Algal Bloom Index Calculator

This index is a heuristic risk score, not a measured concentration. It gives nutrients the largest weight, up to 50 points, because nitrogen and phosphorus are the growth-limiting resources in almost all standing waters; temperature adds up to 20 points scaled linearly between 15 and 30 degrees C, where most bloom-forming cyanobacteria reach maximum growth; light adds up to 15 points; and wind subtracts up to 15 points because mixing breaks down the stable surface layer that buoyant Microcystis and Dolichospermum colonies need to form scums. The reported N:P value is a mass ratio, so the balanced Redfield proportion of 16:1 by moles corresponds to roughly 7:1 by mass. Below that, growth is nitrogen-limited; above it, phosphorus-limited, which is the usual condition in fresh water. For orientation, total phosphorus below 0.01 mg/L is typical of oligotrophic lakes, 0.01 to 0.03 mg/L mesotrophic, and above 0.03 mg/L eutrophic, while the WHO provisional guideline for microcystin-LR in drinking water is 1 micro-g/L. The limitations are substantial. The scoring weights are illustrative rather than calibrated against observed bloom records, so the index ranks scenarios sensibly but its absolute value has no field-validated meaning. The chlorophyll-a output is a fixed linear rescaling of the index, not a prediction from a measured chlorophyll relationship. The model also ignores water residence time, stratification depth, internal phosphorus loading from anoxic sediments, grazing pressure and the legacy nutrients stored in catchment soils, any of which can dominate real outcomes. Wind is treated as purely protective, although a mixing event followed by calm can resuspend sediment nutrients and trigger a bloom rather than prevent one.

The scientific basis was assembled over four decades. Alfred Redfield, working at Woods Hole, reported in 1934 and elaborated in 1958 the remarkably constant carbon, nitrogen and phosphorus proportions of marine plankton, giving limnologists a yardstick for judging which nutrient constrains growth. Richard Vollenweider developed empirical phosphorus loading models for the OECD from the late 1960s, relating external phosphorus input to lake trophic state and providing the first quantitative basis for setting loading limits. The decisive evidence came from David Schindler and colleagues at Canada's Experimental Lakes Area, whose whole-lake experiment on Lake 227 and the divided Lake 226, reported in Science in 1974, produced a bloom in the phosphorus-fertilised basin alone and settled a long argument over whether carbon or phosphorus controlled eutrophication. Those findings drove policy. Phosphate limits in detergents spread through North America in the 1970s, and phosphorus removal at wastewater plants under the 1972 US Clean Water Act helped Lake Erie recover visibly by the 1980s. Robert Carlson published his Trophic State Index in 1977, converting Secchi depth, chlorophyll-a and total phosphorus onto a common scale still used by state agencies. Attention shifted to toxins and diffuse agricultural runoff in the 1990s: the United States passed the Harmful Algal Bloom and Hypoxia Research and Control Act in 1998, and after decades of improvement Lake Erie blooms returned, culminating in August 2014 when microcystin in treated water left Toledo, Ohio, with roughly half a million residents unable to drink from the tap.