Global mapping of freshwater nutrient enrichment and periphyton growth potential

There are severe eutrophication issues in many freshwater systems, but there has been limited analysis, at a global level, of controls and drivers of this. Excessive growth of periphyton (biofilms) is a good indicator of nutrient enrichment. The concentration and ratio of nitrogen (N) to phosphorus (P) can control the growth of periphyton. We predicted the likelihood of periphyton growth globally so that the level of risk could be understood, and management planned within and across jurisdictions. We combined several databases to derive global dissolved and total N and P concentrations (n = 1.4 million), climatic, and catchment characteristics for up to 1957 larger rivers between 2005 and 2012 (stream order 6 and above). After identifying the limiting nutrient, we used nutrient thresholds for total N and P to indicate when and where periphyton was predicted to be accumulating at unacceptable levels. Our models predicted 41 and 60% of the variation in TN and TP but performed well against a validation dataset of 41 large catchments from Australasia. We predicted that 31% of the globe contained catchments likely to exhibit unacceptable levels of periphyton growth. Of this, 23% was caused by P-enrichment impacting on 1.7B people, while 8% was due to N-enrichment impacting 280M people. Much of this P-enrichment occurred in heavily populated catchments dominated by agricultural land in North and South America and Europe. Much of the N-enrichment likely to cause unacceptable periphyton growth occurred in impoverished areas of North Africa, and parts of the Middle East and India. These findings can be used by land owners and policy makers to better target investment and actions at finer scales to remediate poor water quality due to periphyton growth.