Tampa Bay Integrated Science Pilot Study:

Hydrographic and sub-surface mapping and sediment transport modeling

Results/Discussion

Figure 2. Classifying acoustic signal based upon return echo from depth sounder. [view enlargement]

Both of these efforts are currently under development. The benthic habitat mapping effort will provide a baseline for assessing future changes in benthic communities (Fig. 2).

This effort will also directly address the location of the “deep” edges of sea grass beds. Numerical modeling of wave and circulation processes will help to address the physical mechanisms for observed changes of the benthic community. It is anticipated that these models will have predictive capabilities.

Bathymetric mapping:

The initial bathymetric mapping effort focused on the Palmetto quadrangle, which encompasses the Terra Ceia Aquatic Buffer Preserve (Fig. 3). A digital comparison of newly collected to the most recently collected (1950’s) bathymetry data has not yet been made. However, a qualitative comparison to published nautical charts can be assessed.

Figure 3. Bathymetric contours draped over digit ortho photograph of the Terra Ceia Aquatic Buffer Preserve area. [view enlargement]

In general, water depths of the small bays and in the nearshore zone of Tampa Bay remain unchanged. The general shoreline shape and adjacent bathymetry of the area is generally cuspate in shape, reflecting the karst type topography of the region. The linear shape and bathymetry of Terra Ceia Bay and its orientation to the Manatee River suggests that this may have been at one time a tributary into the Manatee River; although, there is no present day evidence of a creek or river feeding into Terra Ceia Bay.

Several uncharted linear channels in Miguel Bay, Joe Bay, Bishop Harbor are less than 0.5 m deep and appear to be either antecedent features or modern channels maintained by tidal currents. Linear bay bars are well defined north of the I-275 causeway and are less well pronounced south of the causeway. They are located approximately 1 km from the shoreline with megaripple surficial expressions. The general water depth from the bars to the shoreline is approximately 0.5 m, with some areas exposed at low tide. Other studies suggest that bay bars protect sea grass beds from extreme wave conditions, and find a correlation between bay bar presence/morphology and sea grass health.

Seismic surveying:

Contamination from ground water sources is a major concern in Tampa Bay. Some estimates are that 20% of the fresh water input into Tampa Bay is from ground water sources; however, it is unknown where ground water seeps into the Bay. The hydrostatigraphy (rock layers and aquifers) of the Tampa Bay region consists of three variable and complex aquifer systems, two of which may be sources of ground water into Tampa Bay due to their proximity to the sea floor.

The goal of the seismic surveying effort is to identify the location and depth of shallow aquifers beneath Tampa Bay. Seismic survey techniques use low frequency sound to identify sand and rock layers beneath the sea floor. For this pilot project, one seismic line was collected parallel to the I-275 Sunshine Skyway Bridge from Boca Ciega Bay to Joe Bay. One seismic line limits interpretation, yet this line reveals much information.

Evidence of a significant sea level low stand and subsequent transgression may be revealed by a sharp contact approximately 20m below sea level (Fig. 4a). At this contact, there is a topographic high feature (identified by strong reflections) that may have been land during the low stand. Channel filling is evident at this contact which may be related to the antecedent bay and tidal processes. Folding of deeper strata is evident throughout the survey line, suggesting dissolution of older sediments has occurred deep beneath the bay (Fig. 4b). The survey line reveals two sinkhole like features that penetrate the contact (Fig. 4c). There is evidence of sediment filling in these depressions.