Improved empirical models for predicting nitrogen retention in lakes and reservoirs

Anthropogenic activities have significantly increased the movement of nitrogen (N) from land to freshwaters and to coastal waters and have led to severe environmental consequences. The flow of N is moderated by retention processes in terrestrial, freshwater and marine ecosystems. Freshwater ecosystems have the highest areal N retention rates. The proportion of N retained in aquatic ecosystems depends on the areal hydraulic load and is described by relatively simple semi-empirical or strictly empirical models. Here I compared the predictive power of several models, that predict the annual mean proportion of total N (TN) and dissolved inorganic N (DIN) retained in lakes and reservoirs and developed an improved version of the models currently in use by inclusion of additional relevant parameters. The study shows that models derived from mass balances describing the proportion of annual mean retention of TN and DIN as a sigmoid function of the areal hydraulic load can be approximated by a linear function on the logarithm of the areal hydraulic load. Stepwise multiple linear regression analyses identified the logarithm of the areal hydraulic load as the main explanatory variable for the proportion of retained TN, followed by the ratio between the DIN and the TN load and the ratio between in-lake concentrations of TN and total phosphorus (TP). The logarithm of the areal hydraulic load, the ratio between the DIN and the TN load and the logarithm of the in-lake concentration of TP explained the largest proportion of retained DIN. Addition of the second and third explanatory variable decreased the normalized root mean square deviation between the observed and predicted proportion of retained TN from 37%, to 31% and to 30% and between the observed and predicted proportion of retained DIN from 39%, to 35% and to 32%.