A common water quality assumption is that water clarity is highly correlated with nutrient concentration due to the enhanced algal growth in eutrophic waters. However, this assumption does not apply to super-polluted water that is inhospitable even for algae. At nitrogen concentrations above 3 milligrams per liter, chlorophyll-a concentrations begin to drop off precipitously. Beyond this threshold, photolysis of nitrate may produce reactive oxygen species that damage DOM and phytoplankton. Reduction in N loading at high P could therefore increase Chl-a and decrease water clarity, resulting in an apparent worsening of water quality. This study suggests that monitoring Chl-a or Secchi depth may fail to indicate water quality degradation by extreme nutrient concentrations. These findings highlight how extreme nutrient regimes in lakes can produce novel relationships between phytoplankton and nutrients.

http://www.tandfonline.com/doi/full/10.1080/20442041.2017.1375176

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