PREDICTION OF DISSOLVED INORGANIC NITROGEN (DIN) CONCENTRATIONS IN DEEP, SEASONALLY STRATIFIED LAKES BASED ON RATES OF DIN INPUT AND N-REMOVAL PROCESSES

Mean dissolved inorganic nitrogen concentrations ([DIN]) in deep, seasonally stratified lakes with comparable DIN inputs can differ by up to a factor of 3 depending on hydraulic and morphometric properties and/or different trophic states of the lakes. In such lakes, net N sedimentation rates were estimated with two independent methods (sediment core analysis and input-output mass balances). They were higher in eutrophic lakes (Mean: 5.1; SD: +/- 1.6 g m-2 yr-1; n = 13) than in oligotrophic lakes (1.6 +/- 1.0 g m-2 yr-1; n = 3), but independent of [DIN]. Gaseous N loss rates to the atmosphere, as calculated from combined N- and P-mass balances from selected lakes, ranged from 0.9 to 37.4 g m-2 yr-1 (n = 10) and were positively correlated with [DIN]. Reduction of NO3- to N2 is assumed to be the main cause for gaseous N losses. A simple one-box mass balance model for [DIN], based on DIN input and rates and kinetics of N removal processes (net sedimentation and gaseous N loss) is proposed, and validated with a data base on [DIN] and DIN input in 19 deep, seasonally stratified lakes of central Europe. The model illustrated that the amount of water loading per unit surface area of a lake (called water discharge height q) is the critical parameter determining mean lake [DIN] relative to mean input [DIN]. Lakes with a q > 50 m yr-1 have average [DIN] similar to the [DIN] of the inflows regardless of their trophic states, because input and outflow exceed lake-internal N removal processes. A high primary production favors DIN removal in lakes with q < 50 m yr-1. It is concluded that measures to decrease primary production, e.g. by means of P removal programs, lead to an increase of [DIN] in lakes.