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Ecosystem History of Southern and Central Biscayne Bay: Summary Report on Sediment Core Analyses - Year Two

U.S. Geological Survey Open File Report 2004-1312

Materials and Methods
Addl. Analyses of Mudbank Cores
Patterns of Change:Military Canal
Patterns of Change: Near-shore Sites
Implications for Restoration
Tables and Figures

G. Lynn Wingard, Thomas M. Cronin, Charles W. Holmes, Debra A. Willard, Gary Dwyer, Scott E. Ishman, William Orem, Christopher P. Williams, Jessica Albietz, Christopher E. Bernhardt, Carlos A. Budet, Bryan Landacre, Terry Lerch, Marci Marot, Ruth E. Ortiz

Abstract/Executive Summary

The Comprehensive Everglades Restoration Plan (CERP) lists restoration of the timing, quantity, and quality of the natural flow of freshwater as one its primary goals.  Before restoration can occur, however, the baseline conditions of the environment prior to significant human alteration must be established and the range of variation within the natural system must be determined.  In addition, the response of the system to human alterations during the 20th century should be evaluated.  Resource managers can use this information to establish targets and performance measures for restoration and to predict the system's response to changes invoked by restoration.

The objectives of the U.S. Geological Survey’s Ecosystem History of Biscayne Bay research project are to examine historical changes in the Biscayne Bay ecosystem at selected sites on a decadal-centennial scale and to correlate these changes with natural events and anthropogenic alterations in the South Florida region.  Specific emphasis is being placed on historical changes to (1) amount, timing, and sources of freshwater influx and the resulting effects on salinity and water quality; (2) shoreline and sub-aquatic vegetation; and (3) the relationship between sea-level change, onshore vegetation, and salinity.

This report compiles and summarizes results on analyses of cores from eleven sites in the Biscayne Bay ecosystem collected from 1996 to 2003.  The following are the significant findings discussed:

  • Southern Biscayne Bay, including Card Sound and Barnes Sound, has experienced distinctive changes over the last century.  The four sites examined, Card Bank (northwest and southeast sides), Middle Key basin, and Manatee Bay, all show increasing salinity over the last 100 years.
    • Card Sound Bank has experienced relatively large swings in salinity, fluctuating between a more restricted upper estuarine environment and a more open estuarine environment, over multi-decadal and centennial timescales.  The amplitude of change exceeds that seen in cores from central Biscayne Bay (No Name and Featherbed Bank).  During the later part of the 20th century, the site has come under increasing marine influence.  (Wingard and others, 2003, corroborated by new data in this report)
    • Middle Key basin has seen a steady increase in salinity that began prior to 1900.  The earliest records from this site indicate a freshwater, limnetic environment, which began to shift toward brackish before the start of the 20th century.  From the 1960s to the present, freshwater supply to Middle Key basin diminished, relative to the rise of estuarine conditions.  Salinities in the basin ranged from mesohaline to polyhaline throughout the 20th century, and there is some evidence of increasingly fluctuating salinities over the past few decades at the site.
    • Manatee Bay also has undergone a change from a freshwater environment at the base of the core transitioning to an estuarine environment at the top (Ishman and others, 1998), similar to that seen in Middle Key basin.  At the Manatee Bay site, however, salinities in the upper part of the core reached upper polyhaline to euhaline levels.
    • Total carbon (TC), organic carbon (OC), total nitrogen (TN), and total phosphorus (TP) concentrations were all significantly high in the core from the Card Bank site, relative to the mudbank cores from central Biscayne Bay.  However, the Middle Key site does not exhibit any significant increase in TP concentration in the upper 20-30 cm, as seen in the other cores examined.
  • The mudbanks of central Biscayne Bay have become increasingly marine and have experienced lower amplitude decadal variability in salinity over the last one hundred years.  (Wingard and others, 2003; corroborated by new data in this report)
    • The continental shelf and open marine influence has increased during the 20th century at Featherbed and No Name Banks.
    • No indications of inter-decadal salinity extremes have been found in cores from Featherbed and No Name Banks; salinities at these sites ranged from polyhaline to euhaline over the last three to four centuries.
    • The downcore total phosphorous (TP) profiles at No Name Bank represent a large increase in TP flux to the sediments, superimposed on the normal diagenetic recycling of P.
  • The near-shore cores from north of Black Point and at Chicken Key record a period of fluctuating salinity during the last 50-100 years or more.  Both sites show an increase in average salinity from mesohaline to polyhaline conditions, and possibly an increase in salinity fluctuations, in the last 10-30 years.
    • The direct influx of freshwater to the site north of Black Point appears to have fluctuated over the period of time represented by the core.  From before 1900 to ~1970, direct freshwater influx to the site seems to have diminished, despite the lowering of salinities during this time period.  The lowered salinities may have been caused by increases in groundwater upwelling, or dilution of the estuarine waters via runoff from other areas and/or by increased rainfall.  In the late 20th century, the significant increase in freshwater gastropods indicates a direct influx of freshwater to the site.
    • At Chicken Key, no freshwater fauna were found in the core, suggesting that the site has not been influenced by a direct influx of freshwater during the time represented by the sediment accumulation.
    • Chicken Key and Black Point North cores exhibit a sharp increase in total phosphorous (TP) concentration above 20-30 cm. This increase appears to be above normal diagenetic trends, and may indicate an increase in TP load to the sediments in recent times.
  • Pollen assemblages from all core sites reflect the long-term dominance of pinelands on the coastal ridge prior to the 20th century, followed by vegetational changes associated with various land-use activities.
    • Comparing the three near-shore cores, the greatest changes were noted at the two northernmost sites (Chicken Key and Black Point North).  These changes are roughly coincident with land clearance, initiation of extensive row-crop agriculture, and canal construction in the area.
    • The two cores from Military Canal indicate that the southern site has always been relatively drier than the northern site, and that the vegetational differences between the two sites pre-date construction of the canal.
    • At the Black Point North site, mangrove abundance peaked in the 1980's, but declines sharply at the top of core, reflecting the impact of Hurricane Andrew in 1992.

A general trend emerges from the multiproxy analyses of all the cores examined - increasing salinity during the 20th century.  Although the timing and onset of increased salinity varies at the different core sites, there are no exceptions to this trend.  In the nearshore sites, the increase in average salinity has been accompanied by an increase in variability of salinity.  In contrast, the central Biscayne Bay sites have shown increasingly stable salinity over the last century, indicated in part by the influx of increasing numbers of marine species.  These trends could be a result of a number of factors, including (1) rising sea level; (2) changes in the natural flow of freshwater into the bay either through surficial or groundwater processes; (3) changes in average rainfall or rates of evaporation; (4) changes in sedimentation rates; or (5) a combination of factors.  The timing of changes at some of the near-shore sites suggests both anthropogenic and natural factors are involved.

In addition to the general salinity trend for Biscayne Bay, the near-shore sites at Middle Key and north of Black Point have illustrated distinct, but site specific, changes in freshwater influx over time.  Our data suggest that sites we assumed had historic point-source inflow of freshwater may not have.  The wetlands cores (near Military Canal) also illustrate that sites in very close proximity to each other have historically been affected by very localized hydrologic regimes.

These results have significant implications for restoration planning.  First, the recognition that Biscayne Bay appears to be evolving toward a more marine environment due to both natural and anthropogenic factors must be factored into the planning process.  Second, generalized performance measures and targets for the near-shore and wetlands areas may not reflect the natural variability seen at these sites.  Third, the nearshore environments are dramatically different from the mid-bay mudbanks, and have been for hundreds of years.  Influx of freshwater into the bay appears to have a subtle or indirect effect on the benthic fauna of the mudbanks.  Changes in flow during restoration may have little effect on the central bay mudbanks.

Examining decadal-centennial trends in a variety of habitats within the Biscayne Bay ecosystem provides a realistic means to set performance measures, predict system response to changes invoked by restoration, and to enlighten the public on what the natural system of the bay looked like. 

Next: Introduction

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