Abstract

Introduction

An essential part of the decision making process is to understand the history of the ecosystem prior to significant human alteration, and to separate natural variability in the ecosystem from human-induced change. The U.S. Geological Survey (USGS), in cooperation with National Oceanic and Atmospheric Administration (NOAA), South Florida Water Management District (SFWMD), the National Park Service (NPS), the Army Corps of Engineers (ACOE), and other Federal, State and local agencies, is conducting research to provide information on the history of the Everglades ecosystem over the last 150-200 years. The distribution of fauna and flora in a series of sediment cores taken throughout the Everglades ecosystem provides information on the biological, physical and chemical parameters of the system over time.

Figure 1. Location of Russell Bank (N25 03.831', W80 37.486'), cores 19A, 19B, and 19C. [larger image]

We would like to thank Robert Halley and Gene Shinn, U.S. Geological Survey, St. Petersburg, FL, for collecting the cores and making them available to us for analysis. We would like to thank our colleagues at South Florida Water Management District, National Oceanic and Atmospheric Administration, and Everglades National Park for their cooperation and assistance in this investigation. We have benefited from discussions with William Lyons, Florida Marine Research Institute, St. Petersburg, FL; and Thomas Scott, Florida Geological Survey, Tallahassee, FL. Our reviewers, Thomas Cronin and John Sutter, U.S. Geological Survey, Reston, VA, provided helpful comments and suggestions to improve this manuscript.

Marci Marot, U.S. Geological Survey, St. Petersburg, FL, prepared the samples for isotopic and calcareous analysis. Assistance in sample processing was provided by Jill D'Ambrosio, Ian Graham, Lisa Weimer, Neil Waibel, Patrick Buchanan, Nancy Carlin, and Steve Wandrei of the U.S. Geological Survey, Reston, VA. Rob Stamm and Patrick Buchanan, U.S. Geological Survey, Reston, VA, assisted in the preparation of illustrations for this report and Jill D'Ambrosio assisted in compiling the final report.

Methods of Investigation

Sediment samples from core 19B were collected at 2 cm intervals and every other sample, starting with 0-2 cm and totaling 34 samples, was analyzed for calcareous benthic fauna. The samples were washed through a 63µm sieve and dried at <50 degrees C. When possible, a total of 300 benthic foraminifer specimens were picked from the sample and mounted on gridded micropaleontologic slides. Large samples were put through a sample splitter to randomly reduce the number of specimens. For samples containing fewer than 300 benthic foraminifer individuals, all of the specimens present were picked. Molluscs were picked from the > = 850µm size fraction. All molluscs, and fragments of molluscs recognizable to the generic level, present in each interval were picked. Species abundances for the benthic fauna were standardized by calculating relative abundances (percent).

Pollen and Dinocysts

Material for palynological analysis was extracted from 1-2 cm sections of core 19A. For each palynological sample, 7-40 g of material (dry weight) was treated in hydrochloric and hydrofluoric acids and processed for palynological studies. All samples were treated with warm KOH for 2-5 minutes, given ultrasonic pulse treatment for 5 seconds, acetolysed, and sieved between 8-200µm mesh. A tablet of Lycopodium marker grains was added to each sample. For most samples, at least 300 pollen grains were counted for calculation of percent abundances and absolute pollen concentration. To calculate absolute concentration of palynomorphs, the marker-grain method was used (Benninghoff, 1962; Maker, 1981, Stockmarr, 1971). For two samples, pollen was sparse enough that fewer than 300 grains were counted. For nine samples, one slide was completely examined for dinocysts and all dinocyst taxa were tabulated. For two samples (0-1 cm and 80-82 cm), two slides were examined completely for dinocysts.

Isotopic Analyses

Benthic Foraminifers

A total of 31 benthic foraminiferal species were identified and counted. The foraminiferal data was standardized to relative abundance (percent of assemblage; Table 1) and was used for all quantitative analyses. Species diversity as measured by Simpson's Index ranged from 0.11 to 0.26, and the number of species ranged from 12 to 19.

The foraminiferal assemblages are dominated by calcareous benthic forms with the dominance patterns alternating between rotaliid taxa, Ammonia parkinsoniana tepida, A. parkinsoniana typica, Elphidium galvestonense mexicanum, E. galvestonense typica, and E. poeyanum; and miliolid taxa, Miliolinella cirlcularis, M. labiosa, Quinqueloculina bosciana, Q. seminulum, Q. tenagos, Q. polygona, Q. poeyana, and Triloculina tricarinata. Other significant species include Archaias angulatus, Peneroplis proteus, Rosalina floridana and Quinqueloculina agglutinans.

Two dominant assemblages can be identified in the cores, an Ammonia-Elphidium (A-E) assemblage and a miliolid assemblage. Observations of the foraminiferal faunal changes throughout core 19B show four intervals dominated by the miliolid assemblage (140 cm, 118-90 cm, 70-42 cm, and 18-0 cm), interrupted by three intervals of A-E assemblage dominance (140-118 cm, 90-70 cm, and 42-18 cm).

Molluscs

Twenty-six molluscan taxonomic categories were recognized and counted in the Russell Bank 19B core. The number of specimens per sample ranged from 2 to 517, so the faunal counts were standardized to relative percent abundance (Table 2). The Simpson's Diversity Index was calculated for each sample (Table 2). Three sections in the core contained few individual specimens (<40), and few molluscan taxonomic groups (1-9): 122-112cm, 62-52cm, 38-28cm. Two species, Brachiodontes sp. and Transennella sp. make up greater than 50 percent of the molluscan fauna in the entire core.

A cluster analysis of Pearsons Correlation coefficient values, calculated for the molluscan percent abundance data and plotted using average linkage method (Figure 2), revealed 3 primary divisions of the molluscan assemblages within the core: 142-68 cm, 68-22 cm, and 22-0 cm. The interval from 142-68 cm is dominated by Transennella sp., Brachiodontes sp., and Cerithium spp.; Bittium varium, Rissoina spp., and Chione cancellata also are present in significant amounts (5%) throughout the interval. The interval from 66-22 cm has the lowest within-group similarity of the three molluscan assemblages in the core and is represented by 2 clusters in the anlaysis. This interval from 66-22 cm contains 2 zones of very low molluscan abundance (62-52 cm and 38-28 cm). Brachiodontes sp. and Cerithium spp. dominate the 66-22 cm interval, with a number of other species present in significant amounts (10%) in individual samples (see Table 2). The cluster containing samples from the interval from 22-0 cm has high Pearsons Correlation coefficient values for within-group similarity, and the largest between-group distances (see Figure 2). This upper portion of the core is dominated by Brachiodontes sp., which comprises greater than 50% of the molluscan assemblage in each sample from this section. Pinctada radiata and Cerithium spp. are present in significant amounts (5%) in some samples from 22-0 cm.

Figure 2: Q-mode cluster diagram of molluscan assemblages from core 19B. Distances are expressed as Pearsons Correlation coefficient values, calculated on the molluscan percent abundance data (see Table 2) and plotted using average linkage method. Shaded areas represent clusters formed by samples from the upper portion of the core (0-22 cm) and the lower portion of the core (68-142 cm). [larger image]

Benthic Faunal Patterns

Figure 3: Plot of percent abundance of benthic foraminifers, molluscs and the two data sets combined that indicate increasing salinity for core 19B. The benthic foraminifers indicate salinity > 30 ppt. The mollusc species have wider salinity tolerances; the species illustrated on this plot are classified as euhaline or euhaline/polyhaline and represent salinities of 20-25 ppt or greater. The positions of the benthic foraminiferal assemblages (Milliolid-gray; Ammonia-Elphidium-white) discussed in the text are shown on the right of the plot. [larger image]

Comparison of the benthic fauna present in core 19B with our modern data set, enables us to interpret the salinity history for the Russell Banks site. The foraminiferal and molluscan records from core 19B show distinct oscillations in salinity, with a gradual increase in salinity toward the top of the core (Figure 3). In addition, a positive correlation ( = 0.52) exists between increasing Simpson diversity indices and decreasing salinity for the benthic foraminfera.

The two dominant foraminiferal assemblages, Ammonia-Elphidium and miliolid, are very similar to the Ammonia-Elphidium and Archaias-miliolid assemblages described from modern Florida Bay sediments (Brewster-Wingard et al., 1996). The distribution of modern benthic foraminifers in Florida Bay shows a strong association between the occurrence of A. parkinsoniana and E. galvestonense mexicanum and typica with salinities less than 25 ppt (Brewster-Wingard et al., 1996). Conversely, miliolids dominate where salinities are near normal marine values (30 ppt). The intervals dominated by the miliolid assemblage (140 cm, 118-90 cm, 70-42 cm, and 18-0 cm) therefore indicate more stable and relatively higher average salinity (32-35 ppt). These segments are interrupted by three periods of A-E assemblage dominance (140-118 cm, 90-70 cm, and 42-18 cm), indicating unstable and relatively low average salinity (<32 ppt).

Molluscs are not as sensitive to variations in salinity as benthic foraminifers, consequently, the molluscan data do not show the degree of fluctuation seen in the foraminiferal data (Figure 3). The general trend of increasing percent abundance of euhaline and euhaline-polyhaline molluscs upcore does follow the general trend of the foraminiferal plot, showing a gradual increase in salinity upcore. No molluscan faunas restricted to terrestrial, fresh water, marsh, or upper estuarine environments are present in core 19B, indicating this area has been relatively open bay over the last century, and the salinity has probably not dropped below 12-15 ppt.

Both the benthic foraminiferal assemblages and the molluscan assemblages show significant changes between 70-66 cm and between 24-18 cm. These zones correspond to shifts from A-E dominated assemblages to miliolid dominated assemblages, but the molluscs do not seem to respond significantly to the decreases in salinity that mark the shift from miliolid assemblages back to A-E assemblages. Almost all of the molluscs present in this core can be classified as polyhaline and are tolerant of a wide range of salinities (from approximately 12 or 15 ppt to 30-35 ppt), which could explain the patterns seen for the molluscs. If, however, the salinity had dropped below ~15ppt during the periods of A-E dominance, then the molluscs would most likely have shown a response. The areas of A-E dominance in the core represent salinities of 15-25 ppt, with the lower number delineated by the absence of species of molluscs found in low salinity waters of Florida Bay and the upper number constrained by the presence of low salinity benthic foraminifera. The miliolid dominated zones represent salinities of 30 ppt or higher.

The suite of molluscs present in core 19B is more indicative of changes in substrate than changes in salinity. Figure 4 illustrates the substrate patterns within the core, based on the percent abundance of molluscs known to prefer those substrate types. The lowest portion of the core, from 142-110 cm has significant numbers of both sediment dwellers and grass dwellers present, with dominance of either group fluctuating. From 106-88 cm sediment dwellers dominate, although grass dwellers are still present in significant numbers. The segment from 88-22 cm is dominated by grass dwellers; above 68 cm the presence of species indicative of a sediment substrate is negligible. The uppermost portion of the core, from 22 cm to the top, is dominated by species that could live on either algae or grass.

The shifts in abundance of molluscan substrate indicators at 88 cm, 68 cm, and 22 cm correspond to changes seen in the benthic foraminiferal salinity indicators, but at this point the significance of this relationship is not understood. However, since the benthic foraminifers used for salinity indicators are not sensitive to changes in substrate, we believe the changing foraminiferal patterns reflect changing salinity.

Figure 4: Area diagram of percent abundance of molluscs indicative of specific substrate types for core 19B. Data plotted at the 116 cm and 28 cm depths represent averages of the overlying and underlying samples; these two samples did not contain enough individual specimens to be statistically valid. [larger image]

Analysis and Discussion of the flora in Core 19A

The percent abundance data on pollen is presented in Table 3. Three pollen zones are distinguishable based on both percent abundance and absolute pollen concentration (pollen grains/gram dry sediment). The bottom zone (140-80 cm) is characterized by the highest percentages of Pinus (pine) pollen (80-90%), and the lowest percentages of Quercus (oak) (<6%), and pollen of the Chenopodiaceae/Amaranthaceae (pigweed family) and Asteraceae (aster family) (Figure 5). No pollen of the Cyperaceae (sawgrass family) is present in this zone. Total pollen concentration in this zone and the middle zone ranges from about 500-1,200 grains/gram, and the concentration of Quercus pollen is lower than the other zones, with <50 grains/gram (Figure 6).

Figure 5. Percent abundance of selected pollen groups for core Russell Bank 19A (note different scales). [larger image]

The middle zone (80-32 cm) is characterized by lower percentages of Pinus pollen (54-73%), higher percentages of Quercus (6-10%), and higher concentrations of Quercus pollen (50-100 grains/gram). In both this and the upper zone, pollen of the Chenopodiaceae/Amaranthaceae and Asteraceae are more abundant, comprising 3% and 2% of the assemblages, respectively. Pollen of the Cyperaceae is present in low percentages in this and the upper zone.

The upper zone has the highest pollen concentrations of the core, with most samples ranging from 1,200-4,600 grains/gram. Several taxa contribute to these higher values, including Pinus, Quercus, Myrica (wax myrtle), Chenopodiaceae/Amaranthaceae, and Asteraceae. Cephalanthus (buttonbush) is present in low abundances (<1%) in this zone, and Liquidambar (sweet gum) is absent from this zone.

Figure 6: Total concentration of pollen (grains/gram) for core Russell Bank 19A (note different scales). [larger image]

Dinocysts

Figure 7. Percent and absolute abundances of dinocysts for core 19A. [larger image]

Figure 8. Absolute abundances of Polysphaeridium zoharyi and Spiniferites spp. in core 19 A. [larger image]

Dinocyst recovery in all samples was low. Most microscope slides are heavily dominated by organic debris and the maximum number of specimens observed in any sample was 214. Absolute abundance ranges from 77.9 to 473.6 cysts/gram. These values are one to two orders of magnitude below those reported by Wall et al. (1977) for samples from the Middle Atlantic Bight and western South Africa and are consistent with high sedimentation rates. These absolute abundances in the sediments are far below bloom concentrations, in which the number of living cells per ml of water may exceed 50 million.

The dinocyst assemblages in the Russell Bank 19A core consist of less than 10 taxa (Table 4). Various species of the genus Spiniferites Mantell dominate all samples (61 to 79 percent). Due to poor preservation and taxonomic difficulties the individual species of this genus were not differentiated; S. mirabilis (Rossignol) Sarjeant, S. ramosus (Ehrenberg) Mantell, S. scabratus (Wall) Sarjeant, and other forms are present. Operculodinium Wall spp. (O. israelianum (Rossignol) Wall or Operculodinium species undifferentiated) are present in all samples in amounts of 5 to 19 percent. Polysphaeridium zoharyi (Rossignol) Bujak et al. is present in all samples and comprises up to 16 percent of the assemblages. Most samples included low numbers of Lingulodinium machaerophorum (Deflandre & Cookson) Wall, Nematosphaeropsis rigida Wrenn, and (or) Tectatodinium pellitum Wall. Only two samples contained a single specimen each of the family Congruentidiaceae.

Most samples from Russell Bank core 19A are quite similar to one another (Figure 7). Throughout the core, the dinocyst Polysphaeridium zoharyi ranges in abundance from 0-16 percent, which is considerably lower than in the Bob Allen 6A (Wingard et al., 1995) and Little Madeira Bay T24 (Ishman et al., 1996) cores. With one exception (80-82 cm), increased absolute abundance of Spiniferites spp. is accompanied by increased absolute abundance of P. zoharyi (Figure 8). This relationship suggests that absolute abundances in this core are a function of sediment supply rather than cyst production. The sample at 80-82 cm is anomalous: the high absolute abundance of cysts is attributable to Spiniferites spp. and fewer than 30 total cysts were found on each slide. This sample may represent a case of an increase in Spiniferites cyst production and an increase in the amount of organic debris in proportion to sediment supply. The lowest samples (132-134 cm and 136-138 cm) show both low absolute abundances of all cyst types and low percentages of P. zoharyi.

Floral Patterns

Figure 9: Pb210 activity in dpm/g for core 19B. [larger image]	Figure 10: 210Pb activity in dpm/g for core 19C. [larger image]

Summary

The molluscan fauna indicate changes occurring in the substrate at the Russell Banks site. Sediment and grass (probably Thalassia) apparently were equally available for habitation from 142-110 cm, or approximately 1878-1904. Although grass was still available, bare-sediment dominated the area from 106-88 cm, or approximately 1908-1922. From 88-22 cm, or approximately 1922-1976, grass was the dominant substrate, and from approximately 1939 (68 cm) to the present, the presence of bare-sediment bottoms was negligible at the site. Significantly, from 22 cm, or approximately 1976 to the present, the molluscan fauna is dominated by species that can live equally well on algae or seagrass.

Brewster-Wingard, G.L., Ishman, S.E., Edwards, L.E, and Willard, D.A., 1996, Preliminary report on the distribution of modern fauna and flora at selected sites in north-central and north-eastern Florida Bay: US Geological Survey Open-File Report #96-732, 34 p.

Davis, J.C., 1973, Statistics and Data Analysis in Geology: New York, John Wiley & Sons, 550 p.

Ishman, S.E., Brewster-Wingard, G.L, Willard, D.A., Cronin, T.M., Edwards, L.E., and Holmes, C.W., 1996, Preliminary paleontologic report on core T-24, Little Madeira Bay, Florida: U.S. Geological Survey, Open-File Report 96-543, 47 p.

Maker, L.J., Jr., 1981, Statistics for microfossil concentration measurements employing samples spiked with marker grains: Review of Paleobotany and Palynology, v. 32, p. 153-191.

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Related information:

SOFIA Project: Ecosystem History: Florida Bay and the Southwest Coast