Although the environmental interpretations based on the molluscan fauna are preliminary, and are likely to be refined with quantitative analysis, the presence of key indicator species of mollusks (Table 3) provides an accurate first look at the broad-scale changes taking place at these sites during the time of deposition of the cores. In addition, comparison of depositional environments in the 2005 cores, with those from the 2004 cores (Wingard and others, 2005) provides additional insights into environmental changes in the significant southwest coastal area.

The inner bay cores collected in 2004 from Big Lostmans Bay, Broad River Bay, and Tarpon Bay were dominated by peat accumulation in the lower portion of the cores (Wingard and others, 2005). With the exception of the core from Lostmans River, Second Bay, none of the cores collected from the Lostmans, Harney or Shark River channels in 2005 contain true peats, although some sections contain peaty muds (Figure 5, Figure 6, Figure 7, Figure 8, Figure 9, Figure 10), nor do any of the six cores contain visible terrestrial gastropod remains. These findings indicate the sediment from Lostmans First Bay and the Harney and Shark River systems represent areas that have been submerged throughout the time of deposition; whereas the inner bays were partially emergent.

Comparing the mid-channel cores to the channel mouth cores, in general, salinity decreases up channel away from the open water of the Gulf of Mexico. Figure 11 illustrates spatial and temporal salinity patterns in the Harney River and Shark River systems. Both mid-channel cores (GLW705-HR2A and GLW705 SR2A) show a very distinctive transition to exclusively freshwater mollusks in the lower portions of the cores (at 86 cm depth in GLW705-HR2A and at 102 cm in GLW705-SR2A). While the transition from exclusively freshwater mollusks to a euryhaline assemblage is separated by a zone of "no visible mollusks" in the Harney River core (GLW705-HR2A)(possibly an artifact of examining unprocessed samples), this is not the case in the Shark River core. The lowermost zone of the Shark River core (GLW705-SR2A, 102-144 cm) contains the remains of freshwater gastropods (planorbellids and hyrdrobiids); these taxa also are found in the overlying transition zone (64-102 cm), which contains a mix of freshwater and euryhaline species. Hydrobiids have been found in portions of all cores collected from the southwest coastal area for this project. They are indicative of freshwater influx, but because hydrobiids are minute (full grown adults rarely exceed 5 mm) they can be rafted on freshwater currents into estuarine waters (see field data available at http://sofia.usgs.gov/exchange/flaecohist/). The much larger planorbellids have only been found where the water column is fresh throughout most of the year. The presence of planorbellids in the sediments up to 64 cm in core GLW705-SR2A suggests substantial freshwater flow through the Shark River system when the lower portion of the core was deposited. The change above 64 cm implies significant salinity changes, although the sediments do not change in this interval. We interpret this change as indicative of reduced freshwater influx, which excluded the planorbellids while maintaining enough flow to sustain or carry the hydrobiids into this section of the river system. The upper portion of the mid-channel Harney River core lacks visible freshwater species.

Cores from the mouths of the Harney and Shark River systems (GLW705-SHR1A and GLW705-SR1A) illustrate temporal fluctuation of salinity (Figure 11) in the estuary. The internal variations within the sedimentary units also indicate a dynamic environment and fluctuating flow regime. Neither core shows any indication of deposition in a strictly freshwater or strictly marine setting. The presence of Polymesoda spp. below 114 cm in the South Harney River core (GLW705-SHR1A) indicates predominantly mesohaline conditions during the time of deposition. This species does not appear to be present in the core from the mouth of Shark River (GLW705-SR1A), but the Shark River core is shorter than the South Harney River core. Until an age model is in place for these cores direct comparisons cannot be made.

Based on a preliminary analysis of the mollusks in the river transects, we have developed a preliminary flow model. Lostmans River shows little visible evidence of freshwater influx throughout the time of deposition. In addition, Lostmans Second Bay contains peat at the bottom of the core and the 2004 core from Big Lostmans Bay was peat except for the upper 10 cm. These preliminary observations suggest that the Lostmans system was more emergent throughout the time of deposition than the systems to the south; however, the Lostmans core observations are based on unprocessed samples and therefore minute freshwater gastropods may have been missed in the examination. The Harney River system, flowing from Tarpon Bay, has been submerged throughout the time of deposition of the cores and has received freshwater fluxes that reached to the mouth of the system periodically over time. The Shark River channels, flowing south out of Tarpon Bay, have received a continuous supply of freshwater at least to the point of the middle core (GLW705-SR2A) throughout the time of deposition, but a significant decrease is indicated at 64 cm, as discussed above. The general pattern appears to be substantial flow in the south (from Tarpon Bay out through the Shark River channels), with diminished flows northward toward Lostmans. These findings are only preliminary, based on the presence of key indicator species of mollusks. Development of an age model and quantitative analyses of the mollusks, ostracodes and pollen within the cores will allow us to test the model suggested here.

Figure 11. Transects of cores in the Harney River and Shark River Systems illustrating spatial and temporal changes in the depositional environments. (See Figure 1 for location of cores). Scale shows length of cores in cm. Data and description of Tarpon Bay Core are available from Wingard and others (2005). [larger image]