MEPS 230:103-112 (2002)  -  doi:10.3354/meps230103

Estimating the spatial extent of bottom-water hypoxia and habitat degradation in a shallow estuary

Christopher P. Buzzelli1,*, Richard A. Luettich Jr.1, Sean P. Powers1, Charles H. Peterson1, Jesse E. McNinch2, James L. Pinckney3, Hans W. Paerl1

1University of North Carolina at Chapel Hill, Institute of Marine Sciences, Morehead City, North Carolina 28557, USA
2College of William & Mary, Virginia Institute of Marine Science, Gloucester Point, Virginia 23062, USA
3Texas A & M University, Department of Oceanography, College Station, Texas 77843, USA
*Present address: Duke University Marine Laboratory, Beaufort, North Carolina 28516, USA. E-mail:

ABSTRACT: Bottom-water hypoxia (≤2 mg l-1 dissolved oxygen [DO]) greatly modifies the benthic habitat of estuaries, depending upon spatial extent, duration, and frequency. Bottom-water hypoxia often develops under conditions of density stratification, which inhibits vertical mixing, and warm temperatures, which enhance biological oxygen demand. Long-term, mid-channel data from the Neuse River Estuary in North Carolina permitted evaluation of how stratification and temperature combined to affect DO concentrations at the bottom. Salinity stratification (ΔS) and water temperature (T) explained respectively 30 and 23% of the variance in bottom-water DO concentrations. The amount of salinity stratification required to induce bottom-water hypoxia declined with increasing water temperature. About 80% of observed hydrographic profiles exhibited bottom hypoxia when ΔS exceeded 5 psu and T exceeded 20°C. Using cross-channel hydrographic surveys as verification, we derived a general set of methods to estimate the lateral extent of low-DO bottom water from mid-channel hydrographic profiles. The method involves cross-estuary and along-estuary extrapolation based on assumption of a flat oxycline. Occasional violation of this assumption resulted in modest overestimation in cross-channel extent of low DO. Application of this method produced estimates ranging from 0 to 116 km2 of bottom area (0 to 42% of the estuarine study area) exposed to hypoxia over all sample dates in summer 1997. The maximal bottom area exposed to hypoxia corresponded closely with an independent estimate of the area (100 km2) that experienced almost complete mortality of Macoma spp. clams, the key benthic resource for demersal fishes and crabs. Consequently, mid-channel hydrographic profiles taken along the mid-channel of the estuary can be employed to assess the spatial scale of bottom habitat degradation due to hypoxia.

KEY WORDS: Habitat degradation · Hypoxia · Dissolved oxygen · Stratification · Hydrography · Benthos · Macoma spp.

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