Sedimentation & Paleoecology

1996 Abstracts

The Biotic Record of Change in Florida Bay and the South Florida Ecosystem.

G. Lynn Brewster-Wingard, Scott E. Ishman and Debra A. Willard, U.S. Geological Survey, Reston, VA; and Robert B. Halley and Charles W. Holmes, U.S. Geological Survey, St. Petersburg, FL.

The Florida Bay environmental record, as indicated by the biotic remains within the sediments, is one of change. Analyses of three cores from Florida Bay and the fringe environments of the Bay show that fluctuations in salinity, substrate, and other critical physical, and chemical parameters, have occurred throughout the history of the bay. During this century, an increase in average salinity and an increase in benthic faunal diversity is recorded.

The principle objective of the U.S. Geological Survey's South Florida Ecosystem History projects is to use paleoecologic tools to reconstruct the history of Florida Bay, Biscayne Bay, and the terrestrial Everglades over the last 150-200 years, as well as the last few millennia. The data gathered from these projects are being compiled to develop a broad regional and temporal picture of changes in the south Florida ecosystem. At selected sites in Florida Bay, data are gathered on modern faunal and floral distributions and environmental preferences, such as salinity, dissolved oxygen, nutrients, circulation, substrate and seagrass conditions. These data are used as proxies for interpreting down-core environmental changes as indicated by the abundance and distribution of faunal and floral remains present in cores collected throughout Florida Bay and the south Florida ecosystem. The chronology of the cores is based primarily on 210Pb analyses; this absolute age control makes it possible to interpret the rate of change of such critical parameters as salinity and substrate, including seagrasses and sediment sources. In turn, changes in salinity patterns provide information on changing freshwater flow, sea level rise, and circulation patterns. In conjunction with geochemical analyses, information on nutrient supplies also can be obtained from the faunal and floral data.

Analyses of Core 6A from the Bob Allen mudbank, central Florida Bay, and Core T-24 from the mouth of Taylor Creek in Little Madeira Bay, eastern Florida Bay, are complete. The benthic fauna were examined at 2 cm intervals in these cores. The 210Pb age model of Core 6A gives a sedimentation rate of 0.86 ñ 0.06 cm/yr. Core 6A records a history of fluctuating salinity over the last 150 years, but from around the turn of the century a general increase in the average salinity has occurred. Benthic foraminifer and ostracode data indicate a significant shift occurred during the 1950's, from a period of relatively lower average salinities (~18 ppt) and relatively low amplitude fluctuations in the salinity, to a period of higher average salinities (~32 ppt) and greater fluctuations in the salinity. The mollusc data show similar patterns. During the period from around 1950 to the present the overall benthic faunal diversity and abundance increased. Patterns of pollen and dinocyst distribution within the core show shifts that correspond to the benthic faunal changes, indicating that the change was not a localized effect. Comparison of the benthic faunal record to precipitation records dating back to 1906 indicates no direct correlation between precipitation patterns and prolonged salinity changes in Florida Bay; however, further investigation is warranted.

The pattern of increasing salinity is even more apparent in Core T-24, which is located at the fringes of the Bay environment. Although a detailed chronology based on radioisotopes has not been developed for this core, the general faunal and floral patterns correspond to those at Core 6A. The relative abundance of oligohaline to mesohaline (~5-18 ppt) benthic foraminifera, molluscs, and ostracodes decreases upward in the core, and the polyhaline to euhaline (18-35 ppt) fauna increases. Benthic faunal diversity increases upward in the core. The dinocyst and pollen assemblages are consistent with the pattern of increasing salinity up-core at this site, with red mangrove and buttonwood pollen increasing and the dinocyst assemblages shifting toward more marine conditions.

Examination of selected samples from a core collected along Mud Creek near Joe Bay on the northern fringe of Florida Bay provides a long-term record of change. 14C analysis dates the base of this core (82 cm) at 2,050 BP, and 210Pb indicates that the last 150 years are included in the upper 20 cm. Preliminary analysis of molluscan fauna indicates a general trend of increasing salinity up-core. Molluscs present in the lower half of the core are limited to fresh water or terrestrial forms, but at 34-36 cm a brackish to nearly freshwater species is found. Pollen assemblages indicate relatively higher abundances of sawgrass pollen from about 600-2000 BP followed by an increasing abundance of mangrove and wax-myrtle pollen; this shift is indicative of an increase in salinity. Because of the slower sediment accumulation rate of peats, less detail is provided for the last 150 years, but cores from the fringe of Florida Bay provide a link between the Bay and the terrestrial ecosystems, and allow us to develop a broad regional and temporal picture of changes to the south Florida ecosystem.

The preliminary examination of three cores from the central and northern margins of Florida Bay indicates a Bay-wide increase in salinity over at least the last century. Examination of historical precipitation records indicate there is no strong relationship between salinity and precipitation patterns, however, other factors such as storm frequency, fresh-water flow, sea-level rise and evaporation/precipitation rates are also important. A comparison of the chronologically-placed data gathered from these cores with historical records of precipitation, streamflow and other critical parameters will facilitate understanding of the causes of salinity fluctuations in Florida Bay. In the upcoming year, paleoecologic analyses of additional cores are planned to determine if the pattern of increased salinity is upheld. Geochemical analyses of shells and sediments from these cores will be conducted to provide absolute measurements of salinity and nutrient fluctuations in the ecosystem. This information, compiled wih data gathered from other ecosystem projects evaluating salinity, circulation, freshwater flow, precipitation and evaporation, will provide modelers and resource managers with information on the causes and enduring effects of salinity change.

Remote Sensing of Water Turbidity and Sedimentation and Their Relationship to Algal Blooms

Richard P. Stumpf and Megan L. Frayer, U.S. Geological Survey, Center for Coastal Geology, St. Petersburg, FL.

A decline in water clarity in Florida Bay has been observed following the seagrass dieoffs starting in the late 1980's. Algal blooms and discolored water have been reported in Florida Bay over the last several years and factors such as resuspension of material and nutrients from the bottom have been suggested as a cause. Monthly monitoring programs by Florida International University (FIU) and Florida Department of Environmental Protection (FDEP) have provided documentation of blooms through chlorophyll measurements. This study is using remote sensing to examine resuspension events, the distribution of turbid water and changes in the patterns of water clarity in the Bay.

The Advanced Very High Resolution Radiometer (AVHRR) on NOAA meteorological satellites has been used in this study. Currently, over 600 usable scenes are available from some 1500 covering a period from December 1989 to the present. AVHRR has a pixel size of about 1.1 km. The data sets are processed for water reflectance, which is related to water turbidity variables such as attenuation, Secchi depths, total particulate matter, and nephelometric turbidity. Tentative relationships with these variables have been made. (Sea surface temperature is also determined.) High reflectance corresponds to high attenuation or particulate loads or shallow Secchi depth. The individual scenes are also processed to obtain monthly and seasonal means, with winter corresponding to the period of December to March and summer to June through September. Initial analyses include the points corresponding to fixed stations occupied at a monthly interval by FIU or FDEP.

In examining the average reflectance of the entire Bay, the satellite imagery does not show a trend between December 1989 and September 1996. The seasonal pattern of high turbidity in winter and low in summer is evident. Trends over the time period appear in subsections of the Bay. A substantial increase in water reflectance is evident in the north-central Bay (which has been documented in field studies), this region includes Rankin Lake and Johnson Key. However these appear to have different phasing, with Johnson Key showing an increase in turbidity about two years earlier than Rankin Lake. Both sites show the decline in both winter and summer. Twin Key, which has the clearest water in the Bay, has shown a slight increase. The southwest portion of the Bay, west of Sprigger Bank, has shown a decrease in reflectance, indicating clearer water.

The project is conducting comparisons between chlorophyll values collected from the shipboard monitoring programs and pre-cruise reflectances to assess whether there is a link between resuspension events and algal blooms. The next stage in the project is to expand the AVHRR data set backward to before the seagrass dieoffs and to incorporate Landsat data for limited high resolution analysis.

The Sediment Record as a Monitor of Natural and Anthropogenic Changes in the Lower Everglades/Florida Bay Ecosystem: A High Resolution Study

Terry A. Nelsen, Michelle Zetwo, NOAA, Atlantic Oceanographic and Meterolocial Laboratory Miami, FL; Harold R. Wanless, University of Miami/Geology, Miami, FL ; Patricia Blackwelder, Peter Swart, Terri Hood, Carlos Alvarez-Zarikian, University of Miami/RSMAS, Miami, FL; John Trefry, Simone Metz, Woo-Jun Kang and Robert Trocine, Florida Institute of Technology; and Lenore Tedesco, Martha A. Capps and Michael A. O'Neal, Indiana University-Purdue University, Indianapolis, IN.

In the absence of historical data and records for the lower Everglades/Florida Bay ecosystem, a proxy-record such as that potentially recorded in the sediments must be evaluated for long-term trends. An understanding of such a proxy-record can be complicated by disruption such as bioturbation and other processes such as storm-induced sediment removal and deposition. To overcome these potential complications, clearly stratified sedimentary sequences were sought for this study. The stratified records vary with respect to continuity of record, persistence of sediment deposition, local reworking, and presence/absence of erosional hiatuses. The nature of the sediment-biotic-geochemical record is dependent on the style of sediment deposition in the area.

The most difficult and critical part of the field program is locating meaningful, stratified sediment sequences. Layered sequences are associated with areas of focused sediment deposition during tidal, winter storm, freshwater flood events, and/or hurricane events. Day to day tidal processes, potentially provide the most detailed sediment record, but do not alone transport and accumulate sufficient sediments in an area to provide a discernible record. The finest scale records found occur where focused tidal flow decreases and deposits sediment provided by winter storms and river flood resuspension events. These records are preserved in areas protected from erosion by winter storm, flood, and hurricane erosion conditions. The least detailed sediment records are formed in areas subjected to hurricane events in which the surficial bio-sedimentary environments is smothered by a hurricane event deposit. These smothering events however, provide a highly detailed preservation of the benthic environment at the time of the hurricane.

This presentation is focused on an example of our work and is of a core selected to document the effect of changes in freshwater discharge from the main drainage system of the Everglades on the adjacent marginal marine water and environment. Using historical aerial photographs combined with our published knowledge of south Florida sediment-body dynamics, we have defined the physiographic features of the Shark River drainage system, Whitewater Bay and northwestern Florida Bay most likely to contain interpretable historical sediment records. Integrating this with field coring reconnaissance, we have defined sites that have striking stratified sequences.

In order to understand the responses of the Florida Bay ecosystem, to natural and anthropogenically-induced changes, we are focusing our work on the following three testable hypotheses:

1. "Anthropogenically-induced changes in freshwater flow across and out of the Everglades, over the last century, have caused changes in the adjacent coastal environment. This has lead to changes in the biogenic community structure which are both recorded in, and interpretable from, the sediment record."

2. "Sedimentary records are interpretable with sufficient resolution that anthropogenic influences can be differentiated from natural changes associated with sea-level rise and natural catastrophic events."

3. "Sediments, from within the Everglades freshwater system seaward into the estuarine environment, record an interpretable time-history of anthropogenic pollutants such as the heavy metals, Hg and Pb."

The core selected for this presentation, 9510-2, is from northwestern Whitewater Bay but related to the lower Shark River outflow system by a side channel which has strong tidal flow. The site is protected to the north and east by very shallow water and by mangrove shorelines less than 50 m away. The sample site is at the southern edge of the very shallow water and is a barren, very-soft muddy bottom that is extending southward as a gently sloping mud wedge. Tidal currents sweeping southward from the Shark River side channel weaken across the broad, deepening flank of the shallow bank. This results in deposition of much of the sediment load carried southward from this side channel. The sample site is protected from the northern and northeastern winds of the waning stages of winter storms, tidal scour and focused wind waves or currents during winter storms and hurricanes.

The core consists of 1-4 mm thick alternating laminae of light carbonate silt- and clay-sized particles and dark, fine, woody organic detritus. The lighter laminae are provided by winter storm resuspension both from the interior Whitewater /Oyster Bay system and from introduction from offshore. Organic laminae are provided by tidal erosion and suspension transport during prevailing conditions. As shown by Pb210 dating, the larger historical hurricanes to affect the area have caused 2-5 cm of erosion of the sequence. Minor burrowing, although visible in X-radiographs of the core and in split sections, are small and scattered and have left the core sequences essentially undamaged by mixing or surface introduction of particles.

The geochronological record for sediment cores from the lower Everglades and northern Florida Bay shows accumulation rates that are typically about 1 cm/yr. High resolution sampling and analysis downcore reveal very good age comparisons between 137Cs and excess 210Pb. In addition to providing a sensitive time-line and accumulation rate information, a variety of fine-structure has been observed in the profiles including sediment erosion/deposition bands from dated hurricane events over the time interval of interest to this study.

Total Hg and Pb concentrations in the sediments range from 5-240 ng Hg/g and 1-30 æg Pb/g respectively. To help identify natural levels of Pb in sediments, Pb vs. Al plots were useful as demonstrated in other areas. However, Hg vs Al relationships were not useful because of high TOC levels in Everglades sediments (up to 35%), thus a Hg vs. [Al+TOC] function was successfully developed. Results showed that sediment with 1% Al and 30% TOC could have about 65 ng Hg/g relative to about 10 ng Hg/g in sediment with 1% Al and 2% TOC. Baseline data for natural Hg levels were used to show that sediment Hg levels were in excess of natural levels by 150 ng/g in sediments deposited since 1950 near the mouth of Shark River Slough and 20-30 ng Hg/g in post-1950 sediment from the lower Shark River and the more isolated Coot Bay. Pre-1950 sediments show no distinct Hg enrichment. At present, lower Hg/TOC ratios in surface sediment cannot be directly linked with recent decrease in Hg inputs relative to diagenetic alterations.

The predominantly organic nature of sediments at this location necessitates multi-step processing for microfaunal analysis. We have developed a new protocol for sample treatments, which has resulted in considerable reduction in processing time. Ostracodes, benthic foraminifera and micromolluscs isolated from this core are being characterized, and selected subsamples are examined for isotopic composition. To establish a salinity/benthic community data base, we have collected surface stained samples, and taken salinity measurements from several localities to examine ostracode and foraminifera benthic community structure/salinity relationships. In surface samples, major shifts in foraminifera community structure are not dramatic, however within the Elphidium genus, and the milliolids, significant species shifts were observed. Most of the members of the ostracode group are salinity sensitive. Even the euryhaline species appear to exhibit morphologic variability related to salinity differences at these sites.

Shifts in vegetation communities or assemblages are useful indicators of environmental change, especially in areas where plant communities are strongly controlled by changes in salinity and hydroperiod. The lower Everglades/Florida Bay ecosystem is such an area. Pollen grains incorporated in sediments during deposition record detailed information about shifts in past vegetation systems. Changes in Everglades floral communities record subtle shifts in the balance between fresh and marine waters. From this laminated sediment core, taken adjacent to the lower Shark River drainage, a detailed, high-resolution record of changes in the watershed and ecological response to changes in the ecosystem is preserved. Sediments deposited during different water management regimes in the Everglades/Florida Bay Ecosystem have been analyzed for contained pollen. Preliminary analysis has focused on samples representing distinctly different water management strategies including (a) unregulated flow through an undisturbed system (pre-1920); (b) unregulated flow through culverts under Tamiami Trail (1925-1960); (c) overland flow partially disrupted by construction of water conservation areas and Hurricane Donna (1960-1962); (d) an assortment of short term management practices (1962-1970); (e) static portion of water allocated to the park each year in the 1970's; (f) S-12 structures left open so water delivery is a function of hydraulic gradients between water conservation areas and the park (1983-1984); and (g) the rainfall plan (1985-present). Pollen analysis, to date, has compared unregulated flow (a) to overland flow prior to and following Hurricane Donna (c) to hydraulic gradient delivery (f) to present management plans. Sediments deposited prior to 1920 have significantly different species compositions compared to those currently being deposited. Sediments deposited circa 1880 are dominated by species representing wet prairies, freshwater marshes and hammocks. Surficial sediments record significant decreases in freshwater species, especially wet prairie and hammock species and increases in red mangrove.

When viewed together, current results and anticipated progress provides a positive outlook for being able to place, within a temporal context, anthropogenic impacts on the lower Everglades/Florida Bay ecosystems, as preserved in the sediment record. We consider our seasoned approaches to this work to have opened the door for fruitful continued detailed paleoenvironmental reconstruction throughout our study area. From this, we anticipate, establishing a pre-anthropogenic paleoecological baseline against which to measure: a) restoration effort; b) paleo-water-column conditions for a historical context in which to place ongoing water-column studies; and c) as a reference for validation of hindcasts, and perhaps forecasts, of water-quality modeling efforts central to larger managerial issues.

Sediment Transport Processes and Sea-Floor Mapping in Florida Bay

Ellen J. Prager, Robert B. Halley, and Mark Hansen, U.S. Geological Survey, Center for Coastal Geology and Marine Studies, St. Petersburg, FL.

Within Florida Bay, circulation and outflows are inherently linked to the processes of sediment resuspension, transport, and deposition. The complex bathymetry of the Bay, a product of sediment production and transport over time, constrains flow and limits mixing. Resuspension events cause increased turbidity, the recycling of nutrients, and facilitates the export of sediment-laden waters. Understanding how sediment transport processes and the overall bathymetry of the Bay respond to processes such as sea-level rise, storm events, and changing flows will allow us to predict future responses, both natural and man-made, and provide important data for hydrodynamic and water quality modeling efforts.

Three projects at the USGS are underway to investigate 1) bathymetric change in Florida Bay, 2) the processes and rates by which sediments accumulate and erode, and 3) to understand the timing and distribution of sediment resuspension. Individually, these projects will provide information essential to understanding ecosystem dynamics, and in combination, they provide an unprecedented baseline of data to evaluate the impact of future storm events on the bathymetry and sedimentology of Florida Bay.

The bathymetry of Florida Bay has not been systematically mapped since the 1890's, and some shallow areas have never been surveyed. In the 1930's, depth measurements were documented principally in the intercoastal channel just north of Key Largo and more recently, spot soundings have been conducted to better delineate the location of cuts and navigational hazards. A modern, digital map of Florida Bay bathymetry is needed to provide numerical modelers with an accurate base map, and to aid in the assessment of long-term patterns of sediment transport and deposition. Through the comparison of historical data with a modern bathymetry data set, areas of net erosion or deposition within the Bay can be identified.

The sea-floor mapping project focuses on the collection of an updated bathymetric data set for Florida Bay, the digitization of historical and modern data for comparison, and the production of quality maps and digital grids of both historical and present-day bathymetry, as well as those changes which have occurred.

Bathymetric data collection with a GPS based hydrographic system began in the summer of 1995 in the northeast quadrant of the Bay. The area east of Russell Key/Upper Matecumbe Key will have been surveyed by the end of 1996. Completion of the bathymetric survey is anticipated in 1998. Digitization of the historical bathymetric data was initiated in 1995. It is anticipated that the majority of digitizing of both the bathymetry and shorelines will be completed in 1996. All relevant information is being archived in a ARC/Info database.

In another project, we are addressing sediment resuspension processes in Florida Bay. Sediment resuspension within the Bay is principally a function of wind-driven waves and the properties of the sediments and sea-floor. A computer simulation of wave development within the Bay is being used to understand the effects of typical wind events. Preliminary results show the importance of the Bay's bathymetry and seagrass cover in controlling wave-driven flow. Current work focuses on the incorporation of varying bottom friction within the model. For this purpose, a map of bottom type and cover is currently being produced based on over 600 sampling sites throughout the Bay. In addition, to delineate the sediment properties controlling resuspension, over 100 surface samples are being analyzed for grain size, mud, water, carbonate, and organic content. Using statistical analysis of the data, a finite number of bottom and sediment types are being identified, these include areas of hard-bottom, dense seagrass, inermediate seagrass, sparse seagrass, open mud areas, a mudbank suite, and shell ridges. We will also quantify the resuspension potential in each of the bottom and sediment types identified using a portable resuspension device.

Using the results of wave modeling, sediment and bottom type analyses, along with the measurement of resuspension potential we expect to quantify the frequency and pattern of sediment resuspension in Florida Bay. This information will aid in our understanding of sediment export events, nutrient recycling, and patterns of turbidity within Florida Bay. Calibration for wave modeling will entail the deployment of a pressure sensor array in the Bay during one or more wind events. Satellite imagery of turbidity events, discussed elsewhere in this conference, will also be used for comparative purposes.

The third study focuses on the quantification of long-term sedimentation and erosion rates through the use of geochemical analyses of cores, multi-year surveys from monitoring stations, and detailed mudbank profiling. Five high-resolution profiles (+ 2 cm) across mudbanks show that they are shaped more like small mesas or plateaus with extremely flat tops rather than the mud mounds they have been compared with in the geologic record. Along each of three of these profiles, five elevation survey stations (+ 3 mm) have been established to monitor very small accumulations or losses of sediment. Stations and profiles provide the basis for determining long-term accretion or erosion and provide a baseline for major storm events. Where appropriate, downcore analyses of lead-210, total lead, and cesium-137 are also used to establish sedimentation rates during the last 100 years. These data together with sea-level rise estimates allow prediction of which portions of the bay will deepen, shoal, or remain the same during the next century. This predicted bathymetry can then be used to evaluate future changes in circulation in the Bay.

Baseline elevation measurements and profiling have been completed. Subsequent measurements will be made at least twice annually over the next two years. Core analyses are underway and results expected in the near future.

Last updated: 2/26/98
by: Monika Gurnée
gurnee@aoml.noaa.gov