Larry Brand , Professor, Division of Marine Biology and Fisheries, Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Cswy, Miami, FL 33149 (305) 361­4138, FAX (305)361­4600, Email: lbrand@rsmas.miami.edu; Alina Szmant, Associate Professor, Division of Marine Biology and Fisheries, Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Cswy, Miami, FL 33149 (305) 361­4138, FAX (305)361­4600, Email: aszmant@rsmas.miami.edu .

I have two projects that will be beginning in the fall of 1995. NOAA is funding a research project (Project 1) by Alina Szmant and me to examine sediment­water column interactions in nutrient­microalgal dynamics in Florida Bay and the Florida Keys National Marine Sanctuary. EPA is funding me (Project 2) to search for and document ephemeral nutrient inputs into coastal waters around the Florida Keys from surface runoff and localized episodic groundwater flushing resulting from large rainstorms.

Project 1

Although it is the extensive phytoplankton bloom that is of present concern, one cannot study plankton and water column nutrients in isolation from the sediments in shallow water embayments such as Florida Bay. Diffusion, resuspension, irrigation by macrobenthic animals, macrophyte translocation and diel migration between the water column and the sediments by dinoflagellates and other phytoplankton can result in significant transfer of nutrients from the sediments to the water column. Whatever the ultimate source of the nutrients that are now generating the phytoplankton blooms in Florida Bay (dead sea grasses, human activities in peninsular Florida or the Florida Keys), it is likely that most of the nutrients within the system reside in the sediments, not the water column. A large pool of nutrients in the sediments could generate phytoplankton blooms for a long time after the ultimate nutrient source is cut off. Any restoration effort must take into consideration the benthic nutrient pool.

Although the data are somewhat sparse, a number of studies in shallow waters have found more microalgal biomass in the surface sediments than in the water column. To understand microalgal dynamics in the water column, we need to also understand microalgal dynamics in the sediments and the coupling of the two. Is it one microalgal community that occupies both the water column and benthic habitat or two distinct microalgal communities, one in the water column and one in the sediment surface, which occasionally get mixed together during resuspension events? Do the microalgal biomasses in the water column and sediments increase and decrease over time and space together, or does an increase in one cause a decrease in the other?

Our overall goal is to determine the extent to which processes in the sediments affect the supply of nutrients and phytoplankton to the water column of Florida Bay, examine some of the mechanisms by which this occurs, and examine how important these may be in enhancing transport of nutrients into the Florida Keys National Marine Sanctuary and its reefs. Although other potential mechanisms such as macrophyte translocation may exist, we will focus on the importance of resuspension of the sediments, diffusion, and diel vertical migration of microalgae between the sediments and the water column as mechanisms for transporting benthic nutrients to the water column.

The degree of coupling between nutrients and microalgae of the sediments and water column is important to the way in which nutrients from Florida Bay may ultimately reach the FKNMS reefs. In addition to hydrographic transport of dissolved nutrients in the water column from Florida Bay to the reefs, one must consider the hydrographic transport of planktonic biomass and resuspended sediments.

The basic core of this research will be to sample 10 diverse sites in Florida Bay and 7 sites along a V­shaped transect from the Long Key Viaduct E and S to the reefs. The exact locations may change as we coordinate our research with that of others working in Florida Bay. The main purpose of the stations in Florida Bay is to examine the relationships among nutrients and microalgae in the sediments and water column. The selected stations include basins which we expect to differ in these variables. The transect from the Long Key Viaduct will be used to examine the transport of the different forms of nutrients from Florida Bay to the reefs.

Each of the stations will be sampled every 3 months to get an estimate of variability on a seasonal time scale. At least twice a year, each of the stations will also be sampled 4 times during a 2 week time series after a major storm event to examine the magnitude and persistence of resuspension and its effects on water column nutrients and microalgae and their transport to the reefs. Our goal is to sample after a winter storm with winds primarily from the NW and after a tropical storm from the SE in the summer.

The goal of the overall sampling regime is to examine the pelagic­benthic partitioning of nutrients and microalgae under a diversity of environmental conditions. Although more frequent sampling is always desirable, quarterly sampling along with two storm event time sequences each year at 17 stations should provide us with a wide range of environmental situations to analyze and compare, which is our primary objective. Budget constraints preclude more frequent sampling.

At each station we will measure 3 replicate water samples for ammonium, nitrate, phosphate, chlorophyll, turbidity, particulate dry weight, and particulate nitrogen and phosphorus. Also at each station 3 replicate sediment cores will be taken for the measurements of benthic chlorophyll; porewater ammonium, nitrate and phosphate; extractable (sorbed) inorganic nitrogen and phosphorus; total nitrogen and phosphorus; porosity; and particle size distribution. Sorbed nutrients constitute a reservoir of inorganic nutrients that are in equilibrium with porewater nutrients. As porewater nutrients are utilized by benthic plants and algae, sorbed nutrients can be released into the porewaters. Further, sorbed nutrients may be a source of enrichment to the water column during resuspension events. Population abundance of selected species of microalgae (species that can be easily identified by light microscopy) will be estimated in both the water column and the sediments. We plan to compare chlorophyll concentrations to turbidity and suspended particulates along with individual microalgal species distributions to estimate degree of resuspension.

Once a year at 5 of the Florida Bay stations, 48­hour diel studies will be conducted on the vertical distribution of microalgae in the water column and sediment (both total chlorophyll and selected individual species) and the vertical distribution of nutrients in the sediments. At the same time, benthic chambers will be used to measure the actual diffusional flux of nutrients out of the sediments, with emphasis on seagrass depauperate areas.

One or two species of dinoflagellates that undergo diel vertical migration in Florida Bay will be isolated into culture and their behavior studied in laboratory microcosms. The microcosms will have a water column over ca. 10 cm of sediment, with different concentrations and combinations of nitrogen and phosphorus in the water column and sediment porewaters as independent variables. This will allow us to examine how diel vertical migration behavior changes with different nutrient regimes under controlled experimental conditions and better interpret our field data from Florida Bay.

Project 2

It is generally agreed that ecological changes are occurring in the FKNMS. One of the suspected causes of some of these changes, increased nutrients, is well known to alter ecosystems. For example, increased nutrients can cause macroalgae to overgrow corals. It has been estimated that there are 25,000 cesspools and septic tanks, 281 injection wells, 4 active and 10 inactive landfills, 182 marinas with 2707 wet slips, and 1410 live­aboard boats in the Florida Keys. These sources along with several sewage outfalls, high nutrient water in Florida Bay, and other human activities are thought to be injecting nutrients into FKNMS waters, but in most cases are not yet proven to be significant. Because in many cases the injection of nutrients is ephemeral or highly local, an extremely intense sampling program in both space and time is needed to detect many of these inputs. To date, such intensive data have not been collected because of the expense. Part of the reason for this is the relatively large number of parameters that are usually measured, which drives up the cost per station.

Spatially intensive sampling is needed to pinpoint local sources of nutrients. Not only point sources such as canals, outfalls, and marinas, but even groundwater may seep out in localized areas because of structural faults and solution holes in the limestone. Because much of the nutrients can be expected to come from surface runoff and groundwater flow driven by rainstorms, sampling needs to be designed around weather events and not just at random. A buildup of nutrients in groundwater of the Florida Keys during the dry season and an increase in nutrients in local marine waters during the wet season has been observed, suggesting that rainfall drives nutrient rich groundwater into local marine waters. The highest concentrations of chlorophyll in Biscayne Bay occur right after the first large rainfall at the end of the dry season, again suggesting rain driven flushing of nutrient rich groundwater or surface runoff. Also supporting this hypothesis was the finding of a strong negative correlation between chlorophyll and salinity, indicating that most nutrients in Biscayne Bay are associated with freshwater flow from land. It has recently been shown that nutrients from sewage injected into the Keys groundwater can make its way into coastal waters rather quickly. In conclusion, frequent and spatially intense sampling is needed to detect many of the fluxes of nutrients into FKNMS waters.

The basic objective is to detect nutrient inputs that may be highly localized or ephemeral. To do this, we need to measure a large number of samples. This will be done by using a rapid, inexpensive method of detecting nutrient eutrophication that can be carried out within a reasonable budget. The method is based on the fact that most nutrients are quickly taken up by plants in shallow tropical waters and phytoplankton increase their biomass quickly in response to nutrients, so that the measurement of chlorophyll as an indicator of plant biomass is a better and more sensitive indicator of nutrient eutrophication than is the measurement of the residual nutrients. The primary method to be used is based upon the fact that in vivo chlorophyll fluorescence is directly proportional to total chlorophyll concentration when the photosynthetic electron transport system is blocked with 3­(3,4­dichlorophenyl)­1,1­dimethylurea (DCMU). The fact that DCMU enhanced chlorophyll fluorescence can be measured much more quickly than can standard extracted chlorophyll concentration or nutrient concentration means that we can greatly reduce cost per sample and therefore greatly increase sample size in space and time with a limited budget.

The primary task will be to collect water samples at 200 stations on three different days following large rainstorms to determine the spatial distribution of chlorophyll and identify localized sources of nutrient eutrophication resulting from rain driven surface runoff or groundwater seepage. All 200 stations will be sampled over a 2 day period using a high speed boat. Half of the stations will be sampled on days 1, 3, and 5 after each rainstorm and the other half will be sampled on days 2, 4, and 6.

At each station, determined with a GPS instrument, 500 ml of water will be taken at a depth of 0.5 meters and temperature, salinity and oxygen will be measured with a Hydrolab system. In vivo chlorophyll fluorescence will be measured with a Turner Designs 10­000R fluorometer, 10­5 M DCMU will be added and fluorescence once again measured. 100 ml of water will be preserved with 5% formalin buffered with sodium tetraborate and another 100 ml will be frozen so that other analyses can be conducted in the future. At 10% of the stations, 3 replicate water samples will be taken, and in addition to measuring DCMU enhanced chlorophyll fluorescence, 100 ml of water from each replicate will be filtered (after adding 1 mg of MgCO3 ) through GF/F glass fiber filters and the filters will be frozen. These filters will be extracted for 30 minutes with 10 ml of dimethyl sulfoxide and then with an added 10 ml of 90% acetone at 5°C overnight and measured fluorometrically before and after acidification for the measurement of chlorophyll and phaeopigment concentrations. The replicates at these stations will be used to assess the accuracy of our methods and to examine the relationship between extracted chlorophyll concentrations and DCMU enhanced chlorophyll fluorescence.

Daniel L. Childers, Southeast Environmental Research Program & Department of Biological Sciences, Florida International University, Miami, FL 33199; John W. Day, Jr. , Enrique Reyes, Center for Coastal, Energy, and Environmental Resources, Louisiana State University, Baton Rouge, LA 70803; David Rudnick, Everglades Systems Research Division, South Florida Water Management District, West Palm Beach, FL 33416.

Background and Introduction:

In 1987, large areas of Thalassia beds began to die off in Florida Bay--a phenomenon that continues today. As a result of this major event, and the secondary ecological effects that have come to be related to it, Florida Bay has become a focus of both research and management energies. The cause or causes of these major changes remain unresolved. It appears clear, however, that the greatly modified hydrologic regime of the greater Everglades watershed has had an effect on the Florida Bay estuary. This modified hydrologic regime has caused a virtual elimination of the freshwater inputs to the estuary. During the summers of 1988 and 1989, salinities in central Florida Bay exceeded 60‰ and temperatures exceeded 40°C. The Everglades Forever Act mandated that more freshwater be directed to flow into the Florida Bay estuary and that the distribution of this flow be as close to historical patterns as possible. It is important that we understand how these changes in the quantity, timing, and [potentially the] quality of this water delivery will affect the Florida Bay estuary. In this study, we are focusing on the mangrove transition zone separating the Bay from the freshwater Everglades marshes to the north. This mangrove transition zone is important not only as a buffer to water inputs but it is also a critical nursery area for the Bay's fish populations and a critical habitat for wading birds.

We are quantifying the nature of the mangrove transition zone with regard to the exchange of nutrients and organics with Florida Bay and the inputs of fresh water to Florida Bay. Our research emphasizes the mechanistic link between freshwater flow and materials exchanges, and the relationship between these ecologically-important processes and environmental forcing. These results will play a critical role in guiding efforts by the SFWMD and ENP to manage Florida Bay in the context of the impending restoration of historical hydrologic regimes. This research effort was initiated in August 1995, and will continue through 1998. To date, we have no results to present.

The purpose of this portion of our overall project is to quantify the exchange of water, nutrients, and organic matter between Florida Bay and the adjacent mangrove wetlands. This will involve developing an understanding of the processes and environmental controls that influence this exchange. This information will help us predict the effect of changing freshwater inflow (e.g. changes in the timing, spatial distribution, quantity, and quality of that inflow) on the status of the mangrove transition zone in particular and the Florida Bay estuary in general. Specific objectives are:

1. To quantify the transformations of water, nutrients, and organic materials as water passes through the wetland forests of the mangrove transition zone. This objective will entail constructing and sampling duplicate throughflow flumes in the mangrove wetlands along the Taylor River.

2. To compare patterns of water, nutrient, and organic matter exchange measured in tidal creeks of the mangrove transition zone with patterns of water residence, nutrient remineralization, and organic matter transformations mediated by the mangrove wetlands themselves. This objective will entail comparing flux data from flume studies with those from our concurrent tidal creek studies.

Methods - The Flume Technique

Throughflow flumes have been used in a number of marsh settings to quantify the exchange of nutrients and organics between an inundated wetland and the inundating water column. The basic flume technique involves removable walls placed on the marsh surface parallel to the normal flow of tidal water. Over the course of a tidal cycle, duplicate water samples are taken from both ends of the flume simultaneously and instantaneous constituent fluxes are calculated. The difference between upstream and downstream flux, or before and after treatment by the wetland within the flume, is assumed to be caused by the experimental wetland enclosed by the flume. Several researchers have used the flume technique in mangrove forests as well. We will construct duplicate side-by-side flumes in the mangrove forest near the Taylor River sampling station in May 1996, and begin sampling them in August 1996. The flumes will be sampled for several tidal cycles during quarterly sampling trips from August 1996 through August 1998.

The flexibility of the throughflow flume technique makes it possible to pursue a number of different hypotheses and approaches for the intertidal exchange portion of this study. For example, we could use the flumes to quantify sheetflow through the mangrove forest towards Florida Bay, or to quantify lateral exchanges between the mangrove forest and the Taylor River. For several reasons, we are pursuing the first question. This will involve constructing side-by-side replicate flume channels across the narrow meander neck of mangrove forest separating Florida Bay from Taylor River near the confluence. The flumes will be approximately 50m long and of variable width (between 2 and 5 m). They will be bounded by the Taylor River to the north and Florida Bay, via Little Madiera Bay, to the south. The flumes will be open at each end. Flume walls will consist of flexible plastic sheets (0.5 m high and 5 m long); they will form 2 parallel vertical walls that prevent lateral water movement as water flows through the forest. The flexibility of the plastic sheets will allow us to install the walls through the Rhizophora mangle prop roots with minimal disturbance. The walls will be supported along the flume length by attaching the plastic sheeting either to mangrove trees or to wooden stakes. The flume walls will be removed after every sampling trip, eliminating long-term impacts on the mangrove forest, preventing the accumulation of litter, and reducing edge effects.

During sampling events, when the forest within the flumes is inundated, we will draw duplicate water samples from both ends of the flumes simulaneously every 45 minutes to 1 hour, as long as the flumes are inundated. Our design will allow us to draw samples from both ends of the flumes by boat. We will analyze each duplicate sample for chlorophyll, total suspended sediments, inorganic nutrients (NH4+, NO2-, NO3- and SRP), DOC, TN, TP, TOC, alkaline phosphatase activity, salinity/conductivity, oxygen content, and temperature (all procedures will follow QA/QC-approved SERP protocols). Whenever samples are taken, we will record the water level at both ends of the flumes and note the velocity and direction of sheetflow currents through the flumes. Water flux will be calculated as the combination of advective flux (from current readings) and water volume change (from water levels). To determine water volume per unit water level, we will survey the microtopography of the mangrove wetland enclosed by each flume. Statistical analysis of flux data will be as per past marsh, mangrove, and intertidal seagrass bank flume studies. Additionally, the replicate flume design will give us more statistical power to discern whether the fluxes we measure are signficantly different from zero and are representative of red mangrove forests in general.

Fran Decker, The Nature Conservancy, Florida Bay Watch Program.

Florida Bay Watch is a volunteer program for people who are concerned about the water quality in Florida Bay and the Keys. The Florida Bay Watch program has a two-fold mission: 1) to collect valid, useful scientific data and information about the health and status of the Florida Bay ecosystem, and 2) to involve concerned citizens in formulating solutions to the problems in Florida Bay.

Water quality in Florida Bay has been deteriorating for many years, as evidenced by marked increases in the size and persistence of algal blooms and declines in seagrass and sponge populations. The deteriorating water quality is affecting commercial and sport fishing, and, in fact, fishing guides and commercial fishers were among the first to recognize and report the problems plaguing the bay.

Monroe County citizens have powerful motives to help develop solutions to the crisis in Florida Bay. Economic and environmental perspectives merge with respect to this issue. The Monroe County tourism economy, heavily dependent on diving and sport fishing as attractions, generated $787 million in sales in 1991. Commercial fishing generated an additional $90 million of economic activity the same year. These industries are already beginning to see impacts from deteriorating water quality.

Florida Bay Watch is a volunteer program designed to help meet the crisis in Florida Bay by ensuring that the knowledge and observations of local people are contributing to restoration efforts. Bay Watch volunteers are trained to collect a variety of data using standard methods, to report their observations of conditions in the bay, and to assist professional scientists. The data collected are passed along to cooperating agencies and, along with reports from other studies in the area, are reported through monthly and quarterly reports. The information collected ranges from qualitative water analysis to observations on the extent of algal blooms and sponge die-offs.

The Nature Conservancy, a private, nonprofit conservation organization, is the managing partner, providing staff support and coordination. Funds are currently being provided by the South Florida Water Management District, U.S. Environmental Protection Agency, Everglades National Park, the Orvis Company, the Yamaha Outboards Miami Billfish Tournament, and many individual donors. The Florida Keys National Marine Sanctuary is contributing office space and other in-kind support for the program, and many other agencies and academic institutions are advising of cooperating with Bay Watch in important ways.

With help from professional researchers at other institutions -- including the University of Miami, Florida International University, Everglades National Park, Florida Marine Research Institute, and the Florida Institute of Oceanography -- Conservancy staff have designed Bay Watch sampling protocols to fill identified data gaps, satisfy quality control demands, and supplement professionally conducted research. A complete quality assurance plan has been filed with the Region IV office of the U.S. Environmental Protection Agency. Currently, volunteers are collecting data on five different projects: 1) An aerial survey is conducted to map the different colored patches of water in Florida Bay. Water samples are then taken from Florida Bay and analyzed to determine algal species and relative abundance, sediment load, turbidity, and salinity. This protocol is being coordinated with Florida Marine Research Institute. 2) Water samples are drawn from 25 fixed locations along the Keys and analyzed for nutrients (nitrogen and phosphorus), salinity, and contaminants. This protocol is being coordinated with Florida International University. 3) Volunteers are collecting physical water quality parameters, such as turbidity and salinity in addition to noting anecdotal observations. A preliminary data collection form is in use presently and development is expected to continue. Various fishermen, dive boat captains and eco-tour captains are participating in this project. 4) Mail-in postcards documenting fish deformities are being distributed to Florida Bay fishers, fishing guides, and resource management staff. Certain deformities are considered indicators of water-born toxins, and the results from this survey are being provided to the U.S. Environmental Protection Agency to augment its study of pollution in the South Florida ecosystem. 5) A SEACAT CTD profiler will be towed between fixed water quality sampling stations in the bay to map salinity and temperature isopleths.

The Florida Bay Watch Program will continue to work with researchers studying the bay. It is important that the program stays responsive and can expand and change as needed. Bay Watch can be expected to conduct long term monitoring of natural processes and restoration efforts.

The Florida Bay Watch Program is committed to reaching the public with current information about the state of Florida Bay. Bay Watch will be publishing results in the form of monthly and quarterly reports. There is a display at the Key Largo library featuring the maps resulting from the monthly flyovers. Bay Watch has plans for on-line access to data through the Internet. There are special events, such as presentations by partner scientists, scheduled to bring Bay Watch results to the public.

Bay Watch has grown from 7 volunteers at the Kick-off in March of 1994 to 134 volunteers presently involved. Community involvement is vital to establish a sense of stewardship. When people care about their environment, they become partners in taking care of the environment and produce constructive changes. The Florida Bay Watch program is an opportunity for the local citizens to become involved in finding solutions to the problems of Florida Bay.

J.W. Fourqurean, Southeast Environmental Research Program and Department of Biological Sciences, Florida International University; R.D. Jones, J. Boyer, Southeast Environmental Research, Florida International University .

Florida Bay, and the mangrove-lined estuaries and bays of the south-west coast of Florida, are unique systems in the US. They are underlain by carbonate bedrock and sedimentary deposits, and are dominated by communities of mangroves and seagrasses. Recent changes in the environments of the marine areas of Florida Bay and the Florida Keys have created a great deal of public awareness of the fragile nature of the south Florida environment. Water quality and closely related issues are at the heart of most of these recent changes. The public perceives that there have been drastic changes in the water quality of nearshore marine waters adjacent to urban and suburban areas, as well as in the waters of Florida Bay, which is more distant from obvious anthropogenic alteration. Directly adjacent to developed venetian canal systems, reduced water clarity, the loss of lobsters and reef fishes, and the loss of seagrasses is causing concern. Recent changes in Florida Bay, including hypersalinity, seagrass die-off, algae blooms, reduced water clarity, sponge mortality and fish kills have focussed national attention of the whole south Florida ecosystem. The changes in Florida Bay have in turn been blamed for recent declines in the pink shrimp harvest from the Tortugas Grounds, and declines in the vitality of the Florida Keys Barrier Reef, as evidenced by reduced water clarity, loss of coral cover, and recent occurrences of coral diseases. The data to document trends in water quality and the potential role of anthropogenic nutrient sources to these phenomena are sorely lacking.

In order to address these data needs, the Southeast Environmental Research Program (SERP) conducts monitoring of concentrations of biogeochemically reactive elements, as well as assays of the size and activity of the planktonic community, on a monthly basis at approximately 100 fixed stations in marine and estuarine areas of south Florida. These stations are located in southern Biscayne Bay, Florida Bay, and the estuarine waters of the southwest coast of Florida. In Florida Bay, sampling began summer 1989. Starting September 1992, we expanded our monitoring network up the west coast of Florida to the Lostmans River. The network was further expanded in September 1994 to include 25 stations in the estuarine areas between Lostmans River to Cape Romano. Funding for this network is provided by a number of federal and state agencies, including the South Florida Water Management District, National Park Service, and the Environmental Protection Agency.

"Water quality" is difficult to define, and means many things to many people. The term itself suggests its qualitative nature. Our program measures specific aspects of the nutrient status of the planktonic system, as well some assays of the size and activity of the microbial community. Specifically, we measure salinity, temperature, dissolved oxygen, inorganic nutrients (ammonium, nitrite, nitrate and phosphate), total nutrients (nitrogen and phosphorus), organic nutrients (carbon, nitrogen and phosphorus), turbidity, chlorophyll-a concentration and alkaline phosphatase activity. These systems typically have low concentrations of SRP (usually < 0.05 µM), high DIN (often > 100 µM), and high DOC (often > 1000 µM). The chief form of DIN in these systems is ammonium. Light attenuation, especially in the mangrove-dominated embayments, is chiefly caused by DOM. Major perturbations have occurred over the period of record, including Hurricane Andrew, poorly understood dieoff of seagrasses, and increased turbidity. Despite these perturbations, phytoplankton biomass is generally quite low (usually < 3 mg/L).

The data generated in this sampling program are proving useful to resource managers, and are providing baseline data to monitor for trends in water quality. In addition to these management-oriented uses, we are using data generated from the network to investigate specific scientific questions. Some topics presently under investigation by researchers at SERP are: regional biogeochemistry in nearshore marine systems; microbial processing of C, N and P in south Florida estuarine and marine ecosystems; budgets of biogeochemically reactive elements for the region; and the linkages between water column processes and seagrass-dominated benthic communities. A sufficiently long record of monitoring data has been collected from Florida Bay and the lower west coast so that analyses of these data are providing us with insight into budgets and processes of biogeochemically active elements. It is to be expected that the data from the recent expansions of the network will be as illuminating of processes as the Florida Bay data.

Data from the Florida Bay portion of the network has provided the basis for a more thorough understanding of the functioning of the Florida Bay ecosystem (Fourqurean et al. 1993). The phytoplankton community of Florida Bay is limited by the availability of phosphorus. On a geologic time scale, the source for phosphorus for Florida Bay has been tidal exchange with the Gulf of Mexico, with the Cape Sable region playing a particularly large role. Nitrogen is abundant in the water of Florida Bay, with ammonium as the dominant species of dissolved inorganic nitrogen. The ratios of total nitrogen to total phosphorus in the system are generally greater than 40:1. Dissolved organic carbon is present in high concentrations throughout Florida Bay.

Principle component analyses have been employed to illuminate the underlying relationships between the measured parameters in the data. The variation of each of the components in the data have been interpreted as indicative of the action of a particular process on the planktonic community. The size and activity of the heterotrophic microbial community is represented by a component variable that describes 21.2% of the original variation in the data set. The spatial variation in this component suggests that the heterotrophic community is most active in the water column of central Florida Bay. Seasonality in the original data set can be represented by a component variable that describes 20.1% of the original data. The primary contributors to the seasonality signal are temperature and dissolved oxygen. A further 13.9% of the variance in the original data was due to the variation in oxidized forms of inorganic nitrogen. Salinity variation accounted for 10.8% of the original variation and ammonium contributed another 7.8 percent of the variation. Variation in the size of the phytoplankton community contributed another 6%. These 6 component variables could explain in total 80% of the original variation.

Declining water quality has been the cause of the loss of seagrasses from coastal areas throughout the world. It has been suggested that nutrient inputs from agriculture in south Dade county may have been responsible for seagrass die-off in Florida Bay. Our data suggest that there is no measurable nutrient impact of agriculture in south Dade county on water quality of Florida Bay. This does not preclude the potential importance of other impacts from agriculture, such as pesticide runoff or freshwater diversion, however. The most severe changes in water quality in Florida Bay that have occurred since we began monitoring have been related to turbidity events, often called algae blooms. These turbidity events began well after seagrass die-off was first noted (in 1987, Robblee et al. 1991).

The data from the SERP near-shore marine and estuarine monitoring network is providing consistent, long term data that is useful to resource managers as well as research scientists. Continuation of this monitoring effort will allow us to detect future trends in water quality, and to answer basic scientific questions about biogeochemical cycling of elements in coastal subtropical systems.

Literature Cited

Fourqurean, J.W., R.D. Jones and J.C. Zieman. 1993. Processes influencing water column nutrient characteristics and phosphorus limitation of phytoplankton biomass in Florida Bay, FL, USA: Inferences from spatial distributions. Estuarine, Coastal and Shelf Science. 36:295-314.

Robblee, M.B., T.R. Barber, P.R. Carlson, M.J. Durako, J.W. Fourqurean, L.K. Muehlstein, D. Porter, L.A. Yarbro, R.T. Zieman and J.C. Zieman. 1991. Mass mortality of the tropical seagrass Thalassia testudinum in Florida Bay (USA). Marine Ecology - Progress Series 71:297-299.

Wayne S. Gardner, NOAA/GLERL, 2205 Commonwealth Blvd., Ann Arbor, MI 48105; Harvey A. Bootsma, Thomas H. Johengen, Peter J. Lavrentyev, Cooperative Institute for Limnology and Ecosystem Research (CILER), University of Michigan, Ann Arbor, Michigan 48104; James B. Cotner, Rosie Sada, Texas A&M University, Department of Wildlife and Fisheries, College Station, TX 77845; Joann F. Cavaletto, NOAA/GLERL, 2205 Commonwealth Blvd., Ann Arbor, MI 48105; Brian J. Lapointe, Harbor Branch Oceanographic Institution, Inc., Big Pine Key, FL 33043.

The definition of water quality [i.e. food web structure/activity] and nutrient cycling dynamics was specified as a major research need in the Science Plan for Florida Bay. To address this goal, investigations were conducted in August 1994 and February-March 1995 to examine (1) nutrient-limitation of phytoplankton and bacteria, (2) sediment-water nutrient fluxes and oxygen demand, (3) water-column nutrient transformations, and (4) lower food web abundance and composition (1995 only), in selected regions of Florida Bay. Detailed bottle and sediment-chamber experiments were conducted over an east-west transect of northern-bay stations (near Duck, Rankin, and Murray Keys), and at a more central station (near Rabbit Key). Seston nutrient composition, dissolved nutrient levels, and lower food web organisms were examined at a total of 12 stations.

Nutrient limitation of phytoplankton in Florida Bay: The severity and spatial distribution of phytoplankton nutrient-limitation were assessed by measuring suspended and dissolved nutrient concentrations and ratios and by conducting nutrient enrichment experiments. Seston stoichiometry suggested that phosphorus limitation was generally more prevalent than nitrogen limitation. However, particulate C:P and N:P ratios decreased in a westerly direction within the bay, and seston stoichiometry suggested co-limitation by N and P in the western portion. Dissolved nutrient concentrations and ratios led to similar conclusions. Phosphate concentrations were very low (0.02-0.07 µM) throughout the bay, but were relatively high in the northwest. Dissolved inorganic nitrogen (ammonium plus nitrates) concentrations ranged from 0.5 µM in the southwest to 28 µM at one northeastern station. Both dissolved inorganic N:P ratios and total dissolved N:P ratios were highest in the northeast and lowest in the southwest. Nutrient enrichment assays also showed similar trends, but indicated that some stimulation of growth by nitrogen may be possible even in northeastern parts of the bay. Water samples from the northern region showed surprisingly small increases in particulate organic carbon (POC) following nutrient enrichment. In the western region, both ammonium and phosphate enhanced phytoplankton growth as measured by an increase in POC. Models predicting phytoplankton response to nutrient loading should therefore account for both N and P availability. Occasionally, enrichment with both N and P resulted in less growth than did enrichment with either nutrient separately. We conclude that other factors (e.g. microbial activities, physical and chemical conditions) can influence the relationship between nutrient availability and phytoplankton production in Florida Bay.

Experiments to determine nutritional factors limiting bacteria were conducted at our 4 main stations. Bay water was filtered (0.8 µm pore size) to remove phytoplankton and bacterial grazers and diluted 1:5 with 0.2 µm pore size filtered bay water. Respective nutrients were added to different bottles and the bottles were incubated in the dark at ambient temperature for about 15 h. Addition of amino acids and P had the greatest effect followed by additions of P alone. The strongest effect was observed at Duck Key and Rabbit Key. These data support the hypothesis that bacterioplankton undergo a transition from P- and/or C-limitation in the northeastern portion of the bay to control by other factors at more western locations.

Nutrient fluxes and oxygen demand at the sediment-water interface. Sediment-water nutrient fluxes were measured both with dark, intact sediment cores (77 mm diameter) and with in situ sediment chambers at our 4 main sites. In the core experiments, bay water was slowly (0.1 mL min-1 ) passed over the cores and differences in nutrient concentrations between inlet and outlet waters were measured. Ammonium was released from the sediments at rates ranging from approximately 2 to 40 moles m-2 h-1. Moderate decreases (< 30%) in added 15NH4+ as the water passed over the cores suggested that partial nitrification of ammonium occurred near the sediment-water interface. Concentrations of phosphate in the water were very low (< 0.1 µM) at the different sites and did not change predictably with passage of the water over the cores. Nitrate (including nitrite) appeared to be dynamic but its concentrations also did not change predictably. Some changes in nutrient concentrations, usually ammonium, were observed during short-term (2-6 h) in situ chamber incubations but the direction of flux was not always predictable. When changes occurred, nutrient concentrations increased more in opaque chambers than in transparent ones, an indication of immediate uptake of released nutrients by photosynthetic organisms. The Rankin and Murray stations had higher ammonium levels and occasionally higher release rates from the sediments than did the Duck and Rabbit stations. A high sediment oxygen demand (SOD = 50-250 mg O2 m-2 h-1 ) was observed at all stations. As expected, net oxygen consumption rates were generally higher in dark chambers than in light ones, especially at Rankin and Murray stations where seagrasses were not well developed or appeared "unhealthy".

Water-column nutrient transformations. Light and dark bottle experiments, with added 15NH4+ or 15N-labeled amino acids, measured autotrophic and heterotrophic nitrogen cycling rates and the response of these rates to additions of phosphorus and site-specific macroalgae. Except for the Rankin station in summer where ammonium removal was complete in a few hours, water column N regeneration rates, in bottles fortified with only 15NH4+, ranged from 0.02 to 0.12 µM h-1. Differences in N recycling rates between light and dark bottles in August were minimal at the Duck, Rabbit, and Murray stations, but were dramatic at the Rankin station in August where a bloom of the cyanobacterium, Synechoccocus was present. Ammonium-15N isotope dilution rates, as well as uptake rates were greatly increased in the light at the Rankin station, indicating a close coupling between autotrophic and heterotrophic processes. The addition of P dramatically increased the uptake and remineralization of labeled amino acids but had a minimal effect on ammonium turnover at Duck Key, an indication of heterotrophic organic-carbon limitation. The presence of macroalgae not only increased ammonium uptake but also enhanced the cycling rates of amino acid-N at Rabbit and Duck Key stations, again suggesting that bacteria were carbon-limited. This conclusion is based on the assumption that these plants release dissolved organic material that in turn fuel bacteria.

Size fractionation experiments supported the hypothesis that bacterial demand accounts for a substantial portion of the P that is recycled and is a major pathway in the P cycle in Florida Bay. Comparison of 33P uptake rates in the 0.8 µm-size class in August accounted for 50-74% of total P uptake at Duck Key, Murray Key, and Rabbit Key but only about 1% of the total uptake at Rankin Key. However, alkaline phosphatase activity was greatest at Rankin Key.

Abundances and compositions of lower food web organisms: Concentrations of chlorophyll and phytoplankton were highest at the Rankin station, where 70% of the autotrophic biovolume were due to the picoplanktonic cyanobacterium, Synechococcus sp. Changes in chlorophyll concentrations between sites were not strictly proportional to the changes observed in phytobiovolume, because of differences in the phytoplankton composition. For instance, dinoflagellates that comprised about 30% of phytoplankton at Duck Key may have had low chlorophyll content or even been heterotrophic. Note that the Rabbit and Murray stations, where POC production was greatly enhanced after nutrient enrichments, were dominated by diatoms. Abundance of both picoplankton and planktonic protists (as well as C:N:P ratio) were highest at Rankin Key. Concentrations of heterotrophic nanoflagellates (HNANO) were positively related with concentrations of picoplankton, suggesting trophic couplings between these organisms. It is possible, that a weak response to nutrient enrichments at the Rankin station was due to top-down effects. Ciliates, that can use both bacteria and algae as food sources, also were most abundant at Rankin Key and least abundant at Rabbit Key, where phytoplankton were dominated by large (> µ30 mm) diatoms. At Duck Key, up to 30% of the ciliate population consisted of chlorophyll-bearing species. The most abundant and diverse assemblage of benthic protists was found at Rabbit Key. In contrast, protist populations were less abundant in sediments of Murray and Rankin Keys where seagrass development and dissolved oxygen concentrations were low.

Table 1. Abundance and composition of the lower food web organisms at four sites in Florida Bay (February 22, 1995).

==================================================================

Variable / Site Duck Rankin Rabbit Murray

-------------------------------------

Plankton Chlorophyll (µg L-1) 0.2 2.9 0.8 1.6

Phytobiovolume (µm3 105 mL-1) 4.7 29.9 7.8 12.6

Cyanobacteria (%) 5 70 2 2

Diatoms (%) 61 22 92 89

Dinoflagellates (%) 33 7 1 3

HNANO (cells 102 mL-1) 3.8 23.2 1.9 2.5

Ciliates (cells mL-1) 9.9 12.2 0.5 8.7

Protist biovolume (µm3 105 mL-1) 1.2 2.6 0.1 0.6

Bacteria (cells 106 mL-1) 6.4 11.2 4.2 5.4

....................................................................................................................................................

Benthos Ciliates (cells 105 m-2) 4.7 3.8 18.8 3.6

Flagellates (cells 105 m-2) 75.3 28.1 236.1 36.0

Conclusions: The degree of phytoplankton P-limitation in Florida Bay decreases from east to west where N becomes a co-limiting nutrient. Internal nutrient cycling in the water, as well as at the sediment-water interface, is a very important supply mechanism for available nutrients. Bacterial uptake accounts for a large fraction of water column phosphorus demand. The microbial food web plays a fundamental role in both nutrient cycling and lower food web dynamics and is an important indicator of water quality degradation and food web changes in the bay.

Enrique Reyes, Coastal Ecology Institute, Louisiana State University, Baton Rouge, LA 70803; John W. Day, Jr., Brian C. Perez, Coastal Ecology Institute, Louisiana State University; Dan L. Childers, Southeast Environmental Research Program & Department of Biological Sciences, Florida International University.

The purpose of this project is to quantify the exchange of water and nutrient between Florida Bay and the adjacent mangrove wetlands and understand the processes that influence this exchange. This information will help us to understand and predict the effect of changing freshwater inflow to Florida Bay on the status of the mangrove wetland and the availability of nutrients in Florida Bay. This project started in August 1995 and will continue for the next 3 years.

We need to understand the response of the salinity transition zone that lies between the Everglades and Florida Bay to changing hydrology in south Florida. This transitional system is important because it may strongly affect the nutrient cycles of Florida Bay. The transition zone is also important because it is an essential nursery for many of the Bay's fish populations and habitat for wading birds, such as the roseate spoonbill.

The transition zone system, which is dominated by mangroves, is connected both to land and sea. Landward connection is provided by the flow of water from the Kissimmee-Okeechobee-Everglades watershed. It is likely that the ecology of the transition zone is highly sensitive to the quantity and quality of water in the watershed and thus sensitive to water management practices. Seaward connection is provided by the inflow of saline water from Florida Bay, which varies with winds and tides, as well as the opposing flow of fresh water.

With the mandate of the Everglades Forever Act, more fresh water will flow to Florida Bay through the mangrove fringe of the Bay. Furthermore, the distribution of this water will change in coming years, with an increase in flow through the Taylor Slough relative to flow through the C-111 basin. Thus, we know that water quantity and distribution is changing. However, we do not know the consequences of these changes on the quality of water flowing into the bay. Furthermore, we do not understand the indirect effects of changing fresh water flow and salinity on nutrient cycling within Florida Bay. Understanding the link between biological and chemical dynamics in Florida Bay and hydropatterns is a necessary precursor to effectively restoring Florida Bay ecological system.

Given the apparent nutrient enrichment of Florida Bay, as evidenced by sustained algal blooms, we need to know the extent to which there is a net transport of nutrients between the Bay and these transition zone ecosystems and how this transport will be affected by water management practices as well as natural forcing functions, such as winter and tropical storms. Mangroves can affect the nutrient cycles of the Bay because they can act as a biological filter, removing nutrients from either fresh water flowing from land or seawater that pulses through the system with changing winds and tides. The efficiency of this filter may differ for different nutrients. Thus, for example, mangroves may remove and retain P, but N may flow from the Everglades into the Bay. Alternatively, with their large store of nutrients (particularly organic P within vegetation and peat) the potential exists for system to be a source of nutrients to Florida Bay. It is likely that nutrients are rapidly flushed from these systems into the Bay during storm events.

1. Define seasonal patterns of forcing functions (including frontal and tropical wave frequency, precipitation, temperature, and evapotranspiration, water levels in freshwater areas of the Everglades) affecting water and materials exchange between central and eastern Florida Bay.

2. Quantify the exchange of water and nutrients between northeastern Florida Bay and fringing mangroves in one major mangrove creek over a period of three years.

3. Compare patterns of water and nutrient exchange in major mangrove creeks along an east to west gradient.

4. Compare patterns of water and nutrient exchange, as measured in mangrove creeks, to these patterns within the adjacent mangrove wetland.

5. Synthesize information of rates of nutrient and water exchange and the factors that influence these rates in the Everglades, the mangrove wetland, and Florida Bay.

Initially, we plan to review and characterize seasonal patterns of important forcing functions which affect water and material exchange between Florida Bay and fringing mangroves. The periods of the year when flux measurements will be carried out will be chosen based on factors such as rainfall, evapotranspiration, mean sea level, water level in the upper Everglades, frequency of frontal activity and tropical wave activity, and temperature. Since the tide range is so low in Florida Bay and climatic factors play such an important role in water exchange between Florida Bay and the fringing mangroves, the sampling design for flux studies must be based on these climatic factors.

Measurements of water and materials exchange at three tidal creeks will be carried out. During each flux study, water flow rate and instantaneous water flux, and concentrations of materials defined in the detailed work plan will be measured every 3 hours. The results of the flux calculations will be analyzed and compared to climatic forcings and management activities to determine the factors responsible for the observed fluxes.

A flume design will be used to conduct measurements of water and materials exchange between the primary tidal creek and adjacent mangroves. During each flume study, instantaneous water flux and concentrations of materials will be measured about every 2 hours when the marsh is covered with water. The results of the flux calculations will be compared to flux measurements in the primary tidal creek and other measurements (such as sediment water fluxes) in order to determine the relative importance of different processes in affecting flux of materials and water.

To integrate all the generated information, concurrently we plan to develop a predictive model of flux of materials between mangroves and Florida bay. We plan to use the model to analyze nutrient behavior under several water management regimes.

D.T. Rudnick, F.H. Sklar, S.P. Kelly, Everglades Systems Research Division, South Florida Water Management District, West Palm Beach, FL.

The main objective of the South Florida Water Management District (District) research program in Florida Bay is to understand and predict the effects of changing freshwater flow on the ecology of the Bay. The focus of our ecological research is on the ecotone between Florida Bay and the Everglades. This mangrove dominated zone receives the inflow of water both from the Everglades and from marine waters and is an area with a highly variable salinity regime. It therefore is an area that is directly affected by changes in the quantity and quality of freshwater inflow and is highly sensitive to water management practices.

This research program began in the summer of 1995 and is a collaboration among researchers from the District and other institutions, including the U. of Florida (C. Montague), Florida Marine Research Institute (M. Durako), Louisiana State U. (J. Day and E. Reyes), Florida International U. (D. Childers), USGS (E. Patino), and Everglades National Park (R. Fennema). The main components of this program include studies of the relationship between freshwater flow and the flux of nutrients across the Bay - wetland interface, internal fluxes of nutrients within the northern Bay and within the mangrove wetland, and the relationship between freshwater flow and submersed macrophyte structure and productivity in the region. This research program will continue for at least 3 years.

Field research in this program will focus on three areas along the north coast of Florida Bay: the Joe Bay-Trout Cove area, the Taylor River-Little Madeira Bay area, and the Seven Palm Lake-Terrapin Bay area. Each of these areas are important sites of freshwater inflow to Florida Bay. Salinity levels within each area potentially span a wide range along a north-south axis and each area encounters wide temporal fluctuations in water flow and salinity. These areas were also chosen because they probably span a wide range of nutrient availability, with increasing nutrients availability from east to west.

Within each area, the continous flux of nutrients over a ten-day period, through major connecting creeks (Trout Creek, Taylor River, and McCormick Creek) will be measured quarterly in association with seasonal changes in forcing functions such as freshwater flow (see abstract by Day, Reyes, and Childers). Flumes within the mangroves adjacent to a creek will measure the exchange of nutrients between the wetland and adjacent waters. Benthic nutrient flux measurements in the Bay and in ponds near these creek sites will be made simultaneously with creek flux measurements. Over a longer time-scale, net sediment accretion or subsidence will be measured at the wetland sites. This combined effort will help to determine under what conditions the mangrove ecotone is a source or a sink of nutrients and the extent to which the nutrient cycles of Florida Bay are affected by external nutrient sources.

Concurrent with these field studies of nutrient dynamics at the margin of the Bay, we will conduct studies of the temporal variability of submersed macrophytes in association with salinity and nutrient variations (see abstract by C. Montague). Salinity fluctuations have been hypothesized to be a major factor influencing the structural dynamics of the Bay's seagrass and macroalgae assemblage. This hypothesis will be tested in laboratory and field experiments.

These studies will be integrated in models of submersed macrophyte populations and the ecotone system. These component models will later be incorporated into the Everglades Landscape Model and thus help us to understand the mechanisms that link Florida Bay to the Everglades ecosystem.

DeWitt Smith, Everglades National Park.

A clear definition of long term variation in key physical variables is essential if we are to understand the estuaries of Everglades National Park. The objective of the Marine Physical Monitoring Project is to define physical conditions in the marine and estuarine parts of Everglades National Park and develop an understanding of the processes that determine these conditions. The approach we have chosen to reach this objective is to establish a series of continuous monitoring stations at key locations throughout marine and estuarine areas of Everglades National Park. We expect these stations to provide a representative long-term record of key physical parameters that will be used by project staff and other researchers addressing a variety of important research and resource management questions. Important physical parameters measured include: salinity, conductivity, temperature, water level, rainfall, wind, and radiation. Data from the project will be used to develop a suite of marine circulation, statistical, and process based landscape models to integrate our knowledge of the system and assist our staff in evaluating management alternatives.

The South Florida Natural Resources Center at Everglades National Park has been operating a network of marine monitoring stations in Florida Bay since 1988. The accompanying map shows the location of both active and proposed sites. Instruments at these sites record water depth every 10 minutes, and at some sites, rainfall, conductivity and temperature every hour. This network is part of a larger system of physical monitoring stations located in both freshwater and marine habitats within Everglades National Park. This project was initiated in October 1987 when nine stage and rainfall monitoring stations, previously operated by the Hydrology Program, were transferred to the Marine Program. Between 1987 and 1988 nine additional stations were added to the Marine Monitoring Network in Florida Bay. All stations were upgraded from chart recorders to digital data loggers between March 1988 and January 1990. During 1991 eleven new continuous monitoring stations were added to the network, a 62% increase in network stations. Four of these stations were placed in Barnes Sound and three in northeast Florida Bay under a cooperative agreement with the South Florida Water Management District (SFWMD). An existing meteorological tower at the Joe Bay station was instrumented as part of the same agreement. The Garfield Bight water quality station was installed to monitor conditions believed responsible for fish kills in the Flamingo area. Three light stations were operated for the seagrass die-off study during 1991 and 1992. These three sites are currently being brought back on line under a cooperative agreement with the Florida Department of Environmental Protection. During this past year four stations in Florida Bay have been equipped with radio telemetry equipment allowing the park staff to check current conditions at any time. Over the next several months twelve new stations will be installed in Gulf Coast estuaries as part of cooperative projects with the SFWMD and National Biological Service Global Climate Change Project.

Peter K. Swart, MGG/RSMAS, University of Miami, 4600 Rickenbacker Causeway, Miami FL 33149; Michael Lutz , MGG/RSMAS, University of Miami, 4600 Rickenbacker Causeway, Miami FL 33149.

Although there has been significant concern and research into the deterioration of water quality in Florida Bay, perhaps the most significant aspect of the factors causative in the decline of water quality has been not been addressed in any significant study. This factor is the extent of the degradation of organic material within Florida Bay. Organic material is important because through its decay it can release nutrients and consume oxygen and sulfate thereby creating anoxia and hydrogen sulfide toxicity. While significant attention has been directed at a restoration of historical salinity levels to Florida Bay, it is possible that salinity is not the problem and that fish and sponge die offs are in fact a result of oxidation of excessive amounts of organic material producing anoxia. There is no information other than that presented in this study regarding the flux of organic material through Florida Bay, the origin of the organic material, its rate of oxidation, and the effect that this oxidation has on the oxygen utilization, H2S, and nutrient levels.

Using data from our own studies as well as literature information, we present three methods with which to calculate a mass balance of organic carbon (OC) and distinguish between the various inputs into Florida Bay. These are (i) the mass balance approach, (ii) estimatation of the organic content of the sediments combined with the age of the bay, and (iii) the geochemical tracer approach.

The mass balance approach relies on combining the estimates of in situ production. Our present estimate is that there is approximately 1011 gms of organic carbon produced per year in Florida Bay. Based an an estimated input of between 50 and 600 km3/yr of freshwater from the Everglades and a particulate organic material (POM) content of 1200 µM/l, we can expect an additional 109 to 1010 gm/yr from this source. A further 10 to 20% organic carbon might be derived from the organic production on the mud-islands and the coastal mangrove fringe, suggesting the Everglades may account for upto 30% of the annual production of OC in Florida Bay. Production in the water column may provide upto 1010 gm/yr. 1.

An alternative approach to examining this question of OC recycling is to calculate the present amount of OC in the bay and compare it to the flux calculated in the first approach. Using these two estimates, an idea of the residence time of organic carbon and the precentage of production actually preserved in the sediments can be ascertained. As a first approximation the OC sinks in Florida Bay can be divided into two reservoirs. First, the mudbanks which comprise the vast majority of sediment contains approximately 3 x 1013 gms organic carbon. This carbon has accumulated over the history of Florida Bay and is relatively immobile. In contrast the basins contain approximately an order of magnitude less OC. This carbon is rapidly recycled and not preserved in the basins themselves, being exported into the marine environment or accumulating to form new or additions to existing mudbanks. Based on the present estimates of carbon in the bay and the estimates of production, it would appear that the carbon has a residence time of approximately 200 years. Our calculations suggest that of all the organic carbon which has been produced over the 4000 year history, approximately 3% has been retained in the sediments. This estimate is significantly higher than normal marine sediments and underlines the importance of better quantifying some of the fluxes involved.

A third approach is to use a combination of the carbon and nitrogen isotopic compositions of organic material together with parameters such as the C/N ratio to constrain the nature and the amounts of organic material contributed by end members to the system. Provided that the C/N ratios and the isotopic compositions of the various inputs are distinctive and the number of geochemical tracers are one greater than the number of significant inputs, then this approach can be used to constrain the contribution of organic material from the various sources. In a preliminary approach we have assumed that there are only two sources of organic material in Florida Bay, terrestrial and marine. The percentage of organic carbon derived from the Everglades can therefore be calculated using the bulk carbon isotopic composition of the sediment as shown in figure 2, and end members for terrestrial carbon of -25‰ and marine carbon of -8‰. Using this approach we estimate that the percentage of OC derived from the Everglades varies from as much as 70% close to Cape Sable to 30% in the center of the Bay. These estimates are higher than we predicted based on the mass flux considerations and underline the need for further research in order to explain this apparent discrepancy.

Work which is presently being carried out involves a better calibration of the various end-members and assumptions made using the previously described approaches. In particular we are (1) carrying out a temporal and spatial investigation of the amount and isotopic composition of POM in the water column, (2) examining methods with which we might characterize the rate of decomposition of organic material, (3) refining a model which will account for the cycling of OC from the Everglades into Florida Bay. This work is essential if we are to understand the links between nutrients, the development of anoxia and the die-off of sea grasses and other organisms in the bay.

Last updated: 07/16/98
by: Monika Gurnée
gurnee@aoml.noaa.gov