The Biscayne Bay region of southern Florida has been affected by natural events such as steadily increasing sea level, droughts and hurricanes. In addition, anthropogenic alterations of the south Florida region, urbanization, increased agricultural activity, dredging of natural waterways, pollution, and water management of the Everglades, have changed the setting of south Florida. These events and modification of the south Florida geography have driven the evolution of the Biscayne Bay ecosystem. However, the magnitude at which each of these factors affects the ecosystem remains unknown.

The "Ecosystem History of Biscayne Bay and the Southeast Coast" project of the U.S. Geological Survey was initiated to determine changes in the Biscayne Bay ecosystem over the last 150 to 200 years and evaluate natural, as well as anthropogenic causal mechanisms. Faunal (foraminifera, ostracodes, and molluscs), floral (palynomorphs) and geochemical data (stable isotope, radioisotopes and trace elements) from modern environments are applied to shallow core sediment samples to infer a variety of historical environmental information (salinity, nutrients, substrate and productivity). This paper presents results from a survey of modern foraminiferal assemblages and their application to the historic record from a 120 cm core (MB1) collected in Manatee Bay to infer environmental changes during the past 150 to 200 years.

Modern benthic foraminiferal distributions in Biscayne Bay were determined to provide modern analogs to which down-core assemblages could be compared. The distribution of modern benthic foraminifera within Biscayne Bay was analyzed using 23 surficial sediment sample sites, each collected in June and August, 1996. The foraminiferal data were compared to hydrologic data (temperature, salinity, nutrients, clarity, dissolved oxygen and redox potential) also collected at each site to determine significant environmental controls on the foraminiferal distributions. A total of 69 taxa of benthic foraminifers common to the North American southeast coast and Gulf of Mexico were identified from the Biscayne Bay modern sediment samples. Species diversity, as measured using Simpson's Index, ranges from 0.080 to 0.493. The Simpson's Index represents diversity by reflecting species dominance with 1 signifying a monospecific assemblage and values decreasing with increased species number and equability. The foraminiferal assemblages are dominated by calcareous forms with agglutinated taxa constituting a minor component in most of the assemblages. Species dominance varies considerably throughout the Bay with Ammonia parkinsoniana forms typica and tepida and Elphidium galvestonense mexicanum constituting over 50 percent of the assemblage in restricted regions of Biscayne Bay. Conversely, Archaias angulatus is dominant (up to 45 percent) in samples collected from the open regions of Biscayne Bay. Other calcareous hyaline forms include Bolivina spp., Elphidium delicatulum, Rosalina floridana and R. globularis. Common miliolids include Articulina mucronata, Miliolinella circularis, M. labiosa, Peneroplis proteus, Pyrgo subsphaerica, Quinqueloculina agglutinans, Q. bosciana, Q. poeyana, Q. polygona, Q. seminulum, Q. tenagos and Triloculina tricarinata. Agglutinated taxa in Biscayne Bay include Ammobaculites sp., Clavulina sp., Pseudoclavulina gracilis, Textularia candeiana, T. conica, and Trochammina spp.

Three dominant benthic foraminiferal assemblages are present within modern Biscayne Bay: Ammonia-Elphidium assemblage, Archaias-miliolid assemblage, and Bolivinid assemblage. The Ammonia-Elphidium assemblage occurs in restricted environments with relatively low salinities (<35 ppt) and regions with frequent point-source fresh water input. The Archaias-miliolid assemblage is associated with marine salinities (37-38 ppt), high water clarity, and well circulated environments. The Bolivinid assemblage occurs in the northernmost Biscayne Bay and is associated with diatomaceous muds that are rich in organic matter, suggesting high productivity.

Six shallow sediment cores were collected along a northeast-southwest coring transect in Biscayne Bay; MB1, MB2, CB1, FB1, BP1, and PB1, in November 1996 and February 1997 for faunal, floral and geochemical analyses. Preliminary benthic foraminiferal analyses were conducted on core MB1 collected from Manatee Bay, Barnes Sound (25 15.69¥ N, 80 24.06¥ W) in November 1996. Forty-one benthic foraminiferal species were identified from 60 samples of core MB1. The assemblages are dominated by calcareous forms and include Ammonia parkinsoniana formas typica and tepida, Elphidium galvestonense formas mexicanum and typicum and Quinqueloculina spp. Three distinct foraminiferal faunal zones are identified in MB1 that correlate strongly with the sedimentology. The Ammonia-Elphidium assemblage dominates (>95 percent) the lowermost part of core MB1 (84-120 cm) where the sediments consist of a yellow-gray peaty marl. The next assemblage shows an increase in species diversity with Ammonia spp. and Elphidium spp. remaining the dominant components (50-90 percent) and occurs from 66 cm to 84 cm in core MB1, within a light olive-gray shelly mud. The third assemblage is the Quiqueloculina spp. assemblage, similar to the modern Archaias-miliolid assemblage in modern Biscayne Bay, that is dominant (>50 percent) from 6 cm to 66 cm within a sandy mud (32-66 cm) through a vegetation-rich, yellow-brown mud (6-32 cm). The upper part of the core (0-6 cm) displays a trend of increasing Ammonia-Elphidium assemblage components.

The foraminiferal trends observed in core MB1 indicate a progressive increase in salinity up-core with the upper-most part of the core showing a reversed trend toward freshening conditions, consistent with the sediment observed. More detailed faunal analyses of core MB1 are underway, as well as floral and geochemical analyses. Radioisotope analyses (210Pb) are currently being conducted to provide an age model for the core. The remaining cores are being processed for similar analyses to provide an overall record of ecosystem/environmental change within Biscayne Bay. These results, in addition to the ecosystem history records from cores collected in Florida Bay will result in a more comprehensive understanding of the South Florida ecosystem.

Back to Project Homepage