Critical Levels of Nutrients
Only in recent years have we have learned just how low nutrients must be to maintain healthy coral reefs. The limits were found independently by two researchers working on opposite sides of the globe, who were not aware of each other's work. By looking at the relative amounts of corals and algae along nutrient gradients from intense land-based sources, namely agricultural fertilizers in Australia and bird droppings on a mangrove island in Belize, Peter Bell and Brian Lapointe independently determined exactly the same limit for acceptable nutrient concentrations. Biologically available nitrogen (nitrate plus ammonia) needs to be below 1.0 micromole per liter (less than 0.014 parts per million of nitrogen), and biologically available phosphorus (orthophosphate plus dissolved organic phosphorus) needs to be below 0.1 micromole per liter (less than 0.003 parts per million of phosphorus). In addition concentrations of chlorophyll (in the microscopic plants called phytoplankton) needs to be below 0.5 parts per billion.
These values are all regarded as extremely low levels, almost undetectable, in coastal waters of temperate and cold zones. For years researchers measured concentrations in this range but thought that values were too low to possibly cause problems to reefs. This was wrong because they used irrelevant standards for acceptable nutrient levels. It is essential that appropriate water quality standards be applied in coral reef ecosystems if they are to be protected against eutrophication. These standards must be below the levels given above. In general, where water quality standards have been applied for tropical waters, they are often based on uncritical adoption of nutrient standards from North America and Europe that are irrelevant to the tropics because cold ecosystems are normally exposed to much higher nutrient levels. Many nutrient water quality standards available are related to human health and are even more worthless for coral reefs because humans can drink water with nutrient levels hundreds of times higher than coral reefs can stand.
