Comparison of the results from the three cores collected in 2002 with cores collected in 1997 has established some patterns and hypotheses that need to be tested in additional cores to be collected in 2003 and in further analyses on the cores already collected.

• The salinity of central Biscayne Bay has become increasingly marine and increasingly stable over the last 100 years.
  • At No Name Bank, prior to approximately 1915, the inter-annual and decadal salinity fluctuations appear to have been greater than after 1915 when salinities stabilized at that site.
  • Continental shelf/open marine influence on the sites has increased during the 20^th century.
  • There is no indication of inter-decadal salinity extremes or periods of hypersalinity.
  • Freshwater and mesohaline salinities have had a minor influence on No Name bank throughout the time of deposition. At Featherbed Bank, the influence is reduced to mesohaline salinities.
• Card Bank has experienced relatively large swings in salinity over multi-decadal and centennial timescales, compared to central Biscayne Bay, but marine influence at the site has increased over the last century. This is consistent with findings from Manatee Bay (Ishman and others, 1998).
• Indications of regional scale patterns have been found, especially in the shell chemistry data and the pollen assemblages. These regional patterns indicate that the changes are not site specific and may not be limited to Biscayne Bay.
  • Changes in Mg/Ca ratios in central Biscayne Bay appear to correlate to coral ¹⁸O records at Elliot Key and to records from eastern Florida Bay.
  • Pollen assemblages correlate between the three cores, and agree with data from eastern Florida Bay.
  • Similar patterns of change can be seen from bank to bank within Biscayne Bay, indicating that the changes seen are not site specific.
• Sub-aquatic vegetation has undergone bay-wide patterns of change over the last 200-500 years, which includes expansion prior to 1900 and declines during the last century.
  • Thalassia has been present on an inter-decadal time-scale in central Florida Bay over the last 200-500 years, but it has undergone natural patterns of fluctuation in abundance.
  • Thalassia appears to have increased at all three core sites sometime between 1550 and 1750 AD.
  • A decline in Thalassia appears to have occurred after 1950 at No Name Bank and slightly earlier in the 20^th century at Featherbed Bank.
  • Card Bank does not appear to have experienced any declines in vegetation on an inter-decadal scale during the 20^th century.
  • Macro-benthic algae appear to have declined at all three sites since the 18^th century, although a slight increase occurred at No Name Bank after 1950 when Thalassia declined.
• Molluscan faunal abundance and diversity have undergone significant changes in central Biscayne Bay.
  • Patterns of change in diversity and abundance are nearly identical at No Name Bank and western Featherbed Bank.
  • No Name and both Featherbed Bank cores show high abundance and diversity in the mid-20th century, an already altered system, suggesting high diversity is not necessarily a good performance measure for restoration.
  • Molluscan infaunal species abundance is highest at the bottom of all three cores (>30%), then declines between 1550 and 1820.
• Indicators of increased organic-rich sediments at No Name occur between 1869 to 1888 and between the 1930's to 1975. These changes may correlate to human activities (settlement, population growth).

These findings represent a first step towards the project's goal to reconstruct the history of Biscayne Bay and they provide us with a working model to be tested at other sites. It is clear from our findings that Biscayne Bay has been a dynamic environment over the last 500 years, with natural changes occurring in salinity and benthic habitats. However, several significant changes have occurred in the 20th century: 1) increased stabilization of marine salinities; 2) declines in seagrass in central Biscayne Bay; and 3) dramatic changes in molluscan abundance and diversity in central Biscayne Bay. The question remains - how do we better differentiate natural cycles of change from anthropogenic change within these observed trends?

In order to best answer this question, the next step will be to collect cores from nearshore locations where changes in freshwater delivery can be clearly detected. Earlier work at Manatee Bay (Ishman and others, 1998) indicates distinctive changes have occurred during the 20^th century and additional cores will be able to determine the spatial and temporal extent of these changes. Additionally, we need to refine our age models by developing a better understanding of the reservoir effects of ¹⁴C in the bay and the variations in background ²²⁶Ra from site to site. Our on going modern site studies will allow refinement of faunal analyses and calibration of Mg/Ca to absolute salinity values.

The preliminary implications from our research are that changes in salinity and benthic habitats have occurred naturally in Biscayne Bay on inter-decadal to centennial scales, perhaps due to climatic changes, changes in sea level, bank migrations, or a combination of factors. However, further work needs to be done to determine which components of change in the 20^th century are human-induced and which are natural. By examining the historical records preserved in the sediments of Biscayne Bay, we can provide restoration trust agencies with the information necessary to set realistic targets and performance measures for Biscayne Bay.

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