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U.S. Department of the Interior
U.S. Geological Survey
Open-File Report 01-390 [View PDF]
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View other reports in the 2001
Tampa Bay Pilot Study Series: |
| 1-2-3-4-5-6-7-8-9 |
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Ecosystem Structure
& Function |
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Task Leader: Carole McIvor
Co-author: Ellen Raabe
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Tampa Bay Integrated Science Pilot Study:
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Wetlands Characterization
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| Results/Discussion |
| Historic analysis reveals major changes to the lower reaches of the Alafia River and its associated wetlands including diversion and dredging of the river mouth and the construction of bridges and other structures near the shore. A more subtle change was the conversion of salt marsh to mangrove, a change possibly coupled to reduction of annual riverine discharge in the Alafia.
Preliminary results from SET monitoring suggest that wetland surface elevation responds to hydrologic regimes and storm events. Preliminary monitoring of small juveniles and forage fish species revealed a community dominated by anchovies, silversides, and menhaden, all schooling species. The presence of small juveniles of mullet, sheepshead, and spot that spawn offshore implies that shallow shorelines lined with salt marsh or mangrove in the lower Alafia River act as a fish nursery or rearing area.
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| Figure 3. Intensive agriculture surrounds circular karst ponds in 1951 (left) on Terra Ceia Aquatic and State Buffer Preserve. Embayments are lined with mangroves, and distinctive berms for field protection are in place. Linear features at photo bottom are windbreaks of introduced Australian pine. In 1999, after years of disuse and addition of mosquito ditches, all karst features but one (pond 24) are now brackish and lined with mangroves (right). Exotic scrub species have colonized the fields, and Australian pine dominates the lower landscape. |
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Wetland distribution in the Terra Ceia Aquatic and State Buffer Preserve is decidedly more complex (Fig. 2). Presently the peninsula consists primarily of brackish ponds of karst origin imbedded in a matrix of upland plant communities. The upland area was previously farmed (Fig. 3, left panel) and mosquito ditches connected ponds to the nearest marine embayment.
Tidal flow through mosquito ditches had a number of effects:
- expansion of mangroves around the interior ponds (Fig. 3, right panel);
- increased pond salinities;
- increased surface water salinity extends into the groundwater in a zone up to 40-60 meters wide and at least 5 meters deep around ditched ponds (Fig. 4);
- the presence of estuarine resident fish that complete their entire life cycle in the ponds or their connecting ditches and are well adapted to highly variable salinities.
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| Figure 4. Apparent bulk ground conductivities from the uppermost 7.5 meters (blue lines) and uppermost 15 meters (green lines) versus distance from the edge of two ponds differing in their hydrological connection to seawater: pond 47 is connected directly to an embayment of Tampa Bay via a mosquito control ditch; pond 101 is connected less directly by natural tidal flooding along a 3-km-long section of Frog Creek (see Fig. 2). Highly conductive ground is associated with seawater saturation. This graph indicates that saline pond water mixes with surrounding fresher groundwater, and that fresher water near the surface is underlain by more saline water. |
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Despite having been connected to tidal flow, these ditched ponds do not appear to be primary nursery areas for estuarine transient fishes like ladyfish or spot that breed offshore in the Gulf of Mexico. Such juvenile fishes are found, however, in Frog Creek and its naturally connected tidal ponds and embayments (Fig. 2). Planned restoration activities for portions of the Aquatic and Buffer Preserve include selective filling of mosquito ditches to convert some brackish ponds to freshwater, thereby promoting a greater diversity of foraging opportunities for birds and bats.
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