Estimation of Inputs to Florida Bay

Inputs from the Atmosphere

July 1998

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Introduction    |     Methods    |   Results   |   Potential Refinements

Introduction

This section develops estimates of atmospheric inputs to the Florida Bay water quality model domain.   The compilation and processing of rainfall data are described separately; this effort has provided a monthly rainfall time series for October 1985-December 1997 which is assumed to represent the entire model domain.

Data Compilation

The following potential sources of data for estimating atmospheric deposition rates have been considered:

The University of Florida and SFWMD data include a broad range of water quality measurements on bulk or wet/dry deposition samples.  The NADP data do not include dry deposition and phosphorus data are not reported (ortho P measurements are made, but results are not reported).  The FAMS data have a limited parameter coverage and are not currently available.

Deposition collectors typically deployed for a period of a week or two are subject to numerous sources of contamination (birds, insects, amphibians).   Various attempts have been made at removing these effects by reviewing records of sample appearance (presence/absence of contaminants of various types) and/or by applying various statistical procedures (SFWMD, 1997).  Suffice it to say that none of these attempts have been completely successful and that the utility of historical deposition data from South Florida are limited (McDowell et al., 1997).

Results

Given the above data characteristics and constraints, estimates of deposition rates for Florida Bay are based upon data collected by the University of Florida at Bahia Honda in 1978-1979.  Because of the apparent importance of terrestrial sources, nutrient deposition rates measured under this study tended to be lower at coastal stations, as compared with inland stations.  Statewide maps of total nitrogen and total phosphorus deposition rates measured under this study are attached.   Nitrogen and phosphorus deposition rates at Bahia Honda (320 and 17 mg/m2-yr, respectively) were the lowest values measured in the State.   This fact, coupled with regional variations in weather patterns and deposition sources, suggests that the relevance of deposition measurements collected elsewhere in Florida decreases rapidly with distance from Florida Bay.

The attached table contains average deposition rates and volume-weighted-mean concentrations for relevant water quality components reported by Brezonik et al (1981) at Bahia Honda for May1978 - April 1979.  Although the study period extended somewhat beyond these dates, the reporting period captures one full year.   Deposition estimates for minor, unmonitored water quality components have been developed using a framework which is similar to that used for estimating loads from the Florida mainland.  Individual measurements used by Brezonik et al (1981) in computing annual deposition rates are not currently available, but are being tracked down.   These data may be useful for characterizing seasonal variations and correlations with precipitation volume.  Initial loading estimates for Florida Bay assume a constant deposition rate for each water quality component.

The following table compares total phosphorus deposition rates and concentrations at Bahia Honda with an estimated derived by Walker (1989) based upon data collected by SFWMD at the ENP Research Center:

Source Units Brezonik et al. (1981) Walker (1989)
Location - Bahia Honda ENP Research Center
Period May 1978-April 1979 Nov 1987 - Nov 1988
Precipitation cm/yr 103 165
Wet Deposition mg/m2-yr 8 1.3
Dry Deposition mg/mg2-yr 15 0.8
Total Deposition mg/m2-yr 17 23 4.1
Wet Conc ppb 5 0.8
Bulk Conc ppb 16 14 3.1

Despite the large difference in precipitation, the volume-weighted-mean concentration at the ENPRC was not significantly different from the Bahia Honda value.    Analysis of recent data collected by SFWMD with refined methods and data screening procedures may provide relevant information on magnitudes and seasonal dynamics of atmospheric deposition rates. 

Walker & Jewell (1997) analyzed wet & dry phosphorus deposition data collected at four locations in Loxahatchee National Wildlife Refuge.   Correlations with season and rainfall rates are shown in the following figures:

Both wet and bulk concentrations are negatively correlated with precipitation rates and show a strong seasonal minimum during the summer.  Total deposition rates (wet + dry) at interior marsh stations, however, are not strongly correlated with precipitation or season.  These results suggest that variations in total phosphorus deposition rate, though substantial, are not systematically related to season or precipitation volume.   This suggests that the annual-average deposition rate represents an unbiased estimate of values in wet and dry periods and in each season.  The transferability of these results to the Florida Bay region and/or to other water quality components is unknown. 

Potential Refinements

The following tasks may provide refined estimates of atmospheric loads:

  • compilation of source data from Bahia Honda measured by Brezonik et al. (1981)
  • compilation and analysis of recent SFWMD data from the ENP Research Center
  • characterization of temporal variations in deposition rate related to season and precipitation volume
  • development of time-variable estimates of deposition rates

Such efforts may be justified if the Bay water quality model is found to be sensitive to atmospheric loads.

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