Near-bottom nutrient fluxes on the Florida Keys Reef tract.

Principal Investigator: James Leichter (Woods Hole Ocean Institute)

Background

Coral reefs need nutrients to grow and thrive, just as all living things need nutrition to grow and thrive. The major question addressed by the current Aquarius mission scientists is: Where do the nutrients come from that support the growth of corals? Results from this work address an important management issue in south Florida related to water quality and the potential problems of nutrient pollution. It is generally understood that coral reefs thrive in tropical waters that are typically low in nutrients. But the picture that is emerging for reefs in Florida is that the story is much more complex.

The Gulf Stream meanders and sweeps just offshore of the Florida reef tract, and it is true that Gulf Stream waters are typically low in nutrients. However, beneath the surface of the Gulf Stream, sometimes as shallow as 100 feet deep, lies a region where the warm low nutrient water transitions to a colder nutrient-rich realm. Nutrients in these deeper waters concentrate as a result of natural processes and don't have anything to do with pollution. Closer to shore in the Florida Keys, right up against the shoreline within the first few hundred meters or so, we can detect the polluting effects of sewage that is mostly discharged into septic systems, cesspits, and shallow water injection wells throughout the region. These shallow inshore waters have increased concentrations of nutrients, no question. But sewage pollution has not been definitively tied to reef degradation that's occurred on reefs in Florida over the last 20 years. Still, there is tremendous interest and concern about the potential polluting effects of sewage on reefs in Florida, and rightly so. But few people are aware of the vast nutrient pool that lies just offshore of the reefs that is part of the deeper Gulf Stream waters. It is this nutrient rich water of the Gulf Stream and mechanisms that bring it to the reef that is the focus of the current Aquarius mission.

Detailed Project Summary

Evaluating how nutrients are delivered to the reef system is an essential element required to understand the function of coral reef ecosystems, and what efforts might be required to help sustain healthy coral reefs. Dissolved nutrients (nitrogen and phosphorus) that reach coral reefs in the Florida Keys may arise from a variety of sources, including: fixation and regeneration within reef environments; movement (flux) out of the reef matrix itself; terrestrial run-off and human-derived inputs from the Florida Keys (especially sewage), outflow from the Florida Bay, and upwelling and transport of offshore waters onto reef slopes (see internal wave simulation as an example of one type of upwelling). But the story is not just about nutrient concentrations. The availability of nutrients to coral reef organisms is governed both by nutrient concentrations and water flow velocities. Thus, estimates of nutrient inputs to the reef depends on knowing both concentrations and flow speeds close to the bottom, and how these vary with time.

The overall flux of nutrients on Florida Keys reefs is not well understood. Recent research has identified the transport of cool, nutrient-rich water onto Florida Keys reefs by internal bores (or waves - see the simulation) as a potentially important source of nutrients. Internal bores are generated by internal tides and breaking internal waves, and their arrival on reef slopes is marked by rapid fluctuations in near-bottom water temperature and density coupled with the onset of strong upslope flows. That is, the flow of water changes direction, current speeds increase, and water temperature drops. The impact of this phenomenon is widespread throughout reefs in the Florida Keys, and new observations show that very large internal waves can be detected near the bottom at depths of about 150 feet about 1 mile seaward of the Keys reef tract. Previous and on-going measurements have been made at scales of one to several meters above the bottom to document the specific affects these larger scale oceanographic features might have on the reef itself. These fine scale measurements closer to the bottom are needed to estimate the exact movements of cool water and dissolved nutrients to coral reef habitats and organisms. A goal of this study is to measure flow velocities and nutrient concentrations close to the bottom on a range of habitats at Conch Reef.