SUMMARY

Compiling our multiproxy analyses of cores collected and analyzed for the current ecosystem history of Biscayne Bay Study (2002 and 2003) with cores collected previously (1996 and 1997) reveals some significant patterns and trends in the history of the Biscayne Bay ecosystem.

Southern Biscayne Bay salinity: Southern Biscayne Bay, including Card Sound and Barnes Sound, has been relatively isolated from direct marine influence for at least the last two centuries due to its enclosed configuration (Figure 1). Currently, this area is less influenced by the urbanization that has taken place to the north, and there are few developed areas along the shoreline. Despite this relative isolation, the area has undergone distinctive changes during the last century. Card Sound Bank has fluctuated between a more restricted upper estuarine environment and a more open estuarine environment over time. The relatively large swings in salinity at Card Sound Bank have occurred over multi-decadal and centennial timescales and 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, however, the site has come under increasing marine influence and fluctuations in salinity appear to have decreased in amplitude and frequency.

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 conditions 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. The absolute abundance of freshwater gastropods indicate that significant pulses of freshwater entered the basin after 1900, but the dominance of estuarine species shows that the environment shifted from limnetic to estuarine prior to 1900. Salinities in the basin ranged from mesohaline to polyhaline range throughout the 20th century and there is some evidence of increasingly fluctuating salinities over the past few decades at the site. Manatee Bay, located south of Middle Key basin, also has undergone a change from a freshwater environment at the base of the core 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.

Central Biscayne Bay salinity: 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. The data presented in this report have corroborated the earlier findings (Wingard and others, 2003) on cores from Featherbed Bank and No Name Bank. No indications of inter-decadal salinity extremes have been found during the 20th century from Featherbed and No Name Banks, in contrast to the high variability seen at Card Bank to the south. Salinities at No Name and Featherbed have ranged between polyhaline and euhaline over the last three to four centuries, and there is no evidence of direct influence of freshwater or mesohaline environments seen in the nearshore cores out on the mudbanks.

Biscayne Bay nearshore salinity: The environments represented by the near-shore cores, located 3.4 to 4.8 km west (2.1 to 3.0 miles) of No Name and Featherbed Banks, bear little resemblance to the mudbank cores. The sediments, the sedimentation rates, and the faunal assemblages of the nearshore cores differ from the mudbank cores. The cores north of Black Point and at Chicken Key record a period of fluctuating environments over at least the last 50-100 years. At both sites the oldest sediments containing faunal remains were deposited under conditions of mesohaline to polyhaline salinities and these were succeeded by a period of lower mesohaline salinity. In the last 10-30 years, an increase in salinity occurs at the sites to polyhaline conditions and an apparent increase in amplitude of salinity fluctuations.

Based on the faunal indicators, the direct influx of freshwater near the Black Point North site appears to have fluctuated over the period of time represented by the core. The lowest segment of the core, deposited prior to 1900, appears to have had a nearby source of freshwater influx into the mesohaline to polyhaline estuarine environment; additionally, the site was very shallow and may have been above mean low tide part of the time. From before 1900 to ~1970, direct freshwater influx north of Black Point 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.

Terrestrial vegetation: Pollen assemblages from all the core sites reflect the long-term dominance of pinelands on the coastal ridge adjacent to Biscayne Bay 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 Military Canal cores indicate that distinct differences occurred in the wetlands, prior to canal construction; the southern site has always been relatively drier than the northern site. At Black Point, mangrove abundance peaked in the 1980's, but declines sharply at the top of core, representing the impact of Hurricane Andrew in 1992.

Geochemistry of sediments: Distinct differences occur in the geochemistry of the sites; however, a clear pattern is not evident. 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, but the Middle Key site does not exhibit any significant increase in TP concentration in the upper 20-30 cm. No geochemical analyses were conducted on the Manatee Bay core to determine if C-111 might be the source of the high values seen at Card Bank. The downcore total phosphorus (TP) profiles at No Name Bank, Chicken Key and Black Point North cores represent large increases in TP flux to the sediments at those sites. These increases appear to be above typical diagenetic levels, and may indicate an increase in TP load to the sediments in recent times, superimposed on the normal diagenetic recycling of phosphorus, but nearby Featherbed Bank does not show this same pattern.