Nitrate addition clearly enhanced phytoplankton production and biomass in all experimental lakes. However, Anne Deininger found that this stimulating nitrogen effect decreased with increased lake organic carbon content caused by the attributed light reduction.
In clearer lakes with low organic carbon content, phytoplankton became so enriched in nitrogen that they became poor quality food items for zooplankton. Decreased food quality resulted in reduced growth of their consumers and less energy being transferred up the food chain. In browner and more organic carbon rich lakes, food quality was not reduced to the same extent as in clear water lakes and the food web was clearly more resilient to nitrogen addition.
Lakes respond differently to nitrogen disposition — ScienceDaily