Influence of Wind Strength and Duration on Relative Hypoxia Reductions by Opposite Wind Directions in an Estuary with an Asymmetric Channel

Ping Wang,Lewis Linker,Harry Wang,Kyle Hinson

doi:10.3390/jmse4030062

Abstract

Computer model experiments are applied to analyze hypoxia reductions for opposing wind directions under various speeds and durations in the north–south oriented, two-layer-circulated Chesapeake estuary. Wind’s role in destratification is the main mechanism in short-term reduction of hypoxia. Hypoxia can also be reduced by wind-enhanced estuarine circulation associated with winds that have down-estuary straining components that promote bottom-returned oxygen-rich seawater intrusion. The up-bay-ward along-channel component of straining by the southerly or easterly wind induces greater destratification than the down-bay-ward straining by the opposite wind direction, i.e., northerly or westerly winds. While under the modulation of the west-skewed asymmetric cross-channel bathymetry in the Bay’s hypoxic zone, the westward cross-channel straining by easterly or northerly winds causes greater destratification than its opposite wind direction. The wind-induced cross-channel circulation can be completed much more rapidly than the wind-induced along-channel circulation, and the former is usually more effective than the latter in destratification and hypoxia reduction in an early wind period. The relative importance of cross-channel versus along-channel circulation for a particular wind direction can change with wind speed and duration. The existence of month-long prevailing unidirectional winds in the Chesapeake is explored, and the relative hypoxia reductions among different prevailing directions are analyzed. Scenarios of wind with intermittent calm or reversing directions on an hourly scale are also simulated and compared.

Highlights

Excessive nutrient and organic matter loads from the watershed and nutrient-driven algal blooms in the spring and summer are the main drivers of summer hypoxia and anoxia in the Chesapeake Bay estuary [1,2]
The model experiments in this work analyzed the impact of wind speed and duration on relative hypoxia reductions for opposite wind directions within the north-south oriented Chesapeake Bay
Besides presenting the reduction of summer hypoxia by wind’s mixing and destratification-related processes, this study further explores another process of hypoxia reduction by wind; this process is primarily associated with enhanced estuarine circulation bringing oxygen-rich seawater to the Bay via winds with down-estuary straining components, i.e., northerly and westerly winds

Summary

Introduction

Excessive nutrient and organic matter loads from the watershed and nutrient-driven algal blooms in the spring and summer are the main drivers of summer hypoxia and anoxia in the Chesapeake Bay estuary [1,2]. Destratification by wind can increase dissolved oxygen (DO) in deep water and reduce hypoxia [3,4,5]. With the north-south oriented (Bay head to mouth) Chesapeake Bay main channel, different wind directions cause different degrees of destratification and associated reduction in hypoxia [6,7,8,9,10]. Hypoxia describes a condition of depressed dissolved oxygen, defined as concentrations less than 2 mg/L [11,12], and is a primary concern for Chesapeake water quality management [13]. This study uses anoxic volume (the volume of water with DO levels ≤ 0.2 mg/L) to measure the extent of the

Methods

Results

Conclusion