Effects of inorganic nutrients in recycled water on freshwater phytoplankton biomass and composition

Planned indirect potable reuse water treated with advanced wastewater technologies (AWWT) to remove pollutants is increasingly being used to augment drinking water and groundwater supplies. While the treatment process substantially reduces the high nitrogen (N) and phosphorus (P) concentrations typically of wastewater, both nutrients can remain at concentrations and in biologically available forms that may stimulate phytoplankton growth in nutrient-deficient systems. This study examined the short-term effect of N plus P additions, at a range of concentrations, on phytoplankton growth and species composition in mesocosm experiments in a subtropical reservoir. Nitrate (NO3) plus orthophosphate (PO4) concentrations as low as 50 and 5 μg L−1, respectively, resulted in significant increases in phytoplankton biomass, with a 3.99 μg L−1 increase in chlorophyll a concentration with every 10 μg L−1 increase in N plus 1 μg L−1 in P. The system was likely to be co-limited because the addition of N or P alone did not result in increased chlorophyll a concentrations compared with the control. However, the toxic cyanobacterium, Cylindrospermopsis raciborskii, had higher growth rates with P addition alone. This study has shown that inputs of AWWT water have the potential to increase the phytoplankton biomass in this subtropical reservoir, at least in the short term. Therefore, the effect of AWWT water on water quality in reservoirs should be further investigated before widespread application occurs.

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► We investigated the short-term effects of N and P addition on phytoplankton.
► Phytoplankton biomass and species composition were looked at in mesocosms.
► Low amounts of NO3 (50 μg L−1) plus PO4 (5 μg L−1) increased phytoplankton biomass.
►  Chl a increased by 4 μg L−1 for every 1 μg L−1 in P and 10 μg L−1 increase in N.
► AWWT water inputs can increase the phytoplankton biomass in the short term.