Subtropical Estuarine Environments: Multidisciplinary Studies II
Presiding: G L Wingard, U.S. Geological Survey; P Swart, Rosenstiel School of Marine and Atmospheric Sciences, University of Miami
B44A-01 15:30h
Tampa Bay Pilot Study: Development of an Integrated Science Strategy for Understanding Estuarine Ecosystems
Many of the nation's estuaries have been environmentally stressed since the turn of the century and will continue to be impacted in the future. The U.S. Geological Survey (USGS) has initiated a research effort in the Gulf of Mexico with the goal of developing reliable predictive capabilities for resource managers based on a system-wide understanding of natural and anthropogenic influences on estuarine ecosystems. Key to the USGS approach is the recognition that estuarine systems function as a result of a dynamic interplay among biologic, geologic, hydrologic, atmospheric, and chemical processes. Therefore, achieving credible results requires a research strategy integrated among scientific disciplines. In order to demonstrate the efficacy of this approach, Tampa Bay was selected for a five-year study. A primary goal of the Tampa Bay Pilot Study is to develop science and management strategies for U.S.G.S. to engage in integrated science that will provide a research template for studies in other Gulf of Mexico estuaries. The Tampa Bay Pilot Study, in partnership with the Tampa Bay Estuary Program and key federal, state, and local entities, has successfully combined regional and high resolution mapping techniques, groundwater and surface water research, wetland and seagrass studies, geological investigations, and information management strategies to assess system-wide impacts of environmental change in Tampa Bay. Successful completion of the two and a half years of this study has provided the foundation to develop predictive modeling capabilities for understanding potential impacts of environmental perturbations in the future.
B44A-02 15:45h
Using Natural Geochemical Tracers to Discern the Dominant Sources of Freshwater into Biscayne Bay, Southeast Florida
Biscayne Bay is a sub-tropical estuary located on the carbonate platform of south Florida. The water occupying Biscayne Bay is a balance of saltwater influx from the open ocean and freshwater inputs from precipitation, surface water runoff, and submarine groundwater discharge. The bays watershed includes a total of 3 million inhabitants, the major urban centers of Miami and Ft. Lauderdale, as well as the Everglades system. With the development of south Florida, the natural diffuse groundwater and stream flow into the bay has been replaced by a large system of canals and levees in an effort to control flooding and drain swampland. The Comprehensive Everglades Restoration Plan includes changes in the freshwater deliveries to Biscayne Bay from point-source discharges via canals to non-point source discharges via wetlands and groundwater flow. The balance of salinity in Biscayne Bay effects sensitive seagrass and tidal ecosystems including numerous species of corals and other biota. A comprehensive understanding of the flow of freshwater into the bay is crucial to future planned developments and restorations. The goal of this study is to use naturally occurring geochemical constituents as tracers to identify and quantify the sources of freshwater, i.e. rainfall, canal flow, and groundwater, discharge to Biscayne Bay. In this study, discrete samples of precipitation, canal water, terrestrial groundwater, marine groundwater, and bay surface water are collected monthly and analyzed for the stable isotopes of hydrogen and oxygen as well as for major cations and anions. Initial results indicate that fresh groundwater has an isotopic signature (del 18O = -2.66 per mil, del D, -7.60 per mil) similar to rainfall (del 18O = -2.86 per mil, del D =-4.78 per mil). In contrast canal water has a heavy isotopic signature (del 18O = -0.46 per mil, del D = -2.48 per mil) due to evaporation. Thus it is possible to use stable isotopes of oxygen and hydrogen to separate canal water from precipitation and groundwater as a source of freshwater into the bay. Other geochemical constituents, such as calcium and magnesium are being investigated to further discern between the sources of canal water, rainfall and fresh groundwater. Both the stable isotopes and ion values will be placed in a mixing model to quantify and discern the dominant sources of freshwater into the Bay in both time and space.
B44A-03 16:00h
Biogeochemical transport in the Loxahatchee River estuary, FL: The role of submarine groundwater discharge
The distributions of dissolved organic carbon (DOC), silica, select trace elements (Mn, Fe, Ba, Sr, Co, V,) and a suite of naturally-occurring radionuclides in the U/Th decay series (222Rn, 223,224,226,228Ra, 238U) were studied during high and low discharge conditions in the Loxahatchee River estuary, Florida. The zero-salinity endmember of this still relatively pristine estuary may reflect not only river-borne constituents, but also those advected during active groundwater/surface-water discharge. During low discharge conditions, with the notable exception of Co, trace metals indicate nearly conservative mixing from a salinity of ~12 through the estuary (This statement contracdicts with what is said in p. 7). In contrast, of the trace metals studied, only Sr, Fe, U and V exhibited conservative estuarine mixing during high discharge. Dissolved organic carbon and Si concentrations were highest at zero salinities, and generally decreased with an increase in salinity during both discharge regimes, indicating removal of land-derived dissolved organic matter and silica in the estuary. Suspended particulate matter (SPM) concentrations were generally lowest (< 5 mg L-1) close of zero salinity, and increased several-fold (~18 mg L-1; low discharge) towards the seaward endmember and this attributed dynamic resuspension the estuary. Surface water-column 222Rn activities were most elevated (> 28 dpm L-1) at the freshwater endmember of the estuary, and appear to identify regions of the river most influenced by active submarine groundwater discharge (where is the data that show this?). Activities of four naturally-occurring isotopes of Ra (223,224,226,228Ra) in this estuary and select adjacent shallow groundwater wells indicate mean estuarine water mass residence times of less than 1 day; values in close agreement to those calculated by tidal prism and tidal period. A radium-based model for estimating submarine groundwater discharge to the Loxahatchee River estuary yielded an average of 1.03 V 3.84 x 105 m3 day-1, depending on river discharge stage as well as slight variations in the particular Ra models used. Such calculated flux estimates are in close agreement with results obtained from a 2-day electromagnetic seepage meter (0.9 x 105 m3 d-1) deployment during high discharge at the confluence of Kitching Creek and the Loxahatchee River, as well as with surficial aquifer recharge estimates. Calculated submarine ground-water discharge rates yield NH4+ and PO4-3 flux estimates to the Loxahatchee River estuary that range from 63 - 1060 Ymol m-2 d-1 and 69 - 379 Ymol m-2 d-1, respectively.
B44A-04 16:15h
Chemical and Isotopic Variations of Three Typical End-Member Organic Materials During Aging Process: Implications for Assessment of Relative Contributions of Various Organic Inputs in Altamaha Estuary
To evaluate the applicability of the end-member model in assessment of relative contributions of organic matter from various sources in estuarine system, we incubated three typical end-member organic materials (C3 marine diatom, C3 land grass, and C4 salt marsh plant) in Altamaha estuarine water over two months. Chemical and isotopic parameters (bulk organic carbon/nitrogen, lipid composition, stable C/N isotopes and lipid stable carbon isotopic ratios) were analyzed for fresh and aged materials. The analytical results showed that the chemical and isotopic compositions of three organic materials varied from material to material and from compound to compound during aging process. TOC and TN contents of C3 marine diatom and C3 land grass dramatically decreased (by 40-60%) while C4 salt marsh plant reduced its TOC and TN contents by less than 13% during incubation. Lipid compositions in all materials were greatly changed by aging process, as indicated by marked drops of unsaturated/saturated fatty acid ratios (e.g., 16:1/16:0 for diatom and 18:2+3/16:0 for higher plants). Bulk Δ13C ratios of aged materials changed in a small range (-0.17‰ to +1.99‰) while their Δ15N ratios shifted more noticeably (from +3‰ to +7‰). Lipid carbon isotopic compositions of three materials changed differently during aging process: some sterols (e.g., 27Δ5 and 29Δ5) had small isotopic shifts (<±1.7‰) while most plant fatty acids showed much greater enrichment (up to +8.3‰) except 16:0 in marine diatom (-2.6‰ depletion). Bacteria-specific fatty acids [iso-15:0, anteiso-15:0 and 18:1(‰7)] became significant components during incubations of all materials and their specific carbon isotopic ratios well corresponded to those of the substrate materials. These results suggest that use of end-member model must consider the age of organic materials and the potential changes in isotopic signals. The Δ13C ratios of bacterial-specific fatty acids also provide insights to the distributions of bioavailable organic matter in estuarine system.
B44A-05 16:30h
Flood Tide Transport of Blue Crab Postlarvae: Limitations in a Lagoonal Estuary
Blue crabs, an important commercial species, spend much of their life in estuaries along the east coast. The larvae spawn at or near the ocean, but the juveniles mature in the lower salinity waters of the estuary. It is generally believed that blue crab postlarvae migrate into near surface waters on flood, possibly cued by increasing salinity, and return to the bottom on ebb. Over several tidal cycles, the postlarvae travel a significant distance up-estuary. This model applies quite well to Chesapeake Bay, which has a strong along-estuary salinity gradient and large tides, but may not apply as well to Pamlico Sound, where circulation and salinity are more wind-driven than tidal. A recently completed study (N. Reyns, PhD), indicates that postlarval blue crabs use flood tides and wind-driven currents to cross Pamlico Sound. This study was based on observations with good spatial coverage, but limited vertical and temporal resolution. We have recently completed a complementary study, sampling crab larvae around the clock at four depths at a single location. Preliminary results from the new study suggest that the crab postlarvae do swim all the way to the surface, on flood only, and that flood currents are strongest slightly below the surface. These observations suggest the utility of flood tide transport in this system. However, near bottom salinity does not seem to be driven by tides; at this point it is unclear what cue might trigger the vertical migration of the postlarvae.
B44A-06 16:45h
Use of a 15N tracer to determine linkages between a mangrove and an upland freshwater swamp
Mangrove forests and adjacent upland freshwater swamps are important components of subsistence-based economies of Pacific islands. Mangroves provide valuable firewood (Rhizophora apiculata) and mangrove crabs (Scylla serrata); intact freshwater swamps are often used for agroforestry (e.g., taro cultivation). While these two systems are connected hydrologically via groundwater and surface flows, little information is available on how they may be biogeochemically or ecologically linked. For example, mangrove leaf litter was once thought to be an important food source for resident and transient nekton and invertebrates, but this value may have been overestimated. Instead, nutrients or allochthonous material (e.g., phytoplankton, detritus) delivered via groundwater or surface water from upland freshwater swamps may play a larger role in mangrove food webs. Understanding the linkages between these two ecologically and culturally important ecosystems will help us to understand the potential impacts of hydrological alterations that occur when roads or bridges are constructed through them. We conducted a 15N tracer study in the Yela watershed on the island of Kosrae, Federated States of Micronesia. K15NO3 was continually added at trace levels for 4 weeks to the Yela River in an upland freshwater swamp adjacent to a mangrove forest. Nitrate and ammonium pools, major primary producers, macroinvertebrates, and fish were sampled from stations 5 m upstream (freshwater swamp) and 138, 188, 213, and 313 m downstream (mangrove) from the tracer addition. Samples were collected once a week prior to, during, and after the 15N addition for a total of 6 weeks. Preliminary results revealed no significant enrichment (< 1 ‰) in the 15N isotope composition of either resident shrimp (Macrobrachium sp.) or mudskipper fish (Periophthalmus sp.). However, the 15N signature of ammonium pools was enriched 10-60 ‰ by the end of the third week. These results suggest that the tracer was present in the mangrove but was either unavailable to higher organisms or was incorporated into organic matter not utilized by shrimp or mudskippers.