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U.S. Department of the Interior A Paleoecologic Reconstruction of the History of Featherbed Bank, Biscayne National Park, Biscayne Bay, Floridaby Jeffery R. Stone, Thomas M. Cronin, G. Lynn Brewster-Wingard, Scott E. Ishman, Bruce R. Wardlaw, and Charles W. Holmes United States Geological Survey, Reston, VA 20192 This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
AbstractUsing multiple-proxy biological indicators, a paleoecological history of the past 550 years of Featherbed Bank, Biscayne Bay, has been reconstructed from a short (2.26 m) sediment core. Paleoecological changes in ostracode, mollusc, and foraminifer assemblages show that core SEI297-FB-1 can be divided into three distinctly different zones, which together provide evidence for distinct changes in historical environmental conditions at Featherbed Bank.
Assemblages from fossil biotic communities within zone 1, representing approximately 1440 to 1550 AD, are characterized by open-marine biota with relatively limited numbers of epiphytic biota. Molluscan faunal indicators suggest the sediment was capable of supporting infaunal organisms and that faunal richness was relatively limited during this time period.
A change in the biotic community occurred around 1550 AD and continued until the late 1800's distinguishing zone 2. Fossil biotic indicators from zone 2 show a strong dominance of epiphytic organisms within all of the biotic communities examined. Foraminifers, molluscs, and ostracodes capable of subsisting in salinities slightly lower than normal marine begin to flourish in this time period, and there is a marked decline in infaunal molluscs.
Zone 2 assemblages are replaced around 1900 AD by increased numbers of organisms that typify open-marine conditions and a return to decreased epiphytic assemblages, similar to zone 1. Zone 3 assemblages, however, show some strong dissimilarities from zone 1, including limited infaunal molluscs, increased abundances of the ostracode Malzella floridana, and a significant increase in molluscan faunal richness.
We wish to thank Gene Shinn, Don Hickey, Chris Reich, and Marci Marot (USGS, St. Petersburg) for assisting in the collection and processing of Core SEI297-FB-1. A special thank you goes to Biscayne National Park for providing permits, which allowed for the collection of this core within the Park as well as data for modern analogues. We would also like to thank Dade-Metro County Department of Environmental Management. Nancy Carlin (USGS, Reston), helped with processing the core, and Jill D'Ambrosio, Ellen Seefelt, and Ian Graham (USGS, Reston) were responsible for picking the ostracodes and foraminifers used in this study. John McGeehin provided analysis of the Ecosystems and Ecosystem History In 1994, the Everglades Forever Act was passed to help stem the effects of anthropogenic influences upon the Florida Everglades, Florida Bay, and Biscayne Bay. Increasing evidence of changes in the ecosystem attributed to anthropogenic forces, such as industrial pollution, over-harvesting of fisheries, and alteration of natural water circulation patterns, sparked an effort to restore the environment to a more natural condition. However, the impact of natural hydrological and climatic forces had not been adequately investigated. The 'Ecosystem History of Biscayne Bay and the Southeast Coast' project of the USGS was designed to help water management agencies with decisions regarding water and land-use management in south Florida by producing a model for recognizing human-induced and natural ecosystem variation using biological proxies. Documenting and correlating historical trends in the ecosystem's variation with the timing of man-made, hydrological, and climatic events can be used to help determine what aspect of the ecosystem has been affected and help distinguish between the factors influencing environmental change.
Multiple-proxy Approach Core Processing Every other sample from the core was examined. Faunal groups and species were identified, counted, and standardized by calculating percent abundance within each sample. Ostracode species were identified using the taxonomy of Teeter (1975), Keyser (1975, 1976, 1977), and Garbett and Maddocks (1978). Cronin et al. (in press) reviewed the ecology of ostracode species. Molluscs were identified primarily using Abbott (1974), Warmke and Abbott (1961), Perry and Schwengel (1955), and Andrews (1971), and taxonomic nomenclature was updated following Turgeon et al (1998). Taxonomy of the benthic foraminiferal species were identified using Loeblich and Tappan (1988). Faunal slides are housed in the Eastern Earth Surface Processes Team, U.S. Geological Survey, Reston, Virginia.
Chronology
Age = 0.004(depth Ages for each faunal sample were computed using this equation and are given in Tables 2, 3 and 4.
One can obtain a slightly different age model by using the original age estimates from the Ostracodes
The second zone (zone 2) occurs from 175 to 65 cm and is characterized by common Loxoconcha matagordensis (20-50 %) and Xestoleberis spp. (20-40 %). Loxochoncha matagordensis is an epiphytal species that lives along Atlantic and Gulf of Mexico coasts of North America on Thalassia, Zostera (in mid-latitudes), and other marine seagrasses (Cronin and others, in press). Many species of Xestoleberis inhabit various types of marine algae in tropical and subtropical regions. In addition to their preference for sub-aquatic vegetation habitats, L. matagordensis and several species of Xestoleberis thrive in environments of reduced salinity, such as those found in coastal bays and lagoons. Therefore, the dominance of epiphytal euryhaline ostracode species in this zone suggests a period of both abundant seagrass and marine algal habitats and also fluctuating salinity between about 20-35 ppt. The age of this zone is estimated to be about 1600-1850 AD (150-400 yr BP). The third zone (zone 3) occurs in the interval from 65 cm to the top of the core and represents the period from 1850 to 1997. The ostracode assemblages consist mostly of carbonate platform species as were found in zone 1, and in addition, 10% or more of Malzella floridana. This latter species is a dominant form in Florida Bay in regions that experience periodic hypersalinity. While its presence in Biscayne Bay during the last 150 years does not necessarily indicate hypersalinity, it does distinguish the assemblages of zone 3 from zone 1. Epiphytal species also decrease significantly in the transition from zone 2 to zone 1. Molluscs The lower portion of Core SEI297-FB-1, from 226 to 175 cm (zone 1), is characterized by the presence of infaunal pelecypods (Pitar sp., Parastarte triquetra, Gouldia sp., Nucula proxima), perhaps indicating softer sediment available for habitation. Epiphytic and epibenthic gastropods (Eulithidium sp., Cerithium spp., Tegula fasciata, Vermicularia spirata, Modulus modulus, and Turbo castaneus) are also present in the lower portion of the core (fig. 5, table 3). Bittiolum varium, a dominant epiphytic species in the upper portion of the core, is nearly absent from zone 1. Average molluscan faunal richness1 is 13 and average abundance is 23 in the lower portion of the core; these numbers do not fluctuate significantly in this portion of the core, suggesting a relatively stable ecosystem. The molluscan fauna indicate near normal marine salinities for zone 1. Samples between 190 and 175 cm mark a transition from zone 1 to zone 2.
The middle portion of the core, from 175-65 cm (zone 2), is dominated by the presence of Bittiolum varium; Turbo castaneus also is relatively abundant. Burrowing pelecypods are sparse in zone 2 and almost completely absent from 118-90 cm, indicating the substrate may not have been suitable for habitation; this corresponds to an increase in organisms that live on hard substrates and/or sub-aquatic vegetation. Average molluscan faunal richness in the middle portion of the core is 12, nearly the same as the lower portion, but abundance is highly variable compared to zone 1, averaging 39 individuals per sample. The molluscan fauna indicate the salinity in this portion of the core may have been reduced periodically (perhaps as low as 25 ppt) compared to zone 1; the presence of several species that require near normal marine salinities, however, discounts sustained periods of lower salinities. Samples between 90 and 65 cm mark a transition from zone 2 to zone 3, sharing characteristics of both. The upper portion of the core, from 65 to 0 cm (zone 3), is marked by a decline in the dominance of Bittiolum varium, and a dramatic increase in faunal richness to an average of 25 faunal groups per sample. Cerithium spp., Tegula fasciata, Vermicularia spirata, Modulus modulus, rissoinids, Nucula proxima, and Transenella sp. increase in numbers in the upper portion of the core. This portion of the core contains a distinct assemblage very similar to the western margin of Florida Bay. The assemblage is dominated by Turbo castaneus and Tegula fasciata with minor percentages of Brachidontes exustus, Pteria longisquamosa, Cerithium muscarum, Chione cancellata, and other species more common in eastern and central Florida Bay. The assemblage requires relatively stable, near-normal marine salinities (Brewster-Wingard, Stone, and Holmes, in press). This evidence is supported by the presence of Halimeda, a green algae common along the western margin of Florida Bay, occurring almost exclusively in the upper 80cm of the core. Faunal abundance, which fluctuates dramatically in zone 3, ranging from 7 to 184 individuals per sample (averaging 74), distinguishes this assemblage from the molluscan assemblage in zone 1. Foraminifers
Ishman (in press) recognized four benthic foraminiferal assemblages from surficial sediment samples collected in Biscayne Bay. The foraminiferal faunas from Core SE1297-FB-1 represent a mixture of the open-bay grass assemblage and the open-bay coarse sediment assemblage, indicating open-bay normal circulation throughout the deposition of this core. From the core bottom to 175 cm (zone 1), an open-bay seagrass form Elphidium delicatulum dominates to the near exclusion of other members of the seagrass assemblage. Within this interval, a variety of open-bay coarse sediment forms of the genera Quinqueloculina (Q. tenagos, Q. poeyana, Q. polygona) and Triloculina (T. triacarinata) and Rosalina (R. floridana) are present to common, suggesting a patchy seagrass-bare substrate environment. From 175 cm to 65 cm (zone 2), Elphidium delicatulum still dominates, but other representatives of the seagrass assemblage, such as Quinqueloculina bosciana, Articulina mucronata, and Miliammina circularis, are present to common, and forms of the open-bay coarse sediment assemblage are still present but less abundant, indicating an environment with fairly healthy seagrass and sparse, patchy bare sediment. Samples between 100 and 65 cm mark a transition from zone 2 to zone 3 assemblages. Foraminifer assemblages, commonly found in minor relative percentages in salinities that range as low as 30 ppt, are present within zone 2, suggesting that salinity conditions may have been slightly reduced when compared with zone 1. From 65 cm to the top of the core (zone 3), the open-bay coarse sediment foraminiferal assemblage attains dominance. There is an increased abundance of most members of the open-bay coarse sediment assemblage in zone 3 and Elphidium delicatulum is greatly reduced, only dominating in a few horizons; other forms of the seagrass indicators remain unchanged or show only a modest increase in abundance. This suggests a bare sediment environment with patchy seagrass gradually replacing the lush sub-aquatic vegetation from zone 2 in the upper part of the core. DiscussionRegional Model for Featherbed Bank
The data from zone 1 (base to 175 cm) indicate that during the 15th and 16th centuries Featherbed Bank had an open-marine carbonate environment. The substrate during this period was patchy sub-aquatic vegetation, with a strong infaunal presence. Key biotic indicators suggest that salinity within zone 1 remained relatively stable and near to normal marine throughout deposition.
Zone 2 (175 - 65 cm) deposited during the 17 Zone 3 (65 - 0 cm) represents the last 150 years of Biscayne Bay history when open-marine species re-colonized and sub-aquatic vegetation decreased. For the first time, the ostracode Malzella floridana became common, indicating possible episodes of hypersalinity and a molluscan assemblage similar to the western margin of Florida Bay appeared. The dramatic change in molluscan faunal richness and abundance in the upper portion of the core is striking (fig. 7). It is possible that this is a taphonomic effect, but preservation quality of molluscan fossil shells is generally the same throughout the core; for example, the lowest portion of the core contains very delicate specimens that are well preserved, and color patterns are still present. These observations indicate that the variation in faunal richness and abundance is real and not a taphonomic bias. The increase in molluscs may be related to the variability in substrate heterogeneity, to fluctuating salinities, or to some other factor such as productivity or increased nutrients.
Bank Migration Model The transition from infaunal and coarse-sediment dwelling biota (zone 1) to prominent epiphytic communities (zone 2) to variable substrates and potentially increased salinity fluctuations (zone 3) is consistent with a model for bank migration; the transition from zone 1 to zone 3 may be explained by gradually shallowing of the environment of deposition, with lush sub-aquatic vegetation (zone 2) representing the side of a migrating bank. Without further investigation, it would be difficult to determine if the changes seen in core SEI297-FB-1 are the result of local bank migration or if they represent regional changes within central Biscayne Bay over the past 5 centuries.
Suggestions for Future Work Ostracode, molluscan, and foraminiferal biological proxy indicators demonstrate that Featherbed Bank experienced three distinct environments. From approximately 1440-1550AD (zone 1), biotic assemblages show that open-bay, relatively stable marine, carbonate deposition dominated the environment. Patchy to limited sub-aquatic vegetation was present. Between 1550 and 1850 AD (zone 2), this environment gave way to open-bay conditions with lush sub-aquatic vegetation with only limited bare substrate. Salinity conditions may have reached as low as 25 ppt within this zone, but probably not for sustained periods. Between 1850 and 1997 AD (zone 3), biotic assemblages, representing open-bay deposition with patchy sub-aquatic vegetation and increasing bare substrate, re-colonize the environment. However, assemblages after 1850 display distinct differences from those in zone 1, suggesting modest changes in salinity regimes and substrate. There is no evidence for proximity to land or sustained periods of lowered salinity throughout core SEI297-FB-1. Although distinct, the changes in biotic assemblages down-core are dominantly substrate-controlled and may represent either bay-wide or local variations in the biological communities at Featherbed Bank.
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