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- Title
- The impact of water management practices in the Caloosahatchee River : mollusk assemblages as indicators of environmental change.
- Creator
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Agobian, Jorge N.
- Abstract / Description
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Because of the selective and destructive nature of taphonomic processes, taphonomic research was originally focused on post-mortem information loss and bias. Over the past twenty years, the focus has shifted towards the inclusion of information gain through paleoecological reconstructions. Paleoecological reconstructions encompass several types of studies, including analyses of palaeosols, macro botany, palynology, ecological parameters, and faunal assemblages. Because of their abundance of...
Show moreBecause of the selective and destructive nature of taphonomic processes, taphonomic research was originally focused on post-mortem information loss and bias. Over the past twenty years, the focus has shifted towards the inclusion of information gain through paleoecological reconstructions. Paleoecological reconstructions encompass several types of studies, including analyses of palaeosols, macro botany, palynology, ecological parameters, and faunal assemblages. Because of their abundance of hard parts, faunal assemblages have proven to be particularly useful in both paleoecological reconstruction studies and live:dead comparison studies. Benthic mollusks, in particular, are remarkably useful taphonomic study subjects. This study seeks to address whether a live:dead molluscan assemblage analysis can be used to discriminate between natural and anthropogenic factors in ecosystem change, and to quantify this change in terms of one environmental variable: salinity. The Caloosahatchee River and estuary is located on the southwest coast of Florida, and represents an extraordinary anthropogenic manipulation of hydrology. Over the past 125 years, the Caloosahatchee has been massively altered, dredged, straightened, and artificially connected to Lake Okeechobee, a freshwater lake in the center of the Florida peninsula. These modifications have changed the natural timing, volume, and frequency of the estuary's freshwater inflow, and must have had a severe effect on its sessile molluscan biota, but documentation of this effect is not available due to the lack of salinity measurements and ecological surveys pre-dating the alterations. The fossil record may provide a record of the pre-alteration conditions, and these data can then be compared to the post-alteration conditions to determine the impact of the alterations on the molluscan fauna of the Caloosahatchee. This study seeks to answer whether the time-averaging resolution of a molluscan live:dead assemblage from the Caloosahatchee is high enough to observe these differences or similarities over a relatively short (hundreds of years) timespan, and whether the estuary's live molluscan community represents an ecosystem effect of lowered salinity when compared to its fossil counterpart. The study took place along four sites within the lower reaches of the Caloosahatchee estuary and incorporated data from a previous study from the Southwest Florida Water Management District. Specimens were collected with a Petite ponar grab and transported to the laboratory for identification. The salinity and sedimentary conditions were recorded for each collection site and replicate. In order to establish the time-averaging of the dead assemblage, the age of selected fossil specimens was determined using radiocarbon dating. The data analyses consisted of calculating biodiversity indices, determining the Pearson product-moment and Spearman rank-order correlations, and carrying out a detrended correspondence analysis (DCA) to detect paleoecological recurrence patterns and determine the principal source of ecological variation in the composition of faunas. The Caloosahatchee estuary's salinity is highly variable at all four sites, and is therefore likely to exert a significant effect on the molluscan communities throughout the study area. The lack of seasonal variation in sediment characteristics does not negate the impact of seasonal variation in salinity. Salinity, and not substrate type, is the environmental factor that determines the molluscan community composition in the Caloosahatchee estuary. The biodiversity indices and Pearson product-moment correlation support this conclusion: overall, the molluscan communities show increased biodiversity with increased salinities. The radiocarbon analysis results represent an ideal scenario regarding the time averaging of the sampled community: a mixture of ''old" (hundreds of years) dead specimens and recent (50 years) dead specimens. The Spearman rank-order analysis validates the short-scale time-averaging results, since significant live:dead agreement was obtained throughout the sampled sites. The DCA ordination plotted the live and dead assemblages for all sites along two axes: Axis 1, the x-axis, represents the principal source of ecological variation. An analysis of the ordinations for the individual sites shows that Axis 1 clearly represents the salinity gradient, with increasing values representing decreasing salinity. Overall, the live communities appear to have been shifted downstream over time: the live communities exhibit lower salinities than their dead counterparts. The significant differences between the pre- and post-alteration molluscan communities faithfully record the change to the Caloosahatchee estuary's salinity regime: the live molluscan community clearly represents an ecosystem of lowered salinity. The comparison of live:dead assemblages within the taphonomically active zone therefore shows great promise as an analytical tool in short geological timespans, and should prove to be particularly useful for restoration work.
Show less - Date Issued
- 2010
- Identifier
- fgcu_ETD_0469
- Format
- Document (PDF)
