Near-shore, shallow waters in the Chesapeake Bay periodically experience episodes of anoxia or severe hypoxia during summer. In order to examine the severity and temporal pattern of hypoxia, and environmental factors that may lead to such episodes, dissolved oxygen, salinity and temperature were measured at 15-min intervals during the summers of 1987 and 1988 in a western shore oyster bed. Bottom dissolved oxygen concentrations averaged lower at a 4-m site than at a 2-m site. At the 4-m site, dissolved oxygen concentrations dropped below 2 mg l-1 during approximately 40% of days and below 1 mg l-1 during approximately 10% of days each summer. However, diel fluctuations in oxygen concentrations were sufficiently large that even on days of the most severe oxygen minima, dissolved oxygen concentrations always reached or exceeded a level tolerable by most estuarine organisms during some part of the day. During episodes of severe hypoxia on the western shore of the Chesapeake Bay, oxygen minima (1) coincided with increased salinities and ebb tides, (2) were preceded by winds from the S-SSE (minima of ≤0·5 1 mg l-1) or SW (minima of 0·6-1·1 mg l-1), and (3) were reached during 22.00-06.00 hours. Severe hypoxia at the study site therefore appeared to result from intrusions of bottom water, which were most effectively driven by southerly winds. Tidal currents were required to provide the final force that brought deep water close to shore. The diel pattern of intrusions is most likely caused by winds, which were often either too strong or had too weak a southerly component to permit intrusions (without near-shore mixing and reaeration) to occur during afternoon-early evening hours, and the diel periodicity of tides. Short-term fluctuations in temperature and salinity were not as great as fluctuations in dissolved oxygen when compared to seasonal fluctuations of each parameter. For many organisms, short-term fluctuations in temperature and salinity may therefore be less likely to influence the habitability of areas such as the near-shore Chesapeake Bay than are similar duration fluctuations in dissolved oxygen concentrations. © 1990.
