Supporting the theory that nutrient levels control the latitudinal boundaries of coral reefs, the team has found a clear increase from south to north in the concentration of phosphorus in forms that can be used by the seaweed and a corresponding expansion of fleshy seaweeds. They also found a complementary decrease in the number of species and extent of coral and reef fishes from south to north. These data were corroborated by analysis of tissue for the dominant seaweed species at each location, which, again, revealed less phosphorus at southern sites and more to the north.
These gradients were much more pronounced during the wet season compared to the dry season, suggesting a significant role for non-point source and other forms of nutrient-rich pollution in controlling nutrient dynamics at the reefs. Lapointe's group has also completed extensive analyses of the chemical signature of nitrogen stable isotopes in seaweed samples and determined that the algae are using mainly nitrogen from land-based sources, rather than from marine sources, further suggesting a tie to human activities.
It is difficult to determine what historical nutrient levels may have been for most reefs due to a lack of monitoring. However, Lapointe has been measuring nutrients at reefs in the Florida Keys since the 1980s and has documented steady increases throughout that period. This is the longest running nutrient time series for any reef in the world, he says.
"Natural and human factors play a role in dictating the dynamics of the waters where we see these harmful algal blooms," says Lapointe," but even 20 years ago the algae was not all over the reefs like it is now, so something has clearly changed over time."
Based on decades of previous research at Florida and other reefs, Lapointe believes that coral-smothering seaweed growth is dependent on a threshold level of phosphorus that in the past would likely only have been reached above the histor
Contact: Mark Schrope
Harbor Branch Oceanographic Institution