Tampa Bay Integrated Science Pilot Study:

A 2001-2002 drought induced imprint on estuarine processes?

Figure 2. Historic Alafia River daily mean discharge at Lithia, Florida. [view enlargement]

The Tampa Bay watershed experienced severe drought conditions in 2000-2001. Long-term flow records (1933-1992) of the Alafia River at Lithia indicate a significant decreasing trend (Fig. 2) in annual mean flows (Stoker et al., 1996) and annual peak flows (Hammett, in press). Annual mean flows in the Alafia River, Hillsborough River, and Sweetwater Creek for the 2000 water year were the lowest on record.

Such reduced riverine inflow resulted in observed elevated salinities throughout large portions of the bay. These trends can be attributed in part to climatic variability, as well as other factors such as drier antecedent conditions, decreased ground water inflow and sea level fluctuations.

Interstitial waters (i.e., pore waters) can provide a powerful forensic tool to interpret post-depositional change in sediments (McKee et al., 1996). Pore water profiles from four sites in Tampa Bay indicate upward diffusion from sediments into the water column for many nutrients such as phosphorus and ammonia.

In contrast, selected metal profiles show a flux into the seabed (Fig. 3). Such divergent pore water profiles influence the availability of these constituents for biologically mediated uptake. A suite of geochemical tracers suggests that exchange across the sediment/water interface at the four sampling sites is limited to recycled seawater facilitated by tidal forcing, rather than submarine groundwater discharge.

Results from a geophysical survey indicate an abrupt change in stratigraphic units, with the deeper layer having either lower porosity or less saline pore waters. The current drought regime suggests that the former explanation is more plausible, and the geophysical resistivity anomalies typically corroborate observed geochemical results.

Estuaries are biogeochemical reactor where a complex suite of reactions and processes transforms riverine constituents (i.e., contaminants) as they are transported out to sea (Swarzenski et al., 1995).

An important effect of these processes is the removal of many particle reactive constituents (Fig. 4) from the dissolved phase onto particles (scavenging), where their fate is bound closely to that of the particle. This implies that when a particle settles onto the seabed and becomes buried, the attached chemical is no longer bioavailable in the water column.

Not all scavenging reactions are irreversible, so that additional processes such as early diagenesis or bioturbation can re-introduce some contaminants to the water column via pore water advection/diffusion. Estuarine processes did not occur in the bay water column during the sampling period, but instead occurred in the Alafia River. How these findings compare to non-drought conditions needs to be evaluated with regard to ecosystem health and sustainability.