Hypoxic Zones

Abstract

“Dead zones” are man-made hypoxic zones (oxygen concentrations of less than 2 mg l⁻¹) that include the seafloor and the water column immediately above it. Such low oxygen concentrations result from the aerobic decay of organic debris from locally elevated primary production that is fueled by anthropogenic nutrient inputs to the coastal ocean (e.g., fertilizer runoff). Other benthic environments with persistent hypoxia unrelated to human activities have been known for decades. Silled basins and cold seeps are examples of natural hypoxic regions. Using microscopy, we have documented disparate, diverse, symbioses-dominated assemblages of protists as well as nematodes from all three of these hypoxic environments. The “dead zone” off the Mississippi River is dominated by flagellates and ciliates. In contrast, Monterey Bay cold seeps, while hosting biomass roughly equivalent to that from the Mississippi “dead zone,” had assemblages dominated by nematodes. The assemblage from Santa Barbara Basin was approximately an order of magnitude higher in biomass and was dominated by foraminifera. In all three hypoxic environments, symbioses were a prominent component of the assemblage and are likely the factor that allows these assemblages to thrive.