Biscayne Bay Nutrient Box Model

Development of Nutrient Load Estimates and

Implementation of the Biscayne Bay Nutrient Box Model

Cetacean Logic Foundation, Inc.

Principal Investigators: Frank E. Marshall and William K. Nuttle

Client: Florida International University

·    Freshwater inputs for each element of water budget were estimated from various sources

·    Time series nitrogen and phosphorous loads were developed from observed data

·    A peer-reviewed hydrology/salinity box model was upgraded and mass balance calculations added

·    Box model works reasonably well based on estimated nutrient loads from the watershed compared to the long-term average of concentrations measured in the Bay.


This study estimates nutrient loads to Biscayne Bay from its watershed and implements mass-balance calculations to estimate the long-term average nutrient concentrations in the Bay based on these loads.  Nutrient loads for Total Phosphorous (TP), ammonium (NHx-N), nitrate-nitrite (NOx-N), and Dissolved Inorganic Nitrogen (DIN) were independently developed for all components of the water budget based on various existing sources of information, including canals, ungauged surface water, groundwater, atmospheric, and Atlantic Ocean contributions.
 Calculations of nutrient concentrations in Biscayne Bay based on the estimated loads extended an existing hydrology/salinity mass-balance model.  The model was calibrated against measured salinity to estimate the water fluxes between each box and validated against an independent set of salinity observations.  Overall the hydrology/salinity model captured well the patterns of spatial and temporal variability in salinity in Biscayne Bay.  Calibration and verification statistics show that the box model accounts for greater that 70 percent of the variability in measured salinity a mean error of between 2 to 3 psu.  Errors are comparable between the calibration and verification time periods and are comparable to results achieved in the previous box model study.

When the model was used for nutrient mass balance calculations, the results showed varied success at characterizing the link between nutrient loads and concentrations in the Bay.  Average values for TP concentrations assuming no loss or transformation of TP exceeded average measured values by between 17 and 44 percent.  Of particular interest, the calculated values of TP concentration reflect the effect of very large loadings of TP caused by hurricanes Katrina and Ernesto in 2005 and 2006.  Average values for NOx-N concentrations without accounting for losses due to net denitrification greatly exceed average measured values.  Calculated values are between about 1.5 and 4 times higher than measured values.  The disparity between calculated and measured values indicates the degree to which uptake, removal, and wash-out of nitrogen species may exert a significant influence over NOx-N concentrations in the Bay.  When an average net denitrification rate of 0.3 month-1 was applied Bay-wide in the DIN calculations, calculated values for DIN concentrations were equal to or substantially less than observed values in all but one of the sub-basins.  In general this means that the net denitrification rate in most areas of the Bay is capable of being estimated by the default rate or a lower rate. 

The mass-balance calculations provide an incomplete description of the processes contributing to the variation of nutrient concentrations in Biscay Bay. Even so, this study showed that the mass-balance nutrient calculations work well for an evaluation of estimated nutrient loads from the watershed by comparing the long-term average concentrations calculated based on these loads to the long-term average of concentrations measured in the Bay.