Atmospheric Deposition of Nitrogen and Phosporus to the South Florida Bay Ecosystems

Principal Investigators:
Pai-Yei Whung (University of Miami, Rosenstiel school of Marine and Atmospheric Science, Cooperative Institute for Marine and Atmospheric Studies)

Tilden P. Meyers (NOAA/ARL/ATDD)

Collaborating Investigator:
Steven E. Lindberg (Oak Ridge National Laboratory)

I. Introduction

Historically, most of Florida Bay is considered to be a nutrient limited system. Nutrients such as nitrogen and phosphorous 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 includes 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 phosphorous to South Florida including Florida Bay ecosystems, is largely unknown. Dry deposition of nitrogen can be in the form of both gas-phase (HNO3), 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. Identified as a primary objective in the Strategic Science Plan by the Oversight Panel for the Interagency Florida Bay Science Program, the role and importance of the relative contribution of nutrients into the bay needs to be determined.

II. Background and Justification

Growth in South Florida has expanded the regional population to nearly 14 million inhabitants. Land is not only being developed to accommodate the growing need for housing, agricultural production activities, including both ranches and farms, have also increased dramatically. This, however, 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 sign)ficantly in recent years.

Nutrient bioassay studies in Florida Bay Research Programs 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 the Florida Bay through wet and dry deposition processes. In addition, the atmospheric input of phosphorous 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.

The atmospheric deposition of nutrients has been identified by the Oversight Panel of the Interagency Florida Bay Science Program as a major unknown. In order to fully assess the magnitudes of the atmospheric sources of phosphorous and nitrogen to the Florida Bay region, season and annual dry deposition rates of phosphorous need to be quantified. A program to assess the weekly, seasonal and annual dry deposition of nitrogen and phosphorous is proposed. 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 (HNO3 and 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.

III. Research Work Plan

A sample strategy and methodology similar to that currently employed in the NOAA Dry Deposition Research Network is proposed (Hicks et al., 1991; Meyers et al., 1991). With this approach, year-round monitoring of atmospheric phosphorous and nitrogen will be initiated at a few selected locations. These locations will be selected based on regional representativeness, local technical support, safety and security considerations and site logistics. To date, four candidate locations for the land based measurements have been identified; Shark River, Taylor River, Flamingo stations, and Keys Marine Laboratory. Throughout the year, shipboard samples will obtained in collaboration with the zooplankton group (Dr. Peter Ortner) at the Ocean Chemistry in NOAA. Atmospheric sampling systems will be taken on board at the beginning of each season for 12-hour continuous samples for 5 days to establish a first-order data base for atmospheric nitrogen and phosphorous over the Florida Bay itself. We will also join other research groups (South Florida Water Management District and Florida International University) for short term shipboard measurements.

With the weekly monitoring approach, the so-called inferential method, the dry deposition is determined as the product F = Vd [C] where Vd is the deposition velocity and [C] is the concentration of the chemical species of interest. This approach is adopted because the current state of the art methods for direct flux measurements of phosphorous
and nitrogen dry deposition are not well suited for long term operation. In this approach, weekly concentrations of the nitrogen and phosphorous will be measured with filterpacks (Anlauf et al., 1988; Quinn et al., 1989; Williams et al., 1992).

The deposition velocity (Vd) will be determined from a calibrated model that uses meteorological data and critical information on the properties of the surface (Hicks et al., 1987; Meyers et al., 1997). The application of this approach for the quantification of the dry deposition of airborne particulate will initially require that both the large (> 2 um) and small (< 2 um) aerosols be considered since little is known about the particle size distribution of phosphorous and nitrogen in the Florida Bay region. The distinction between the coarse and fine aerosol fractions must be made because the deposition velocity for the coarse fraction is greater than the Vd for the fine. The modeling framework that will be applied to the monitoring data, will determine the dry deposition flux (Meyers et al., prep.)

Meteorological data will also monitored at each of the land based monitoring sites. The meteorological data collected at these sites will provide the necessary input for both model calculations of the deposition velocity and for the regional scale models (ARPS) that will be used to provide the regional estimates of nitrogen and phosphorous deposition into the Bay. Parameterizations of the deposition process based on standard meteorological data taken at the land based sites will continue to be refined. This refinement will occur under an parallel program (sponsored by SFWMD) that is scheduled to commence in June of 1997.

Under this program, several intensive field experiments are planned for 1997. Micrometeorological flux and chemical sampling systems will be augmented to include measurements of HNO3, nitrate aerosol, and phosphorous from which fluxes will be determined. The mod)fied Bowen-ratio approach, which has been used successfully to determine fluxes of mercury (Meyers et al., 1996; Lindberg et al., 1997) and HNO3 (Meyers et al., 1989) will be used to determine fluxes of HNO3, NO3- and PO4-. Aerosol concentrations of aerosol nitrogen and phosphorous will be determined for two separate size fractions. From these intensive field campaigns in which the dry deposition is quantified, parameterizations of the deposition process based on standard meteorological data will be used to model the deposition velocity for each size fraction.

IV. Methodology

A. Sample Collections

1. Wet Deposition

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.

2. Drv Deposition

The techniques 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), one nylon filter (1 ~m pore size) and followed by four Whatman 41 filters. The Teflon filter, nylon filter and Whatman filters collect particulate nitrogen and phosphorous species, HNO3 and NH3, 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 24 hours at an averaged flow rate of 10 l/mint After the samples are collected, filter-packs will be sent back to the laboratory for analysis.

B. Sample 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 30 mins. 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).

V. Coordination Efforts and Data Implementations

Under the sponsorship of the South Florida Water Management District (SFWMD), a similar effort to monitor the dry input of phosphorous and nitrogen is planned. Three sampling sites are planned for 1997 with the first site to be installed in April 1997. This location will be within the Everglades Nutrient Removal (ENR) project area. A second system will be located further to the south just east and north of the Loxahatchee Wildlife Refuge. A third system is planned to be installed at the NADP site located at the Everglades National Park. The information from this project, can be used to examine north-south differences in airborne concentrations and deposition rates.

Proposed sampling sites in this proposal are chosen to parallel other ongoing research projects funded by FBRP. Our data can be compared and used by related research teams namely Sediment Retrospective Analysis Group, Water Quality Model and Nutrient Dynamics. The results from our field measurements will also be integrated into the Atmospheric Regional Prediction System (ARPS) model developed by Dr. Craig Mattocks at AOML/NOAA. The ARPS model can then estimate the point net fluxes using Lagrangian trajectory technique and, gradient net fluxes using Eulerian transport scheme for the atmospheric nitrogen and phosphorous species in Florida Bay. We will be able to obtain a first-order estimate of the atmospheric deposition of nitrogen and phosphorous and, better understand the controlling mechanisms for the nitrogen and phosphorous loading in Florida Bay.

VI. Progress to date

Two monitoring stations (Keys Marine Laboratory and NW Florida Bay monitoring platform site) are expected to be up and running by the first week of February and early March, (1998) respectively. The filterpack systems, dry-wet deposition collectors, meteorology information and data logging systems hae all been tested. The 10 meter towers with pulleys for changing sample filters are under construction and should be ready at the end of January, 1998. We are also still working on installing the power lines at the Keys Marine Lab site. The power was scheduled to have been installed by mid-December, but there has been an unexplained delay by the Marathon Electric Sign and Light, Inc. After debugging the setup at the Keys Marine Lab, we are currently working with FIO on the logistics of installing a second air monitoring systems at the NW corner of the bay.

Page last updated on January 25, 1998.