PATTERNS OF CHANGE AT NEAR-SHORE SITES

The near-shore cores were collected in 2003, in areas near historical freshwater drainage, in order to determine changes in freshwater influx to Biscayne Bay over time.

Middle Key Basin

Middle Key core (GLW603-MKA) is located southwest of the Card Sound Bridge in a relatively isolated shallow basin (Figures 1 and 14).  No development has taken place in the basin or in the immediate drainage area west to US Route 1 and north to the Florida Sand and Gravel pits.  Compared to other core locations, Middle Key represents a relatively undisturbed site.

An examination of the sediments in the core reveals primarily gradational transitions from a mixed matrix of carbonate mud and limestone rubble from the underlying bedrock in the lower portion of the core, to peaty material with very high organic content and few preserved shells in the middle of the core, to a carbonate mud with abundant shell material at the top of the core.

Figure 14. (A, top) Aerial photograph of Middle Key area, 1940, from Smith (2002); and (B, bottom) digital orthophoto quadrangle, 1999, from EarthExplorer (http://edcsns17.cr.usgs.gov/EarthExplorer/). Core site is indicated with white dot. [larger version]

The lack of abundant, well-preserved shell material in the lower portion of the core prevented us from obtaining carbon-14 analyses in the initial phase of sampling (future analyses are planned on wood and some shell material found during processing).  The excess (or unsupported) lead-210 reaches background levels at 22-24 cm in depth (Figure 15).  The first Casuarina pollen appears in the sample from 20-22 cm.  As discussed in the Material and Methods section, lead-210 typically reaches background levels in the early 1900s and Casuarina was introduced in south Florida around the turn of the 20th century.  The correlation of these two events in the core (Figure 16), the relatively good fit between the lead-210 data and the logarithmic decay curve (Figure 15D), and the lack of any obvious event horizons indicate that the age model for the 20th century is sound.  The upper 20-25 cm of the core, therefore represent deposition in the 20th century; however, the relatively low sedimentation rate means each 2-cm core sample represents approximately seven years of sediment accumulation, preventing any precise correlations to anthropogenic or natural events.

The only method we currently have for estimating the age of the lower portion of the core is to extrapolate the sedimentation rate based on the lead-210 data backward.  Using this approach, the base of the core may be around 1600, but it could be much older.  Radiocarbon analyses corrected for the local reservoir effect should provide better data on the age of the lower portion of the core.  The lower segment of Middle Key core, however, shows a distinct lead-210 anomaly beginning between 66 and 74 cm.  This anomaly is similar to anomalies found in Florida Bay cores examined by the authors and is believed to be due to groundwater upwelling (Holmes, 2001).

Figure 15. Lead-210 data for Middle Key core (GLW603-MKA). A. Projected calendar year at depth. B. Loss on ignition (% dry weight). C. Total lead-210 activity in decays per minute per gram (dpm/g), with error bars. D. Excess (or unsupported) lead-210 activity (dpm/g). Data in yellow, logarithmic decay curve in blue. [larger version]

 

Figure 16. Composite of age information for 2003 nearshore cores. Average sedimentation rate is based on lead-210 data (see Figures 15A, 24A, 32A); the lowest position of the yellow line in the core marks the point where lead-210 reaches background level - approximately 1900. Projected sedimentation rate assumes lowest measured sedimentation rate is constant to bottom of core. Chicken Key core has a change in sedimentation rate at 32 cm. Samples submitted for carbon-14 show position in core and estimated age without local correction factor (data from Table 2, see text for full explanation). Carbon-14 samples measured as having modern carbon indicate the shell grew after 1950. First occurrence of Casuarina shows position in core and assumes an age of ~1900-1925 (Langeland, 1990). [larger version]

Results

Figure 17. Percent abundance of key ostracode taxa in Middle Key core (GLW603-MKA) plotted against depth in cm. No ostracodes were recovered below 28 cm core depth. Note different percent abundance scales. (Data in Appendix I.) [larger version]

Ostracode assemblages in Middle Key core show distinctive changes occurring in the 20th century (Figure 17; Appendix I).  The most significant change is the decline in non-marine species from about 20 to 10 cm core depth (~1934-1969).  Sediments from the interval 27-13 cm core depth were deposited in close proximity to freshwater environments where non-marine taxa, such as Candona, Heterocypris, and cyprids, live.  The disappearance of non-marine taxa near 10 cm core depth (mid-late 1960s) is accompanied by small increases in Loxoconcha matagordensis and Malzella floridana, as well as increases in Mg/Ca ratios of M. floridana shells from approximately 25-30 mmol/mol at 5 cm core depth (1980s) (Appendix B).

Three distinctive shifts occur in the molluscan assemblages in Middle Key core (Figure 18; Appendix J).  First, from 90 cm to approximately 30 cm (pre-1900) the assemblage is predominantly freshwater fauna (hyrdrobiids, minute gastropods that can be found floating in freshwater currents, and Physidae and Planorbella, relatively large bottom dwelling freshwater gastropods).  Besides the freshwater gastropods, the only other mollusks found below 42 cm are scattered terrestrial gastropods (Polygyra and Pupillidae).  Portions of this lower section of the core do not contain statistically significant numbers of mollusks (from 82-72 cm > 5 individuals are present; from 56-70 cm >75 individuals are present), however, all the mollusks present are indicative of freshwater and terrestrial environments.  The transition to the second assemblage is gradual, beginning with the sample at 42 cm (pre-1900), where Cyrenoida floridana, a clam that lives in very low salinity waters (<10 ppt and typically <5ppt; Brewster-Wingard and others, 2001) makes a rare appearance.  Above 40 cm core depth, the freshwater gastropods begin to decline steadily.  At 24 cm (near the turn of the century to 1920s) typical near-shore estuarine species (Acteocina canaliculata, Anomalocardia auberiana, Bittiolum varium, and Brachidontes exustus) begin to increase.  Diversity and absolute abundance also increase above 24 cm (data in Appendix J), consistent with a transition from freshwater to more estuarine or mixed salinities.  The final shift occurs above 10 cm (late 1960s to mid-1970s) where freshwater gastropods drop to <13%, the oligohaline to low mesohaline species decline (Cyrenoida and Polymesoda), and species that can tolerate significant fluctuations in salinity increase (Anomalocardia, Brachidontes, Bittiolum, Parastarte and Cerithium).  Also, the typically mesohaline to polyhaline clam, Transennella spp. increases in the upper 10 cm.

Two samples were quantified for foraminifers from Middle Key (0-2 cm, and 16-18 cm) (Figure 19; Appendix K); the six samples examined from the lower portion of the core were essentially barren.  A comparison of the two samples indicates that the Ammonia and Cribroelphidium- Elphidium groups, which are more abundant in freshwater mixing zones, are relatively higher in abundance in the lower sample (16-18 cm; 1940s).  The three groups more indicative of marine influence (Miliolinella, Quinqueloculina, and Triloculina) are higher in abundance in the upper sample (0-2 cm; ~1996-2003).  The absence of foraminifers from the lower portion of the core is significant because they are indicative of estuarine and marine environments.

Only two pollen assemblage zones are found in the Middle Key core (Figure 20; Appendix L).  In Zone I (22-114 cm), Pinus pollen comprises < 90% of pollen assemblages, and pollen of Myrica and Chenopodiaceae/Amaranthaceae typically are present.  In zone II (0-22 cm), Casuarina pollen is present; Quercus, Myrica, and Chenopodiaceae/Amaranthaceae pollen each are more abundant than in zone I, mangrove (Laguncularia and Rhizophora) pollen is present, and Pinus pollen comprises <90% of the assemblages.  Pollen concentrations are reduced in Zone II.

Geochemical analyses of the total phosphorus (TP) concentrations in the Middle Key core are shown in Figure 21 (Appendix C).  A significant TP anomaly occurs in the lower portion of this core from 100-70 cm (pre 1900), where TP levels reach 0.0588% in the 72-74 cm sample.  In the upper 30-20 cm of the core TP levels exhibit a normal exponential decrease in TP with depth due to diagenetic recycling of P from sedimentary organic matter.

Discussion

Several anomalies occur in the lower portion of Middle Key core.  Below 70 cm (pre-1900) total lead-210 concentration increases significantly and below 90 cm pollen concentrations are greatly reduced (Figures 15 and 20).  Both of these factors may be related to the underlying bedrock.  Upwelling of groundwater through the limestone allows radon to seep into the over-lying peat where is it adsorbed and then decays to lead-210, altering the total lead-210 concentrations for the bottom of the core.  The occurrence of limestone rubble in the bottom of the core reduces the quantity of sediments and therefore the quantity of pollen grains.  The total phosphorus (TP) anomaly occurs from 100-70 cm (Figure 21) and may be related to the presence of the limestone and carbonate muds.  Carbonates often seem to have somewhat higher TP values due to formation of calcium fluorapatite mineral phases that do not recycle very readily.  Alternatively, the high TP values could represent a period of high aquatic productivity.  The samples where TP is highest (72-74 and 80-82 cm) correspond to an essentially barren faunal zone.  Whether these two factors are related is unknown.

The mollusk relative assemblage data illustrate a clear pattern of increasing salinity and shifting environments upcore (Figures 18 and 22).  Below 40 cm (pre-1900) freshwater gastropods, with a few terrestrial mollusks are the only faunal component in the core (Figure 22, #1).  The pollen data, the presence of large amounts of wood debris and organic material, and the peaty nature of segments of the lower core are consistent with a freshwater environment.

A gradual shift to more saline conditions occurs before the 20th century at approximately 42 cm core depth as indicated by the appearance of oligohaline to low-end mesohaline species (Figure 22, #2).  A significant drop in pollen concentrations occurs at this point in the core, and the sediments shift to a carbonate mud.  A steady decline in the relative abundance of all freshwater gastropods occurs above 30 cm (Figure 22, #3).  Absolute abundance data of mollusks indicate significant pulses of freshwater influx above 30 cm, but the high numbers of estuarine species show an increasing influence of saline waters relative to freshwater at the site.  Ostracodes typical of non-marine environments near freshwater influx are present in samples from 28-12 cm (late 1800s or early 1900s to ~1960) (Figure 22, #4).  The Mg/Ca calibration developed for Malzella indicates the salinity for the sediments deposited between 28 and 12 cm (Figure 22, #5) was about 14-18 ppt.¹ Oligohaline and mesohaline mollusks continue to increase above 24 cm (1920s) (Figure 22, #6) and Pinus pollen decreases in abundance, while pollen of other trees and weedy species became more abundant.  Foraminifers from the sample at 16-18 cm (1940s) are consistent with the mollusk and ostracode data; species typical of freshwater mixing environments are intermixed with species more indicative of higher salinities.

The upper 10-12 cm of the core, representing deposition from approximately the mid-late 1960s to the present, shows a significant decline in the relative abundance of all indicators of freshwater influx and an increase in more mesohaline to polyhaline fauna (Figure 22, #7).  Several species of the mollusks and ostracodes present in this segment of the core can tolerate wide fluctuations in salinity from mesohaline to hypersaline.  The Mg/Ca data also indicate increasing salinities in this segment of the core with values of 17-21.5 ppt¹.  The foraminifers from the top of the core are consistent with a shift toward increasing salinities.  Mangrove pollen (Laguncularia and Rhizophora) appears above 12 cm providing additional evidence of the increasingly brackish estuarine nature of the environment.

Taken together these patterns indicate increasing salinities and decreasing freshwater influence throughout the time of deposition of Middle Key core – a typical transgressive sequence.  These changes could potentially be explained by rising sea level alone.  Estimates of sea level rise in south Florida range from 2.27 mm/yr (http://co-ops.nos.noaa.gov/sltrends (FL: Key West Station Data); and Smith, 1997) to 3-4 mm/yr (Wanless, 1989; Wanless and others, 1994).

The pre-1900 shift towards increasing salinity at approximately 42 cm is most likely due to natural causes; however, the timing of some of the significant changes in the upper segment of the core indicates other factors may have influenced the changes seen at Middle Key.  The decline in relative abundance of freshwater gastropods, the increase in brackish fauna, and the increase in trees and weedy species that occurs in the early 20th century could be explained by several events.  The construction of the Flagler Railroad 4 km (2.5 miles) to the west of the site around 1906 may have reduced freshwater influx to Middle Key basin coming from the eastern Everglades.  The first Card Sound bridge and a roadway leading to the bridge were constructed in the 1920s (http://www.keyshistory.org/osh.html); although this probably had less affect on the freshwater supply than the railroad construction, clearing for the road would account for a reduction in the pine and an increase in weedy species.  Depending on the actual bridge construction, it could potentially have affected tidal flushing of the Middle Key basin.  An unnamed category 4 hurricane in 1926 (Pielke and Landsea, 1998) also could explain changes in the flora.  The increased salinity, the appearance of mangroves, and the sharp decline (both absolute and relative abundance) in freshwater fauna in the upper 12-10 cm of the core begin in approximately the 1960s and could be a response to the hydrologic changes of the Central and Southern Florida project (Light and Dineen, 1994).

The preliminary results from Middle Key can be compared to the Manatee Bay core collected in 1996 (SEI96-MB1), 2.8 km (1.7 miles) to the south.  The age model for Manatee Bay (Ishman and others, 1998) is based on the first occurrence of Casuarina; lead-210 analyses were not done on the core so a direct comparison between the sites is difficult.  The first occurrence of Casuarina in both cores, however, serves as a datum, indicating that the sedimentation rate at Manatee Bay is significantly higher than at Middle Key (Casuarina first appears at 65 cm in MB1 and at 22 cm in MKA).  In general, a similar sequence is seen at both core sites of change from freshwater environments at the bottom of the core transitioning into increasingly estuarine environments in the upper portion of the core.  The Manatee Bay site, however, records a change to a diverse polyhaline to nearly euhaline assemblage at the top of the core that is not seen at Middle Key.