When are nutrient levels too high?

Posted on July 20, 2015

Although dissolved nutrients drive freshwater productivity, lakes with excessively high nutrient levels tend to have dense concentrations of algae and microbes, low oxygen availability, and low biodiversity. So at what point do nutrient levels become so high that management intervention is required? European researchers looked at the relationships between algal chlorophyll (a proxy for nutrient concentration) and different biodiversity indicators in 99 small lakes in the Dombes region of France over a two year period. The lakes had a mean depth of 0.7 m and a mean surface area of 0.13 km2. As expected, chlorophyll a levels were positively related to nutrient concentrations, and ranged from near zero to over 1000 µg/l, with a mean of 87 µg/l. Biodiversity measures declined at a decreasing rate as chlorophyll increased. Critical chlorophyll levels (above which there was a significant drop in the number of species) were 37-64 µg/l for aquatic plants and 46-55 µg/l for invertebrates. These findings showed that, when measured weekly in spring, chlorophyll a is a good, cheap indicator of biodiversity and of likely biodiversity trends later in the year. Coupled with measurements of water transparency, which give information on the effects of weather events that increase sediment resuspension and turbidity, chlorophyll monitoring provides a useful early warning system for the management of dissolved nutrients.

Reference: Robin, J. et al. 2014. Biodiversity in eutrophicated shallow lakes: determination of tipping points and tools for monitoring. Hydrobiologia 723, 63–75. http://link.springer.com/article/10.1007%2Fs10750-013-1678-3#page-1