Recent negative trends have been observed in the ecosystem of Florida Bay, including algal blooms, seagrass die-offs, and declining numbers or shellfish, adversely affecting the fishing and tourist industries. Many theories of cause and effect exist to explain the adverse trends, but these theories have not been scientifically tested. Prior to finalizing plans for ecosystem restoration, the relative roles of human activities versus natural ecosystem variations need to be established. This project addresses this need by focusing on two primary goals. First, to determine the characteristics of the ecosystem prior to significant human alteration, including the natural range of variation in the system; this establishes the baseline for restoration. Second, to establish the extent, range, and timing of changes to the ecosystem over approximately the last 150 years and to determine if these changes correlate to human alteration, meteorological patterns, or a combination of factors. In addition, data on recovery times of certain components of the ecosystem will be obtained allowing biologists to estimate responses to proposed restoration efforts. This project is planned as a five year study, to be completed in 1999.

This project is one segment in a group of coordinated USGS projects examining the biota, geochronology, geochemistry, sedimentology, and hydrology of southern Florida, Florida Bay and the surrounding areas. Data are being compiled from terrestrial, marine, and freshwater environments in onshore and offshore sites in order to reconstruct the ecosystem history for the entire region over the last 150 years.

Methods: In cooperation with other USGS projects, and other state and federal agencies, a series of shallow piston cores (~1-2 m) have been collected in the central and northeastern areas of Florida Bay. The cores are x-rayed and examined to determine the degree and extent of sediment disruption. Cores that appear to contain relatively undisturbed sediments are submitted for 210Pb analysis to determine the age and degree of disruption of the sediments. An independent support for the age model is obtained by analysis of the pollen in the core; key introduced species include Casuarina, Schinus, and Melaleuca.

Cores that have a good stratigraphic record are sampled at closely spaced intervals (2 cm) for all macro- and micro-fauna and flora present. The primary biota analyzed are 1) benthic foraminifera, 2) ostracodes, 3) mollusks, 4) dinoflagellate cysts, 5) pollen and macro-plant material. The faunal and floral groups are compared by means of quantitative down-core assemblage diagrams. Determinations of salinity, bottom conditions, nutrient supply and various other physical chemical parameters of the environment are made for each sample based on the fauna and flora present.

Examination of the diversity and distribution of the fauna and flora over time allows us to infer the nature and extent of changes that have occurred in the ecosystem. In the marine environment, benthic foraminifera and ostracodes are used to suggest changes in salinity and substrate; micromollusks indicate substrate changes and general changes in salinity patterns; dinoflagellate cysts may indicate algal blooms and current patterns; pollen reflects the composition of onshore vegetation, therefore providing an indication of regional changes, such as climate, hydroperiod or nutrient supply.

Data from all cores are being integrated to search for regional patterns of change in diversity and distribution of the fauna and flora; data from Florida Bay will be correlated to data obtained in the corresponding USGS onshore ecosystem history project. The integrated data set will be analyzed to see if detected changes in biota correlate to alterations in physical parameters and/or historic records of human-induced modifications of the environment.

In addition, modern core-top living assemblages are being collected twice a year (in the wet and dry seasons), over a period of several years, from the central and northeastern portions of the bay, in order to provide data on seasonality, habitat distribution, and preferred substrates of the living biota for interpretation of the down-core assemblages.

Preliminary Results: Two shallow cores have been examined to date: 1) Bob Allen #6A Core (172 cm total depth), located on a grass covered mud bank near the southern end of the Bob Allen Keys; 2) Taylor Creek Core T-24 (90 cm total depth), located near the mouth of Little Madeira Bay. The age model developed using 210Pb established a 1.1 cm/year sedimentation rate for the Bob Allen #6A core; the ages discussed below are estimates based on that rate.

The Bob Allen #6A core can be divided into five zones based on the benthic fauna. The ostracodes, mollusks, and benthic foraminifera in the lower portion of the core (172-135 cm; ~1850-~1890) show moderate diversity and abundance; they indicate that the salinity ranged from 20-30 ppt and the subaquatic vegetation was present in moderate amounts. The dinoflagellate cysts were low in abundance and the terrestrial vegetation, as indicated by the pollen, was pine-dominated upland forests. The portion of the core from 135-75 cm (~1890-~1930) is characterized by a very low diversity, low abundance, benthic faunal assemblage dominated by sand dwellers; subaquatic vegetation was low to absent, and the salinities ranged from 15-25 ppt. In contrast, dinoflagellate cysts were relatively abundant in this portion of the core, and the assemblage was dominated by one species, Polysphaeridium zoharyi. The terrestrial vegetation began to shift to typical wetlands species, with more salt-marsh, slough, and hardwood pollen present. All five biotic elements examined show corresponding changes at approximately the 135 cm and 75 cm sections of the core.

Above 75 cm in the Bob Allen #6A core a great deal of fluctuation occurs within the benthic fauna. This portion of the core is divided into three zones: 75-35 cm (~1930-~1970); 35-10 cm (~1970-~1985); and 10-0 cm (~1985 to the present). The 75-35 cm and 10-0 cm zones are characterized by moderate to high benthic faunal diversity and abundance, and low dinoflagellate cyst abundance. The benthic fauna indicate a period of extreme fluctuations in salinity, ranging from 20->50 ppt, and abundant subaquatic vegetation. The 35-10 cm zone is distinguished by a return to lower diversity and abundance for the benthic fauna, and low to moderate abundance for the dinoflagellate cysts. The salinities in this portion of the core appear more stable, fluctuating between 15-30 ppt, and the subaquatic vegetation was low. The pollen for the upper portion of the core, from 75-0 cm indicates greater abundances of onshore wetlands environments, dominated by hardwoods, mangroves and buttonwoods.

The benthic fauna seen in the upper 60 cm in the Taylor Creek T-24 core show the same kinds of fluctuations as those seen in the upper 75 cm of the Bob Allen #6a core. The dinoflagellate cysts show a significant increase in the dominance of Polysphaeridium zoharyi at 75 cm in the Taylor Creek T-24 core, which may correspond to the increase seen at ~ 75 cm in the Bob Allen #6a core (although the Taylor Creek T-24 core has a higher sedimentation rate). Taylor Creek T-24 core has approximately 20% more Polysphaeridium zoharyi than Bob Allen #6a throughout the core; this is consistent with the more restricted marine setting of Little Madeira Bay. The pollen record for Taylor Creek T-24 shows more subtle changes in the onshore terrestrial environment, primarily because of its closer proximity to land, but the general patterns are the same. The basal portion of the core (90-80 cm) indicates a pine-dominated upland, and above 80 cm wetland vegetation becomes more abundant.

Our results indicate that major environmental changes occurred in the Florida Bay and Everglades ecosystems over the last 150 years. Periods of decreased salinity correspond to intervals of decreased subaquatic vegetation, lower benthic faunal diversity, and lower benthic faunal abundance. Periods of increased salinity correspond to intervals of increased subaquatic vegetation, higher benthic faunal diversity, and higher benthic faunal abundance. The pollen and dinoflagellate cyst records show that changes in the terrestrial habitat and the pelagic realm are roughly synchronous to the marine benthic faunal record. This apparent synchroneity between the marine and terrestrial realm implies changes occurred in parameters that affect both habitats. Rainfall and/or human-altered hydroperiods may be possible explanations for the changes seen. Presently, there is no evidence of hypersalinity in Florida Bay prior to ~ 1930. The fluctuating salinities and periods of hypersalinity that we see are consistent with controlled discharge through the canal system and inconsistent with natural sheet flow.

Future Work: Analysis of cores and samples collected to date will be completed, and monitoring of the modern environment for our core-top data base will continue. In addition, we hope to begin geochemical analyses of the shell material. Initial efforts have been concentrated in the northern and eastern portions of the bay, the areas presumed to be most impacted, but as patterns and trends begin to emerge, we plan to continue sampling along rough transects moving west and south through the Bay. Coordination with other USGS projects focused on terrestrial biota, geochronology, geochemistry, sedimentology, and hydrology should enable reconstruction of the regional ecosystem over the last 150 years by the time the project is terminated.

(This abstract was taken from the Florida Bay Science Conference Proceedings, 1995)

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