Input of nutrients to reefs in the Florida Keys by upwelling. Dr. James Leichter (Woods Hole Oceanographic Institution).

Evaluating the transport of nutrients to coral reefs is an essential element in understanding how coral reef ecosystems function. Nutrients reach coral reefs in the Florida Keys from a variety of sources, including fixation and regeneration within and on the reef itself, flux out of reef matrices, terrestrial run-off and anthropogenic inputs (sewage, run-off), outflow from Florida Bay, and upwelling and transport of offshore waters onto reef slopes. However, the availability of nutrients to organisms living on the bottom (such as corals and seaweeds) depends on knowing both concentrations and flow speeds close to the bottom, and how they vary with time. The overall input and export of nutrients on Florida Keys reefs is not well understood. Recent research identified that the transport of cool, nutrient-rich water onto the reefs by internal bores is a potentially important source of new nutrients to the reef. These internal bores are generated by internal tides and breaking internal waves, and their arrival on reef slopes is marked by rapid changes in water temperature, density, and strong upslope current flows. The impact of internal tidal forcing is widespread throughout reefs in the Florida Keys, and recent observations show that large internal bores can be detected near the bottom at 50 m depth at least 1.5 km seaward of the Keys reef tract.

While previous and ongoing studies of internal tidal forcing have been made at scale of one to several meters above the bottom, fine scale measurements very close to the bottom are needed. Without these measurements closer to the bottom it is not possible to estimate the actual delivery rates and concentrations that determine availability of nutrients to benthic plants and animals. This project focuses on how flow velocities and nutrient concentrations close to the bottom are affected by the high frequency variability introduced by internal tidal forcing. Small scale measurements of nutrient change, correlated with small scale measurements of flow, will be made under internal bore and non-bore conditions. These data will be used in nutrient modeling efforts to determine overall nutrient availability for reefs in the Florida Keys.

The project has three objectives. First, boundary flow conditions will be measured at a variety of reef sites in association with the arrival of internal tidal bores and non-bore periods using acoustic Doppler velocimeter (i.e. a current meter). Second, nutrient concentrations (nitrate, nitrite, ammonium, and soluble reactive phosphate) will be measured 1, 10, and 100 cm above the bottom in association with the fine scale flow measurements. Third, information on measurements made close to the bottom will be integrated within the context of tidal forcing, nutrient transport, and availability on Florida Keys reefs. Results will provide a unique, high frequency view of nutrient dynamics on a Florida reef.