Atmospheric Deposition of Nitrogen and Phosphorus to the
South Florida Bay Ecosystems
1315 East-West Highway, Room 3151
Silver Spring, MD 20910
Tilden P. Meyers and Steven Lindberg
P.O. Box 2456
Oak Ridge, TN 37831
Historically, most of Florida Bay is considered to be a nutrient limited system. Nutrients such as nitrogen and phosphorus are effectively recycled by both plants and animals. Additional loading of nutrients into the Bay could trigger an imbalance in the natural cycling process of nutrients. Nutrient inputs into the Bay include flow from coastal fringes and estuaries and resuspension of decomposing material. Also considered sources are the wet and dry deposition of micro-nutrients, which can directly impact the ecophysiology and nutrient balance of the various sub-ecosystem types within Florida Bay. Measurements of nutrient input from wet deposition processes are limited in South Florida, with only one station located at the Everglades National Park.
Although it may be important to the nutrient balance, the dry contribution of both nitrogen and phosphorus to South Florida including Florida Bay ecosystems, is largely unknown. Dry deposition of nitrogen can be in the form of both gas-phase (HN3), and particulate form (NO3- and NH4+) in both small and coarse aerosols. To fully understand the nutrient cycling within the Florida Bay area and assess the relative importance of the atmosphere as a source of nutrients, the dry input of nitrogen and phosphorous needs to be assessed. The Oversight Panel identifies this as a primary objective in the Strategic Science Plan for the Interagency Florida Bay Science Program.
Growth in South Florida has expanded the regional population to nearly 14 million inhabitants. To accommodate the growing need for housing as well as increased agricultural production, land is being developed at an alarming rate. This has put a large demand and strain on the available fresh water resources. Water conservation and management practices have been adopted to meet the growing demand for fresh water in South Florida. The Everglades and Florida Bay regions have and continue to be directly impacted by current water management practices and the availability of fresh water which can impact the nutrient cycling of these ecosystem. These impacts may be manifested in the magnitude of phytoplankton blooms in Florida Bay which have reportedly increased significantly in recent years.
Nutrient bioassay studies in Florida Bay Research Programs have identified that nitrogen is one of the three major limiting nutrients in the western and central regions of the Bay. Since most of the natural wetlands in South Florida are nutrient deficient, the atmosphere may directly impact the nutrient cycling in Florida Bay through wet and dry deposition processes. In addition, the atmospheric input of phosphorus and nitrogen via dry and wet deposition processes may also be episodic, due to the nature of the wind and precipitation regimes in South Florida.
Because of the relatively few monitoring sites in Florida, the atmospheric input of nitrogen and phosphorous into South Florida ecosystems is largely unknown. Previous attempts to quantify dry deposition (using buckets) were unsuccessful largely because of the sampling problems associated with contamination by bird droppings. In addition, had the dry bucket data not suffered from contamination problems, interpretation of the data would have been difficult.
Our research will attempt to assess the weekly, seasonal and annual wet and dry deposition of nitrogen and phosphorous into the Bay. Implementation of this project will yield the following information:
(A) Measurements of particulate and gaseous nitrogen compounds in representative regions of the Bay. These measurements will include particulate (NO3- and NH4+) and gaseous nitrogen (NH3).
(B) The weekly, seasonal and annual deposition rates of phosphorous and nitrogen to the Florida Bay region.
(C) Potential relationships between the distribution of atmospheric nitrogen concentrations and that in the water columns in the central and western bay.
(D) A preliminary assessment of the episodic nature of phosphorous and nitrogen dry deposition in the Florida Bay region.
A. Sample Collections
Rainwater samples will be collected in plastic buckets using an automatic wet-dry collector similar to that used in the National Atmospheric Deposition Program (Galloway and Likens, 1976). Water droplets on the rain sensor served to activate the system and open the bucket cover during periods of precipitation. A heater attached directly to the sensor grid rapidly evaporates droplets so that the system closes shortly after a precipitation event. Rainwater samples will be filtered, transferred to pre-cleaned polyethylene bottles, and kept frozen. Samples will be shipped back to the laboratory for analysis.
The technique used in this study is the filter-pack method (Anlauf et al., 1988; Quinn et al., 1989; Williams et al., 1992). Duplicates and field-blanks will be collected during the experiment. Each filter pack device included one Teflon filter (1 ~m pore size) and followed by three Whatman 41 filters. The Teflon filter and Whatman 41 filters collect gaseous and particulate nitrogen and phosphorous species, respectively. The Whatman 41 filters will be washed with Mill-Q water and then vacuum dried prior to sampling. The sampling rate for the filter-packs is one integrated sample per week at an averaged flow rate of 10 l/min. After the samples are collected, filter-packs will be sent back frozen to the laboratory for analysis.
Sample filters will be unloaded in a Class 100 clean hood once arrive in the laboratory. The filters will then be extracted using Milli-Q water in pre-cleaned plastic containers. The extraction will be done in a sonicator for 20 min. The extracts will then be for nitrate (NO3-) by ion chromatography (Dionex) and for ammonium (NH4+) by phenol hypochlorite colorimetric technique (Solarzano, 1969) with a Technicon Autoanalyzer. The sample filters for phosphorous will be analyzed by the Illinois State Water Survey which also perform the wet deposition sample analysis for the National Atmospheric Deposition Program (NADP).
The monitoring of nutrient deposition to Florida Bay is now underway. A 10 meter meteorological tower (Figure 1) was installed at the Keys Marine Laboratory on Long Key in April of 1998. Equipment on the tower includes filterpack sampling systems, anemometry for wind speed and direction, air temperature, relative humidity, and solar radiation (Figure 2).
Atmospheric nitrogen include gaseous ammonia (NH3), particulate ammonium (NH4+) and particulate nitrate (NO3-) were sampled using treated filter-packs. The preliminary results for the gaseous nitrogen species during the period of April, 1998 and March, 1999 showed that the NH3 concentrations varied greatly (between 0.034 and 0.76 ug/m3) with relatively higher concentrations in the Summer season. The averaged particulate NH4+ and NO3- concentrations were 1.20 ug/m3 and 2.17 ug/m3, respectively. The observed atmospheric NH3 concentrations in South Florida Bay were higher than the averaged NH3 concentrations in other coastal regions (such as Tampa Bay and Chesapeake Bay).
Since late October of 1998, aerosol monitoring of phosphorous and wet deposition of nitrogen has also been underway. The sampling protocol, sample analysis and shipping logistics have been worked out in collaboration with the laboratory at the Illinois State Water Survey who conduct the analysis for the National Atmospheric Deposition Program (NADP). Samples collected through mid-February have been analyzed. For this period, the average dry deposition rate of ammonium aerosol was estimated to be about 0.18 mg/m2/wk, compared to an average wet deposition rate of (0.1 mg/m2/wk). The averaged weekly wet deposition rate of nitrate was on the order of 0.7 mg/m2/wk. For Phosphorous, there has only been one week in which the wet deposition data were above the detectable limit. However, measurable air concentrations have been obtained each week in order to obtain a dry deposition rate. At this time, the averaged weekly PO4- deposition rate was estimated to be about 0.1 mg/m2/wk. The analyses for organic contributions (that do no show up as PO4) have revealed some suggestive trends. For the last four sampling periods, total PO4 (which includes the organic fraction) have been generally 30% higher than the inorganic fractions. Further analyses of elemental ratios will be used to determine the possible origin of this organic fraction.
In order for the data to be utilized by the Key Marine personal as well as for our research purposes, a remote data link was installed in October, 1998. This device links the data logger on the tower to a computer in the laboratory. Data from the tower can then be easily download via telephone modem. This will provide a near real time access to the meteorological data for QA/QC.