Abstract Introduction Acknowledgements Material and Methods Results Discussion > Changes in Salinity - Changes in SAV and Benthic Habitats - Other Indicators of Change - Correlation Between Sites Significant Findings, Implications And Future Work References Appendices PDF version

Changes in Salinity

The changes in benthic faunal and floral assemblages and shell geochemistry illustrate important changes in the environment of Biscayne Bay over the last 350-550 years. Determining changing patterns of salinity over decadal to centennial time scales is particularly important to restoration of natural freshwater flow in Biscayne Bay. The results of our analyses indicate that the salinity of central Biscayne Bay has become increasingly marine.

Figure 21 illustrates some of the significant salinity indicators for No Name Bank. These data highlight several points. Prior to approximately 1915, the inter-decadal and decadal salinity fluctuations appear to have been greater than after 1915 when salinities stabilized at No Name Bank. Although present in minor amounts (<10% relative abundance) throughout the core, the ostracode Malzella floridana and the mollusk Brachidontes exustus are more abundant in the lower portion of the core and fluctuate with greater frequency and higher amplitude. Both of these species are tolerant of fluctuating salinities, and can tolerate hypersaline conditions. The fluctuations in the Mg/Ca ratios, although not yet calibrated to actual salinity values for Biscayne Bay, can be interpreted as relative increases or decreases in salinity. The pattern of fluctuation in Mg/Ca ratios show increased frequency and amplitude of fluctuations prior to approximately 1915, than compared to post-1915. The large sustained shift in Mg/Ca between ~1844-1850 corresponds to a significant shift in relative abundance of many of the faunal groups and in diversity and absolute abundance of the mollusks.

Figure 21. Key salinity indicators in No Name Bank core (GLW402-NNB). Percent abundance of indicator species and 3-point moving average Mg/Ca in mmol/mol plotted against depth on the left and calendar year on the right. See text for explanation. [larger version]

There are several indications of increasingly stable marine salinities during the 20^th century at No Name Bank: 1) less variation in the Mg/Ca ratio; 2) a progressive increase in the marine ostracode group, the bairdiids; 3) increases in the benthic foraminifers Archaias and Articulina, both indicative of continental shelf influence; and 4) increases in mollusks (for example, Turbo castaneus) typically found in marine salinities (30-40 ppt). The progressive increase in bairdiids, is supported by the presence of several other normal marine ostracode taxa (i.e. Neocaudites, Cytherella, Radimella). In contrast, there is some evidence of minor freshwater influence on No Name Bank, presumably from discharge, throughout the time of deposition of the core as demonstrated by the minor (< 4%) amounts of the freshwater hydrobiid gastropods and the presence of the mesohaline mollusk Polymesoda maritima and foram Ammonia beccarii var. parkinsoniana.

At Featherbed Bank Malzella floridana is present in minor amounts (<16% relative abundance in SEI297-FB1) (Figure 22) and Brachidontes exustus is nearly absent (SEI297-FB1 and GLW402-FBB). The marine ostracode bairdiids are present in relatively high abundances in the lower and upper portions of the core (SEI297-FB1). Archaias and Articulina increase in the upper half of the core (SEI297-FB1), similar to their pattern at No Name. Featherbed Bank also shows some minor indications of influx of mesohaline waters at the 1997 core site (SEI297-FB1), with the presence of Polymesoda, Cyrenoida and Ammonia. No freshwater gastropods are found at either Featherbed Bank site. The ostracode Mg/Ca ratio (SEI297-FB1) shows less variation than at No Name (Figures 17 and 22), but follows a similar trend with increased frequency and amplitude of fluctuations in the lower portion of the core. At Card Bank (SEI297-CB1) Malzella floridana is present in higher abundances and experiences greater fluctuations in abundance, and the bairdiids increase sometime between 1720 and 1840.

Figure 22. Comparison of No Name Bank (GLW402-NNB) and Featherbed Bank (GLW402-FBB and SEI297-FB1). Left section shows changes in molluscan absolute abundance (total number of individual specimens excluding worn and fragments) and number of faunal groups (a rough measure of diversity). Middle section shows changes in percent abundance of benthic faunal habitat indicators; the No Name sub-aquatic vegetation data include mollusks, forams and ostracodes (a sum of all data shown on Figure 23, plus additional molluscan species) but the Featherbed Bank data are mollusks only. Right section shows changes in salinity indicators - the ostracode Malzella floridana and three point moving average for Mg/Ca data (see figure 17 for full data plot). NN: No Name core GLW402-NNB. FBB: Featherbed Bank core GLW402-FBB. FB1: SEI297-FB1. [larger version]

Examining Malzella trends at the three sites indicates that over multi-decadal and centennial timescales, Card Bank has experienced large swings in salinity, whereas central Biscayne Bay has been comparatively stable. There is no indication of interdecadal salinity extremes at Featherbed and No Name banks and periods of hypersalinity are not in evidence. The increases in bairdiids at the three sites indicate increasing marine influence throughout the Bay, and the increase in Ammonia and Archaias at the two central Biscayne Bay sites indicate increasing continental shelf influence. It is too early in our study to determine the causes of these changes, however, the increases in Archaias and Articulina appear to have come shortly after the 1947 reduction of freshwater flow into Biscayne Bay as a result of changing water management practices. These findings are consistent with the record from Manatee Bay (Ishman and others, 1998). The fauna in the Manatee Bay core record an increasingly marine influence throughout the 1900's, with an increase in the amplitude of salinity fluctuations occurring around 1940.

Correlation of the Mg/Ca records to other published data indicates that regional trends may be emerging. The inter-annual to multi-decadal Mg/Ca (salinity) oscillations at No Name Bank have a strong correspondence with wet/dry oscillations interpreted from the coral ¹⁸O record off Elliot Key published by Swart and others, (1996). The upper portion (66-0 cm) of the Featherbed Bank Core (SEI297-FB-1) Mg/Ca record shows a pattern that is similar to the Mg/Ca record from Russell Bank in eastern Florida Bay (Dwyer and Cronin, 2001). The Mg/Ca changes at the Russell site are thought to primarily reflect changes in salinity, which in turn are largely driven by changes in freshwater runoff/discharge into Florida Bay. Temperature variations likely amplify the Russell Mg/Ca changes. The similarity between the Mg/Ca records at Russell and Featherbed, suggests that comparable, synchronous ecosystem changes occurred in Florida and Biscayne Bays and that the changes are driven by a regional (or larger) scale forcing mechanism.

Additional faunal and geochemical analyses and refinement of the age model are needed to more thoroughly understand the salinity history of Biscayne Bay and the minor differences seen between No Name and Featherbed Banks. Completion of the analyses at Card Bank will provide confirmation of the patterns noted here.