Topical Area: Water Circulation and Currents

Patrick A. Pitts and Ned P. Smith, Harbor Branch Oceanographic Institution, Ft. Pierce, Florida

Many of the environmental issues involving Florida Bay and the Florida Keys stem from concerns that Florida Bay water may have a negative impact on water quality in the Florida Keys National Marine Sanctuary, especially the reef tract system. To address some of these issues, an understanding of how water moves between the bay and the reef tract is required. Hawk Channel provides an important connecting link between the bay and the reef tract. An understanding of transport processes that move water along and across Hawk Channel is fundamental to understanding the export of dissolved and suspended material from Florida Bay to the reef tract.

Hawk Channel is an elongated, shallow basin that serves as the continental shelf on the Atlantic Ocean side of the Florida Keys. It is approximately 10 km wide, and it extends 230 km from Fowey Rocks off Key Biscayne to Sand Key off Key West. Water depths along the center of the channel are 7-8 m off the Upper Keys, and generally between 12-15 m off the Middle and Lower Keys. By comparison, Florida Bay is significantly shallower so its waters respond more readily to evaporations and rainfall resulting in a distinct hydrographic identity.

In the study presented here, hydrographic data are used to describe salinity gradients across Hawk Channel near Long Key. We test the hypothesis that Florida Bay water retains its hydrographic characteristics in near-bottom layers as it is transported across Hawk Channel, and that it can be detected through its hydrographic properties at the reef tract. Results will show the extent to which Florida Bay water is diluted by mixing as it moves seaward across the shelf. The analysis is based on historical data collected in 1991-92 and new data collected in March 1997.

Across-shelf hydrographic transects were made during adjacent ebb/flood tidal cycles from the Long Key Channel bridge due south to the reef tract. Hydrographic profiles were taken from surface to bottom every 0.5 nautical mile across Hawk Channel. Data were taken at slack water following the ebb to show how far into Hawk Channel Florida Bay water penetrated. Another cross-section was taken at slack water after the preceding or ensuing flood to determine the extent to which Florida Bay water had retreated into the bay through Long Key Channel. Spacing the transects throughout the year provided information that reflected a variety of wind and rainfall conditions.

To document the existence of significant salinity differences between bay and Atlantic shelf water, and to justify the use of salinity as a natural tracer for tide-induced exchanges, a time series of salinity recorded in Long Key Channel near the bridge during an approximately 11-month time period from late August 1994 to late July 1995 is provided. The site is well positioned to document salinity differences between Florida Bay and Hawk Channel. The data show time periods with large fluctuations in salinity, as much as 6‰ within a tidal cycle. Largest salinity fluctuations occur in midwinter months. Current meter data recorded at the study site during this time indicate that low salinity water from Florida Bay was moving through Long Key Channel during the ebb. For relatively short time periods throughout the study, either very weak tidal exchanges or very small salinity differences result in negligible fluctuations at the study site in Long Key Channel.

Results of the hydrographic transects indicate that when sufficient salinity differences exist between Florida Bay and shelf waters of the Atlantic, bay water can be readily tracked across Hawk Channel. Under those conditions the data show that Florida Bay water often penetrates 2-4 km into Hawk Channel during the ebb half of the tidal cycle. Profiles during these times indicate that the water column is well mixed on the landward side of the channel and isohalines are nearly vertical. Across the middle of Hawk Channel however, there is usually an indication of vertical stratification. Here, Florida Bay water continues seaward in the lower half of the water column when it is higher in salinity than shelf water and higher in the water column when it is lower in salinity than shelf water. During several sampling periods, hydrographic profiles across Hawk Channel indicate minimal salinity differences between Florida Bay and shelf waters. On these occasions using salinity as a natural tracer to track the seaward movement of bay water toward the reef tract is ineffective.

Unfortunately, details of the movement of Florida Bay plume water cannot be explored in this study, if only because we can never be certain whether the transect remained in the plume or left it at some point. Also, studies of tidal currents in Long Key Channel have shown that diurnal inequalities in ebbs and floods can be significant and the technique used here can be influenced greatly by this phenomenon—although the interpretation can take this into account. But while the use of salinity as a natural tracer has its limitations, data from this study indicate that under the right conditions this can be a convenient and low-cost approach to tracking Florida Bay water as it enters Hawk Channel.

In conclusion, the results from this study show that Florida Bay water often has distinct hydrographic properties as it emerges from Long Key Channel. But unless the salinity of Florida Bay water differs appreciably from that of Hawk Channel, the seaward movement of bay water, even at the mouth of Long Key Channel, cannot be tracked with salinity cross-sections. Results indicate that Florida Bay plume water moves 2-4 km across the shelf on a given ebb and it will favor near-surface or near-bottom layers depending on density differences between bay and shelf waters. Using salinity as a natural tracer, we found no evidence of Florida Bay water at the reef tract. This is not to suggest that Florida Bay water is not reaching the shelf break, however. It almost certainly arrives at the seaward side of Hawk Channel in a diluted state, but unless the plume can be tracked as it moves across Hawk Channel, it will not be possible to determine at what rate and to what degree dilution occurs.