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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
ADDITIONAL ANALYSES OF MUDBANK CORES
The following analyses have been completed on cores from No Name Bank, Featherbed Bank and Card Bank since the publication of Wingard and others (2003). These cores record the patterns of change in the more open-water areas, removed from the direct influence of freshwater inflow.
Featherbed Bank Foraminiferal Analyses
Relative abundances of key groups of foraminifers are shown in Figure 5 and the specific data are shown in Appendix D. Cribroelphidium-Elphidium and Quinqueloculina are the two most dominant foraminifer groups in the core, showing alternating patterns of dominance. In the uppermost portion of the core (above 32-34cm or since 1957) Cribroelphidium-Elphidium drop below 24% abundance, reaching a low of 3.48% in the top sample. These genera are typically more abundant in environments with greater freshwater mixing in which there are seasonal periods of reduced salinity. Fluctuations in relative abundance of Rosalina generally mimic the Cribroelphidium-Elphidium assemblage.
The relative sparseness of Cribroelphidium-Elphidium assemblage near the top of the core is the inverse of the trend seen in Articulina and Archaias. Articulina and Archaias are both marine species that are only present in minor amounts in the core, but their increases in abundance are indicative of increased oceanic influence and possibly more stable salinities at the site. Archaias is most abundant in the bottom of the core and Articulina in the top, both intervals where Cribroelphidium-Elphidium decreases in abundance.
In contrast, Quinqueloculina are relatively abundant (25-31%) and stable in the lower portion of the core (146-197 cm). Quinqueloculina reach their peak abundance (40%) in the sample from 88-90 cm (1885). A rapid decrease in abundance occurs between 1885 and 1903, reaching a low of 5% at 72-74 cm (1885). Quinqueloculina increase in abundance again beginning at 56-58 cm (1925) to >24% in the upper 40 cm (since 1947). Miliolinella also are relatively abundant in the Featherbed Bank core, occurring in every sample. Significant low abundances occur at 138-128 cm (1835-1844), 72-74 cm (1903), and between 58 and 48 cm (1925-1936). The peak abundance of 35% occurs at the bottom of the core (194-197 cm). Miliolinella is an epiphytal species so the fluctuations in abundance are indicative of fluctuations in seagrass abundance.
Relative abundances of Bolivina fluctuate throughout the core, appearing to be almost cyclic with peaks ranging between 7 and 16% at approximately 16-18cm, 48-50cm, 72-74cm, 104-106cm, 128-130cm and 164-166cm; these alternate with periods of relatively low abundance (3-5%). Bolivina typically increase when nutrients and/or organics increase, thus indicating fluctuating nutrient supply at the site over the last 1783. Triloculina are most abundant in the lowermost and uppermost portions of the core. Ammonia, typically a low salinity genus, are present in very low amounts (<1%) throughout the core.
Featherbed Bank Ostracode Analyses
Card Bank Ostracode Analyses
Card Bank Molluscan Analyses
Nineteen samples, at 8-cm intervals, were analyzed for total molluscan faunal remains from the Card Bank Core collected in 1997 (SEI297-CB1). The mollusks were classified into 63 faunal categories (excluding worn and fragmented specimens). The predominant species are Bittiolum varium, Schwarziella catesbyana, Carditamera floridana, Laveicardium mortoni, Transennella sp., and Crepidula spp. (Appendix G). The majority of these species live on some type of sub-aquatic vegetation. Figure 8 illustrates the distribution of epiphytal mollusks downcore. These data indicate that some type of sub-aquatic vegetation has been present at the CB1 site throughout the time of deposition. The sub-aquatic vegetation indicators only drop below 50% of the total molluscan fauna at three intervals in the core: 42-66 cm, 96 cm, and 138 cm. Fauna that live almost exclusively on Thalassia are present throughout most of the core (exceptions at 42-50 cm and 106 cm) at levels between 10-20 % of the total molluscan fauna (excluding worn and fragmented).
The distribution of the infaunal mollusks at CB1 follows the trend seen at the Featherbed Bank cores (GLW402-FBB and SEI297-FB1) and at No Name Bank (GLW402-NNB) (see Wingard and others, 2003, fig. 22 and discussion). As discussed in the earlier report, the decline in infaunal mollusks in the later half of the 20th century needs to be investigated more thoroughly. There is a possibility that this is an artifact of sedimentological and biological processes. However, the repetition of this trend in several cores lends support to the hypothesis that real declines have occurred in the infaunal mollusks and the occurrence of this pattern at Card Bank means it is not limited to central Biscayne Bay.
Figure 9 illustrates the downcore distribution of some of the important molluscan indicator species present in the Card Bank core. Bittiolum varium is by far the dominant species in this core. Bittiolum is a minute (full grown adults can be <5mm) gastropod that can be found on any type of sub-aquatic vegetation. Its abundance in the upper part of the core is in contrast to the central Biscayne cores (No Name, and Featherbed; see Wingard and others, 2003) where Bittiolum decreases significantly and almost disappears in the upper portion of the cores. In Florida Bay, Bittiolum is common in the eastern and central portions of the Bay, in mesohaline to polyhaline salinities (Brewster-Wingard and others, 2001). Carditamera floridana is more commonly seen in the western portions of Florida Bay in polyhaline to euhaline salinities. The relative dominance of Bittiolum and Carditamera alternates, illustrating that Card Bank seems to be a transitional area between a more restricted upper estuarine environment and a more open estuarine environment. Brachidontes and Anomalocardia are both tolerant of wide fluctuations in salinity. The near absence of these two species in the upper part of the CB1 core is in agreement with results seen in the central Biscayne Bay cores (Wingard and others, 2003) - that salinity in Biscayne Bay is becoming increasingly stable and increasingly marine during the last century.
No Name, Featherbed Bank and Card Bank Geochemical Analyses
Results of geochemical analyses of sediments from these cores are shown in Appendix C, and the data are plotted in Figure 10. Total carbon (TC) contents ranged from 11.6-17.0% in these cores, inorganic carbon (IC) from 7.81-10.1%, and OC from 1.68-9.19%. Both TC and organic carbon (OC) concentrations were significantly higher in the core from the Card Bank site, and IC contents were lowest at Card Bank. The range of total nitrogen (TN) and total phosphorous (TP) contents in sediments from these cores are: 1.16% to 0.157%, and 0.030% to 0.0063%, respectively. As with OC, both TN and TP concentrations were higher at the Card Bank site. Significant downcore trends were observed at all sites, representing both diagenetic recycling of nutrient elements, as well as historical changes in the flux of nutrient elements to the sediments. Perhaps the most interesting historical trend is the large increase in TP concentrations in surface sediments at Card and No Name Banks, beginning in the 1970s. The downcore TP profiles at both sites represent a large increase in TP flux to the sediments, superimposed on the normal diagenetic recycling of phosphorous. Since the apparent increase in TP is largest in the south (Card Bank) and lowest in the north (Featherbed Bank), a source of the excess nutrients from the Miami urban area seems unlikely. It is possible the apparent increased TP flux to the sediments resulted from inputs from canal structures in the southern part of Biscayne Bay, especially the C-111 canal.
Discussion of Additional Analyses of No Name, Featherbed Bank and Card Bank Cores
The results reported here from additional analyses of the 1997 and 2002 cores are in agreement with the findings reported in Wingard and others (2003) and provide additional evidence that a bay-wide series of faunal events has occurred over the past 2 to 5 centuries. The addition of the geochemical data provides further insight to the system-wide changes taking place in Biscayne Bay.
Comparison of ostracode trends between the two Featherbed Bank cores (GLW402-FBB and SEI297-FB1), located 1.6 km (1 mile) apart, indicate remarkably similar faunal patterns in the upper two meters of sediment. Molluscan faunal trends at the two Featherbed Bank sites also are very similar (Wingard and others, 2003). The repetition of results provide a high level of confidence that the temporal trends in the faunas seen at the Featherbed sites are representative of the ecological history of this part of Biscayne Bay. Results of the foraminiferal analyses of the 2002 Featherbed Bank core are in agreement with the conclusions of the 2003 report and with ostracode and mollusk data. The foraminifers indicate a stronger marine influence in the upper portion of the core. Epiphytal indicators also are in agreement - there has likely been a natural variation of seagrass since at least 1814, and the most recent data from the top of the core suggest that the coverage at this location is somewhat near the average of the past two centuries.
Comparison of the ostracode trends between the two Card Bank cores (GLW402-CBB and SEI297-CB1) also indicates similar faunal patterns in the upper 2 meters of sediment from the two sites located about 2 km (1.3 miles) apart, and these trends also are very similar to the patterns seen at Featherbed Bank. Molluscan analyses of the 1997 Card Bank core illustrate that some type of sub-aquatic vegetation has been present at the site throughout the time of deposition. Card Bank has been a transitional area between a more restricted upper estuarine environment and a more open estuarine environment, fluctuating between these environments over time, but the site appears to have become increasingly stable and increasingly marine during the last century. These data are in agreement with the ostracode data from Card Bank, and with the conclusions of Wingard and others (2003) for Biscayne Bay as a whole.
Geochemically a shift appears to have occurred at Card Bank and No Name Bank around 1970, with a large increase in total phosphorus (TP) occurring. Since the apparent increase in TP is largest at Card Bank in the south and lowest at Featherbed Bank, a source of excess nutrients from the Miami urban area seems unlikely. It is possible that the apparent increased TP flux to the sediments resulted from inputs from canal structures in the southern part of Biscayne Bay, especially the C-111 canal.
U.S. Department of the Interior, U.S. Geological Survey
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