Scientists Identify Important Source of Nutrients to Florida's Coral Reefs

Coral reef scientists were surprised to learn that the deep ocean is the source of 20-40 times more nitrogen and phosphorus on the outer coral reef than nutrient pollution from sewage and storm water runoff.

A new study published this month documents a large and not previously quantified source of nutrients to the coral reefs of the Florida Keys " the deep ocean. Lead author Dr. James Leichter, Scripps Institution of Oceanography, said, "We studied upwelling of cool, subsurface water at multiple sites along the Florida Keys reef tract. Our results show that this natural source of nutrients can deliver as much as 20-40 times more nitrogen and phosphorus to the outer reef tract than estimates of nutrient pollution from sewage and storm water runoff."

While these numbers are remarkable it's important to provide context. Co-author Dr. Steven Miller, University of North Carolina at Wilmington, added, "I'm sure some people will try to use these numbers to claim that changing sewage disposal practices in the Keys is unnecessary, but they would be wrong. What our results show is that a major nutrient pump exists offshore. However, nothing in our study contradicts the fact that we also have a nearshore pollution problem."

The study was published this month in the journal Limnology and Oceanography and was a collaboration among scientists from Scripps Institution of Oceanography, the University of California at Berkeley, and the University of North Carolina at Wilmington (UNCW). Much of the work conducted during this study utilized saturation diving and the NOAA Aquarius underwater laboratory in Key Largo.

"What I like about this study is that it provides a balanced approach to a complicated issue. They acknowledge that while upwelling is a significant source of nutrients to the offshore reefs, they don't dismiss the need to better understand the dynamics of nearshore nutrient pollution making its way offshore," said Dr. Brian Keller, science coordinator for the Florida Keys National Marine Sanctuary.

The large amounts of nitrogen and phosphorus are brought to the reef by an oceanographic process called upwelling, in this case a specific, high frequency form of upwelling caused by internal tidal bores. Leichter explained, "It's a little like waves sloshing back and forth in a bathtub, but in the ocean at much grander scales and in ways that sometimes cause surges of deep water to move into much shallower areas. When this happens in the Keys, nutrients brought to the reef can increase 10 to 100 times over background levels."

The upwelled water is rich in nutrients due to natural processes. When plankton and other organisms produced or living in surface waters die, they often sink to the bottom. Leichter further explained, "As these materials sink, they start to break down, they get eaten and excreted, and the end result is the slow and continuous addition of nitrogen and phosphorus to waters with increasing depth."

Interestingly, the upwelled water makes it to the reef a lot more often than people realize, sometimes several times a day. "The presence of this water on the reef for extended periods of time has the potential to significantly affect the biology of corals, sponges, and algae," added Leichter.

"This study illustrates the importance of assessing water quality over different time scales," said Dr. Joe Boyer of Florida International University. Boyer manages the largest and longest running water quality monitoring program in south Florida. "Our sanctuary-wide, quarterly monitoring efforts occasionally pick up these events, but are not designed to quantify this type of detail," he added.

Another important result of the study documents that a special form of nitrogen in the upwelled water is also present in samples of algae collected from the reef. This part of the work, led by Hannah Stewart, University of California - Berkeley, suggests that the algae are directly using nitrogen from the upwelled water. "This is particularly interesting because some scientists believe that this special form of nitrogen is a sewage signal " a smoking gun " for pollution, while our work clearly suggests otherwise for the offshore reefs," added Miller.

These results are the culmination of work first started in the early 1990s, initially with single deployments of oceanographic equipment that detected the surprising frequency and intensity of the upwelling events, and later with equipment deployments throughout the Keys to document the regional significance of these events. "While our study focused on a large natural source of nutrients for the Florida Keys reef tract, things like coral disease, global warming, and overfishing are also important. The impacts of humans in this system are clearly dramatic and complex. That's why it's important to accurately assess variability in this system that is a normal part of the system," added Leichter.

The paper by James Leichter, Hannah Stewart, and Steven Miller, is titled "Episodic nutrient transport to Florida coral reefs" (Limnology and Oceanography Vol 48:1394-1407) and is available free at http://aslo.org/lo/toc/vol_48/issue_4/1394.pdf.

This research was funded by NOAA's National Undersea Research Center at the University of North Carolina at Wilmington (UNCW). The Aquarius underwater laboratory is owned by NOAA and is managed by UNCW (http://www.uncw.edu/aquarius).