oxygen). BOD is an important variable because it represents carbon, nitrogen, and oxygen kinetics simultaneously.

Dissolved oxygen. Panel D on Figure 11.3 shows model and data comparisons for dissolved oxygen (DO). The model results are shown as ten-day average concentrations in the surface layer (solid line) and in the bottom layer (broken line). Model results for surface maximum and bottom minimum are shown by the dashed lines. The model averages agree well with the mean observations, shown by various symbols, and the model maximums and minimums illustrate that the model is capturing the variability over time in the observed data.

Other variables. Figures 11.3 and 11.4 show model and data comparisons for other water quality variables including phosphorus (panels O through R), silica (panels M and N), and light extinction (panel E). The agreement between SWEM calcu lations and measurements of light extinction and additional nutrients simultaneously is further evidence that SWEM is useful for understanding the causality of primary production.

Primary productivity. A limited number of primary production measurements were collected for comparison to SWEM results. Primary productivity measurements were obtained by deploying a series of bottles containing surface water at various light levels and measuring dissolved oxygen production. Such a deployment measures net production as some of the oxygen that is being produced by algae within the bottles is also lost to respiration. By also deploying dark bottles, respiration could be estimated directly. A summation of light and dark bottle results yields gross primary production. Not enough primary production measurements were made to test the ability of the model to capture spatial gradients. Further, the data were difficult to interpret as dark bottle measurements showed oxygen production in some cases.

Figure 11.5. SWEM 1994-95 temporal calibration for primary production.

Figures 11.5 and 11.6 show summary model and data comparisons temporally over the course of a year for several locations as indicated on the map for chlorophyll and dissolved oxygen. These model and data comparisons demonstrate that primary production and eutrophication for the Hudson River Estuary and contiguous waterways is accurately calculated by SWEM. The model results and data at the locations shown are representative of both light/residence time limiting (i.e., Hudson River) and nutrient limiting (i.e., western Long Island Sound) conditions. The ability of SWEM to perform equally well under both conditions demonstrates model robustness and suitability for predictive purposes. The chlorophyll calibration represents modeled primary production in biomass units and is evidence that algal dynamics, nutrients, light, and physical conditions are all properly accounted for in the model. Further, dissolved oxygen (like chlorophyll) is an integrated system response. The dissolved oxygen calibration is a measure of the ability of the model to capture all features of primary production as well as additional source and sink terms in the dissolved oxygen balance including bacterially mediated decay processes. In this regard, dissolved oxygen may be viewed as the most important endpoint for calibration. Results on Figure 11.5 show that SWEM picks up much of the variability observed in near surface measurements. Results on Figure 11.6 show that SWEM captures observed vertical stratification.

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