Subtropical Estuarine Environments: Multidisciplinary Studies I
Presiding: G L Wingard, U.S. Geological Survey; P Swart, Rosenstiel School of Marine and Atmospheric Sciences, University of Miami
B43A-01 13:30h
Application of Multidisciplinary Research to Estuarine Restoration: Examples From South Florida
Estuaries are complex integrated systems susceptible to physical, biological, and chemical changes; and this vulnerability is amplified by their typically shallow and enclosed nature. The stressors that affect these systems can be anthropogenic and natural. The passage of the Estuary Restoration Act of 2000 focused national attention on the need to restore these vital bodies of water. Currently, many estuaries around the country, for example Chesapeake Bay, San Francisco Bay, Puget Sound, and Galveston Bay, are undergoing large restoration efforts. In South Florida, the restoration of Biscayne Bay and Florida Bay is a component of the larger South Florida restoration effort, guided by the Comprehensive Everglades Restoration Plan. Critical components of the restoration of any estuary are understanding the natural cycles of change that have occurred over decades and centuries, and determining the extent to which human activity has altered those natural patterns. Scientific surveys do not extend far enough back in time to provide the types of data necessary to reconstruct several hundred years of natural variation, and anecdotal information, when available, only provides undocumented snapshots of the estuary. In South Florida we have successfully integrated biological, paleoecological, geochronological, geochemical, biochemical, and sedimentological data to reconstruct the ecosystem history of the estuaries. Shallow sediment cores have been collected at multiple locations within the estuaries, dated using Pb-210 and C-14 geochronology, and analyzed for faunal and floral remains, sediment geochemistry, and shell biochemistry. Core assemblage data are compared to modern census data from different salinities to provide ecological interpretations of down-core salinity proxies. In Florida Bay, we have determined that 1) salinity is more strongly correlated to rainfall than to water management practices or any other single factor; and 2) anthropogenic influences play a secondary, although still very important role, in determining salinity for Florida Bay. Declines in species diversity and increases in the predominance of salinity tolerant species have occurred since the 1980's in several benthic invertebrate groups. In central and southern Biscayne Bay, all cores examined to date indicate increasing salinity during the 20th century. In the nearshore sites, the increase in average salinity has been accompanied by an increase in variability of salinity. In contrast, cores collected from the mudbanks in central Biscayne Bay show an increasingly marine environment, with fewer salinity variations in the 20th century compared to the 19th century. The results of these multidisciplinary analyses allow restoration managers to establish realistic targets and performance measures for restoration. Given the limited resources available for restoration, it is essential to focus efforts on restoring the components of the system that have been altered by human intervention. Attempting to alter changes caused by natural variations is not cost-effective or sustainable.
B43A-02 13:45h
Determining the Rate of Environmental Change in the Subtropical Florida Bay.
Over the past few decades, short-lived isotopes (7Be, 210Pb, and 137Cs) have been used extensively to define the rates of environmental changes. In a 10-year study, short-lived isotopes were used to establish historical records and baseline information at 102 sites in the southern Everglades and Florida Bay. The most profound discovery was the recognition of environmental changes in the lakes and mud islands along the northern boundary of the bay. Prior to 1950, the bay floor was rock: a hardbottom habitat. Beginning around 1950, concurrent with decrease in freshwater flow, the environment changed from estuarine to marine. With this shift, sediment began to accumulate over hardbottom surfaces, creating a soft-bottom ecosystem. In addition, because of the subsurface geology of South Florida and the nuances of the short-lived isotope systematics, it was determined that subsurface freshwater retreat had coincided with the estuarine-to-marine change. The increasingly marine nature in the northeastern portion of the bay also affected that part of the bay by increasing carbonate production in the region. As a result, sediment accreted to the mud islands, extending tidal flats. The increase in sediment closed the passes between islands, restricting circulation. In the central bay, the sediment accumulation record showed that deposition was not as affected by the change in hydrology but was controlled by variations in progressive sea level rise. The sea level record at Key West shows that the rate of rise has not been constant but has varied, with periods of relatively rapid rise followed by periods of no change. The response to this sealevel variation is a change in accommodation space: rising sealevel increases the accommodation space and increase sediment accumulation; during periods of stable sea level, the accommodation space rapidly decreases, leading to a decrease in sediment accumulation.
B43A-03 14:00h
A Comparative Study of Florida Bay Benthic Foraminiferal Distribution Change in 30 Years and its Environmental Significance
Florida Bay, the southernmost extension of Everglades National Park, is a unique ecosystem that provides sanctuary to a diverse flora and fauna. Decades of modification to the fresh water delivery and coastal development of Florida Bay have resulted, in part, to significant changes in sub-aquatic vegetation and a variety of marine invertebrate distributions. This led to major research efforts by federal, state and local agencies, as well as academic institutions culminating in the Comprehensive Everglades Restoration Plan (CERP). As part of the research effort, a broad array of surface sediment samples was collected from Florida Bay in the mid 1990's. Benthic foraminiferal analyses of these samples indicate distinct spatial distributions of species related primarily to salinity and sea grass distributions. Benthic foraminiferal, molluscan, and ostracode data from sediment cores collected at various localities throughout Florida Bay indicate historical variability in salinity, with several salinity events associated with land and water management changes in southern Florida. Results are presented of a comparative study between foraminiferal data of surficial sediment samples collected in the 1960's and the more recent data collected in the 1990's. Multivariate statistical analyses of the distribution data from the two data sets are used to determine the magnitude and spatial extent of changing salinity and sea grass conditions that lapsed over an approximate 30 year period. Results of these analyses are then compared to results from the core analyses to determine their consistency.
B43A-04 14:15h
Distribution of Epiphytic Diatoms in a Sub-Tropical Estuary
Within estuaries, seagrasses may represent an order of magnitude greater surface area relative to sediments for the colonization and growth of diatoms. Fossil diatom distributions have proven useful in inferring paleoenvironmental conditions. The strength of these inferences is dependent upon defining the environmental relationships of contempory diatom compositions. The present investigation characterized the modern epiphytic diatom flora on the seagrass Thalassia testudinum at seven sites in the sub-tropical Florida Bay estuary and at one Atlantic Ocean site east of the upper Florida Keys. These sites were sampled six times between March 2000 and April 2001. Diatom species composition was related to water quality parameters using multivariate statistics. 338 diatom species were identified. The seven most abundant species from pooled samples were Cocconeis placentula, Brachysira aponina, Nitzschia liebetruthii, Hyalosynedra laevigata, Amphora cf. tenerrima, Mastogloia crucicula, and M. pusilla. These seven species collectively accounted for 51.7 percent of all valves counted and occurred in at least 85 percent of all samples. Analysis of similiarity and NMDS ordination of species relative abundances revealed four distinct diatom communities across the study region. The spatial variability of these communities was correlated with salinity and water-column nutrient availability. Summertime communities were significantly different from winter-spring communities, but these communities showed a gradual temporal progression with much overlap. The temporal variability was correlated with temperature. Indicator species analysis identified many species significantly influencing the four spatial groups. The Atlantic marine site was characterized by many different Mastogloia species and some epipsammic (sand-grain associated) diatoms (i.e., Cymatosira lorenziana, Dimerogramma dubium, and Neofragilaria nicobarica). Mastogloia pusilla, Rhopalodia pacifica, and Cocconeis woodii were strong indicators of the Gulf of Mexico marine site. Reimerothrix floridensis was particularly abundant in the western interior of Florida Bay (i.e., sites 2, 3, and 4) during summer months. The eastern interior of Florida Bay was characterized by high relative abundances of Brachysira aponina and Nitzschia liebetruthii. The optima and tolerance of these indicator species relative to individual water quality parameters were also determined.
B43A-05 14:30h
Interpreting Recent Water Quality Changes in Florida Bay in a Paleoecological Context Using Diatoms and Linked Organic Biomarkers
Recent alterations in the quality and quantity of freshwater flowing into Florida Bay from the Everglades and surrounding landscape of south Florida has altered the salinity and nutrient regime of the Bay. To interpret these changes with respect to natural variability in this ecosystem we conducted a multiproxy paleoecological investigation at several sites in the Bay. Sediments were cored to bedrock, chronologically calibrated using conventional radiometric methods and sampled at 2 cm intervals for a variety of paleoenvironmental proxies including diatoms, organic biomarkers and nutrient concentrations. Interpretations of paleoenvironmental settings were based on habitat specificities of diatoms and their relationships to biomarkers and nutrients determined by a survey of the modern environment. Signatures in basal peat deposits suggest fresh to brackish-water mangrove-dominated communities existed in central Florida Bay as little as 2000 YBP. Overlying marl contains benthic diatom assemblages indicative of a shallow, seagrass-dominated marine environment that experienced large, decadal-scale fluctuations in salinity. Recent sediments (<100 years) reveal a third zone that contains a flora that is compositionally dissimilar to those occurring in the past. Together with organic biomarkers, phosphorus and nitrogen data, the recent diatom record generally suggests a transition to conditions favoring proliferation of planktonic algae over benthic epiphytes, suggesting displacement of primary production from seagrass beds (which have declined in aerial extent) to the plankton as water-borne nutrients become more available. Several sites show an increase in diatom-inferred salinity after 1990, while the amplitude of fluctuation in salinity has declined.
B43A-06 INVITED 14:45h
Coastal Ecosystems and Climate Change: Is Modeling and Monitoring Enough?
Many coastal ecosystems are severely degraded due to a variety of human factors, requiring large and expensive monitoring and modeling efforts for restoration and management. Climate variability, including abrupt climate change, is seldom factored into coastal ecosystem management despite growing evidence for climate forcing of precipitation, river discharge, water quality, salinity, turbidity, faunal and phytoplankton dynamics, dissolved oxygen, and other ecosystem processes. We will review evidence from long-term monitoring records, multi-proxy paleoclimatic and paleoecological records, and climatic modeling that suggests that the effects of climate can override local and regional human activities and may potentially diminish the success of restoration efforts. Because ecosystem restoration often involves long-term objectives requiring decades to achieve, our focus will be on examples from sub-tropical and temperate estuaries in North America that show ecosystem response over decadal timescales to variability related to El Niño-Southern Oscillation, the Pacific Decadal Oscillation and the North Atlantic Oscillation. Climatic variability evident from paleo-records of the past few centuries exceeds that recorded in most 20th century monitoring records. This raises issues about the efficacy of local and regional ecosystem and hydrodynamic models designed to simulate ecosystem response to anthropogenic changes in sediment and nutrient input, fresh-water discharge, and land-use because such models, though tested with rigorous validation procedures, use calibration data sets limited to a few years. Thus, they might not be appropriate for simulating response to climatic extremes on the scale and duration of past events outside their calibration range. Understanding the complexities of ecosystem response to climatic forcing, especially in the context of local and regional ecosystem disturbance, raises formidable challenges, but attempts to integrate climate variability would be a positive step in progressive adaptive management.