It's not just temperature damaging coral reefs - Tue 16 Jul 01:03 GMT

US study points the finger at nitrogen – and people. Nick Carne reports.

  Nitrogen loading is as much to blame as increasing temperatures for coral bleaching in the Looe Key Reef in the lower Florida Keys, US, a 30-year study suggests.

  Writing in the journal Marine Biology, a team led by Florida Atlantic University's Harbor Branch Oceanographic Institute says improperly treated sewage, fertilisers and top soil are elevating nitrogen levels, which are causing phosphorus starvation in the corals, reducing their temperature threshold for bleaching.

  The researchers say their study represents the longest record of reactive nutrients and algae concentrations for coral reefs anywhere in the world.

  They also monitored living coral, collected abundant species of seaweed for tissue nutrient analysis, and monitored seawater salinity, temperature and nutrient gradients between the Everglades and Looe Key.

  "Our results provide compelling evidence that nitrogen loading from the Florida Keys and greater Everglades ecosystem caused by humans, rather than warming temperatures, is the primary driver of coral reef degradation at Looe Key Sanctuary Preservation Area during our long-term study," says senior author Brian Lapointe.

  A key finding is that land-based nutrient runoff has increased the nitrogen-to-phosphorus ratio (N:P) in reef algae, which indicates an increasing degree of phosphorus limitation known to cause metabolic stress and eventually starvation in corals, the researchers say.

  Concentrations of reactive nitrogen are above critical ecosystem threshold levels previously established for the Florida Keys, as are phytoplankton levels for offshore reefs as evidenced by the presence of macroalgae and other harmful algal blooms due to excessive levels of nutrients.