Conceptual Model of Aquatic Plant Decay and Ammonia Toxicity for Shallow Lakes

A conceptual process model was developed to examine the potential for late summer ammonia toxicity in shallow lakes. Processes represented in the model were macrophyte decay; growth, death, and sedimentation of phytoplankton; growth and death of zooplankton; nitrification; volatilization; and chemical equilibria (carbonate and ammonium systems). Peak NH3 concentrations occur at the peak of the phytoplankton bloom that develops about 2 weeks after macrophyte decay starts, when the pH is elevated. Ammonia peaks are highly transient, lasting <1 day. It is hypothesized that late summer ammonia toxicity following macrophyte senescence may be a common but generally unrecognized phenomenon in shallow lakes.