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 Sub-Aquatic Vegetation and Benthic Habitat

Sub-aquatic vegetation (SAV) and benthic habitats change naturally over time. It is important to restoration of the estuarine environments to determine what recent changes may be due to anthropogenic influences and what may be due to natural cycles of change. The distribution of epiphytal species within the cores provides an indication of the presence of sub-aquatic vegetation and in some cases, the type of vegetation. While many factors can control the distribution of an individual species (salinity, food supply, substrate, water quality), by looking at multiple proxies, we can get a fairly accurate picture of long term trends in SAV and benthic habitats (Cronin and others, 2001; Brewster-Wingard and Ishman, 1999; Ishman, 2001).

Figure 23. Sub-aquatic vegetation indicators in No Name Bank core (GLW402-NNB). Percent abundance of key indicator species, plotted against depth on the left and calendar year on the right. Increases in epiphytal species are indicative of increases in the vegetation they live on. Use of multiproxies increases the reliability of this method. See text for full explanation. [larger version]

Figures 23 and 24 illustrate changes in SAV, based on the abundance of epiphytal indicator species of foraminifers, ostracodes, and mollusks at No-Name Bank (GLW402- NNB) over the last 350-400 years. On an inter-decadal scale SAV appears to have been present throughout the time of deposition of the core, but the relative abundance of the indicators has fluctuated over time, indicating that the SAV itself has fluctuated. In general, Thalassia was most abundant between approximately 1700 and 1950 as indicated by Loxoconcha matatgordensis and Miliolinella spp. The dominant molluscan Thalassia indicator Turbo castaneus reaches a peak between 1900 and 1950 probably responding to a combination of stabilized salinity and Thalassia availability. A decline occurs in almost all of the indicator species for Thalassia after 1950, indicating a decline in seagrass abundance. Macro-benthic algae, as indicated by Brachidontes, Bittiolum and Xestoleberis was most abundant at No Name Bank prior to ~ 1700, but increased slightly after 1950 when Thalassia declined.

Examining epiphytal species in the other cores reveals some bay-wide patterns of change. All three cores reveal a significant increase in Loxoconcha matatgordensis in the lower portion of the cores between 1550 and 1750 AD. A decrease in L. matagordensis and Turbo castaneus occurs at Featherbed Bank beginning in the late 19th century continuing into the 20th century. This decline at Featherbed begins earlier (based on the current age model) and is more pronounced than the decline at No Name. At Card Bank (SEI297- CB1), however, L. matagordensis did not decrease in abundance during the 20th century. Based on the ecology of these indicator species, these patterns indicate that there was a substantial increase in seagrass habitats throughout central and southern Biscayne Bay 450-250 years ago, but a decline occurred in central Biscayne Bay during the last century. Southern Biscayne Bay-Card Sound seems to have retained healthy seagrass communities during the last century, but additional existing core data and new cores need to be examined to confirm this trend.

Figure 24. Indicators of environmental change at No Name Bank (GLW402-FBB) as shown in Figure 22, plotted against age and depth. Three left plots show percent abundance of key indicator species. Data for Thalassia and macro-algal epiphytes are a sum of all indicator species shown in Figure 23, plus additional molluscan species. Data for number of molluscan individuals (absolute abundance) and number of faunal groups (a rough measure of diversity) is given in Appendix G. Mg/Ca data is shown as three point moving average (see figure 17 for full data plot). [larger version]

All three cores also record a decrease in abundance in the ostracode genus Xestoleberis since the 18th century, although a slight rise in abundance during the last 50 years is observed at No Name Bank as discussed above. Bittiolum varium matches the trends for Xestoleberis at both Featherbed Bank cores. Xestoleberis abundance is extremely variable over inter-decadal time scales at Card Bank over the last 800 years, however, the environmental significance is not clear.

The patterns of change in the infaunal molluscan assemblages are nearly identical (all though offset on the timing based on current age models) in the No Name core and the 2002 Featherbed Bank core (GLW402-FBB) (Figure 22) and similar to the 1997 Featherbed Bank core. The abundance of infaunal species is highest at the bottom of all three cores (>30%), then declines between 1550 and 1820 (based on the current age models). In No Name and the 2002 Featherbed cores the infaunal species increase somewhat again in the middle of the core and then decline in the later half of the 20th century, corresponding to the decrease in Thalassia. This pattern is the opposite of what has been observed in Florida Bay cores (Brewster-Wingard and others, 1998, 1999, 2001). Typically when epiphytal molluscan species decline, infaunal species increase in relative abundance. The environmental significance of the decline in infaunal mollusks is not currently understood.

Stone and others, (2000) concluded that the sequence of changes in faunal zones at Featherbed Bank (SEI297-FB1) was consistent with a model for bank migration and gradual shallowing of the bank. The flank of the migrating bank was characterized by the abundant sub-aquatic vegetation in zone 2 in the core. The conclusion of Stone and others, (2000) was that it would be difficult to determine if the changes at Featherbed represented regional scale change, or local bank migration. While bank migration can still not be completely ruled out, the commonality of trends seen at No Name and both Featherbed cores (Figure 22) implies there are regional-scale factors at work.