Diatoms as Paleoecological Indicators of Environmental Change in the Florida Bay Ecosystem.

Topical Area: Paleoecology

Jacqueline K. Huvane, Laura Pyle and Sherri R. Cooper, Duke Wetland Center, Duke University, Durham, NC

During the 20th century there have been large-scale anthropogenic modifications to South Florida ecosystems. The effects of these changes on Florida Bay and its biota are of both political and scientific interest. This study is part of a larger effort by the USGS to reconstruct the environmental history of Florida Bay using paleoecological techniques. The goal is to establish past changes in salinity, nutrients, and other water quality parameters to determine whether these changes may be related to anthropogenic activities and to determine the natural variability of the ecosystem prior to these anthropogenic disturbances.

We are using diatom indicators preserved in Florida Bay sediments to infer long-term water quality, productivity, nutrient, and salinity changes within the bay. Diatoms are microscopic algae that form a siliceous frustule that is generally well preserved in sediments. Diatom taxonomy is largely based on the morphology of the frustule, allowing species level identification of diatom assemblages derived from sediment cores. Diatoms and diatom assemblages are very useful as integrated indicators of environmental change because species distributions are closely linked to water quality parameters including salinity and nutrient status. Diatom species also have affinities for particular physical habitats. For example, planktonic diatoms are found in the open water and benthic diatoms are bottom dwellers.

Sediment cores were collected by the USGS and sectioned at 2 cm intervals. To date we have examined diatom remains from two cores collected at Pass Key and Russell Bank. These cores were 74 and 138 cm in length respectively. A measured amount of dried material (ca. 0.5g) from select samples in the cores was used for the diatom extractions. Samples were digested with H2O2, NO3 and potassium dichromate, and HCl. A size fractionation step was included to remove large particles. The resultant diatom slurry was washed in distilled water and permanently mounted on microscope slides with Naphraxâ . Diatoms were identified and counted at 1000x with the aid of a light microscope equipped with Nomarsky optics. For the Pass Key core, 15 samples were examined and 300-500 diatom valves have been identified from each sample. For the Russell Bank core, 20 samples were examined, but only 100 valves have been identified per sample to date. Counts will be increased to at least 300 valves per sample.

Centric: pennate (C:P) ratios were calculated for each sample counted in both cores. The centric diatoms identified from the two cores were primarily planktonic and the pennate forms were generally benthic. Therefore, this ratio is useful as a general indicator of the relative availability of planktonic (open water) and benthic habitats (Cooper, 1995). A stratigraphically constrained cluster analysis (CONISS) was used to define major floristic shifts in the diatom assemblages within each core (Grimm, 1987). For the Pass Key core samples, diatoms were grouped by salinity preferences. For example, diatoms reported in the literature to occur in habitats with salinities greater than 30 ppt were classified as marine.

Preliminary ²¹⁰Pb analyses suggest that the Pass Key core is relatively young, with a high sedimentation rate (C. Holmes, pers. comm.). 95 species and varieties of diatoms have been identified from the Pass Key core. The five most abundant taxa were Cyclotella cf. litoralis, Nitzschia granulata, Mastogloia crucicula, Cocconeis placentula cf. var. euglypta, and Fragilaria tabulata. A cluster analysis defined major shifts in diatom community structure at 23 and 47 cm depths in the core. C:P ratios were low in the bottom of the core and peaked in the middle. There was an overall trend of increasing C:P ratios towards the top of the core. These data suggest that benthic habitats have been reduced over the time period represented by the core. Marine species were less abundant in the bottom portion of the core and peaked between 40 and 20 cm. Marine taxa declined above 20 cm until the very top of the core where they increased in abundance. Overall there was a gradual increase in marine taxa towards the top of the core. Increases in marine taxa suggest increased salinity during the last few decades. Analyses of foraminifera, molluscs, and dinoflagellate cysts by USGS researchers (L. Brewster-Wingard, pers. comm.) also show salinity declines in the middle of the core, followed by an increase up-core.