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We examined the rate of photoammonification in 16 lakes from Saskatchewan, Canada. Lakes were selected to encompass a broad range in dissolved organic nitrogen (DON) (269-1,435 lg l -1 ). Lake filtrate (\0.2 lm) was exposed to artificial solar radiation for 4 h. Rates of photoammonification were significant in 7 of the 16 study lakes. Ammonia (NH 3 ) concentrations increased 0.84-2.85 lg l -1 over control values. This is a 4-92% increase in NH 3 concentration and a conversion of 0.18-0.3% of the DON pool to NH 3 . We developed an empirical model to predict photoammonification rates across aquatic ecosystems. Photoammonification rates and ancillary parameters (i.e., pH, DOC and DON concentrations, DOC:DON ratios, and a350:DOC) were obtained from published studies to expand our dataset for model development.Model selection was conducted with Akaike's Information Criterion (AIC). DON concentration and pH were selected as model predictors by AIC. Our model explains 49% of the variance in photoammonification rate across a diverse set of aquatic systems. This model may be useful in estimating photoammonification rates in other aquatic systems.

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