Linking watershed loading and basin-level carrying capacity models to evaluate the effects of land use on primary production and shellfish aquaculture

Aquaculture production of hard clams, Mercenaria mercenaria, in the lower Chesapeake Bay, Virginia, U.S.A., has increased dramatically within the last decade. In recent years concern has been raised that some growing areas may be approaching the exploitation carrying capacity for clam production. Preliminary calculations indicate that large-scale intensive clam aquaculture may be controlling nutrient and phytoplankton dynamics in this system. To date, carrying capacity models have not been applied to this system, but we are in the process of building models for that purpose. Moreover changing land use in the watersheds surrounding the clam-producing areas raises the need for an improved understanding of how these changes will affect water quality, primary production and shellfish production. We describe an ongoing project linking a watershed-based loading model with a physical transport-based water quality model to simulate primary production and predict carrying capacity for clam aquaculture. Extensive calibration and verification of the water quality model has demonstrated its utility for simulating primary production and water quality parameters in the Chesapeake Bay. In our present efforts, watershed loading models have been developed and tested for predicting both surface and groundwater inputs into the coastal waters. We are currently coupling the water quality and watershed loading models, and developing clam physiology and population-level sub-models. Also, under development is a sediment deposition/resuspension sub-model. Each of these components will be linked to estimate exploitation carrying capacity for clam production in this system. Our goal is to use the coupled models to predict how varying land use scenarios impact water quality, primary production and shellfish carrying capacity of coastal waters

Assessment of Hypoxia and its Relationship with Nutrient Loads and Wind, and Implication to the Chesapeake Bay Management

Statistics are applied to analyze the correlations of summer hypoxia in the Chesapeake Bay with watershed input and wind conditions based on nearly three decades of monitoring data. The Pearson correlation coefficients indicate that the averaged summer hypoxia has strong positive correlation with watershed nutrient load and discharge, and moderate negative correlation with summer average wind speed. Nutrient inputs and the subsequent decay of organic matter are the primary factor that controls the oxygen demand causing summer hypoxia, while episodic wind can partly erode stratification and hypoxia. The interannual variation of hypoxia is mainly controlled by watershed input, but wind plays an important role in modulating hypoxia, such as variations of hypoxic volumes in individual summer months. Although the extent of hypoxia reduction is different with different wind directions, a faster wind speed (above certain strength) causes stronger destratification and hypoxia reduction than weaker speeds, which is generally more important than the effect due to wind directions. Computer modelling is used to obtain dissolved oxygen conditions in finer temporal and spatial scales to supplement the discrete observations in scattered monitoring stations to better understand hypoxia development under episodic wind events, which enhances the understanding on physical relationships among the concerned constituents beyond the statistical analysis

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

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