Abstract:
Local point source pollution and/or nutrient enrichment has been a recognised source of stress to coral reef habitats for a number of years. It has been documented to lead to increased algal growth and an overall reduction in habitat resilience. These local sources, although potentially contributing to regional eutrophication phenomenon, generally appear to only have effects on a local level. Of greater concern are larger regionally influential sources of nutrients from heavily populated areas of South America and the Gulf states. Recently there has been growing evidence of previously understudied nutrient sources emanating with seasonal river plumes from the Amazon, and to a lesser extent the Orinoco. These plumes, laden with nutrient rich water accentuated by deforestation and agricultural run-off, have recently been suggested as the cause of large algal blooms that spread over vast areas in the Caribbean in 2009 and 2010. Other studies have also documented these plumes as the source of unprecedented amounts of Sargassum that washed ashore fouling local beaches in 2011 and again in 2014/2015. Although the long-term ecological effects of these events are unknown, and the short term picture is that excessive nutrients are metabolised and ultimately sink to the sea floor, it does highlight for the first time this significant contributor to regional eutrophication that up until now was largely attributed to local sources. Furthermore, water current patterns appear to be at least partially recirculating nutrient rich water back into the Caribbean, or at times inhibiting its departure from the region. It is theorised that this may be causing an overall nutrient build up, explaining why the river plumes now produce an observable bloom, or ‘green water event’, that would previously have mixed with ocean waters and dissipated without causing such an event. The secondary effects of these nutrients are largely detrimental in nature and create tertiary stresses, some of which may interact synergistically with other multiple stressors, and lead to increased habitat degradation and contribute to the ultimate loss of a once diverse ecosystem. Although management of regional nutrient sources will involve multi-national intervention, mitigation measures on a local level can be introduced by managers, even in small island nations with limited financial and logistical resources.
