Atmospheric Deposition of Nitrogen and Phosphorus to the
South Florida Bay Ecosystems
Principal Investigator(s):
Pai-Yei Whung
NOAA/ARL
1315 East-West Highway, Room 3151
Silver Spring, MD 20910
Tilden P. Meyers and Steven Lindberg
NOAA/ARL/ATDD
P.O. Box 2456
Oak Ridge, TN 37831
Introduction
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
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.
Dry
Deposition
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.