Seasonal Changes in Nitrogen Compartments of Lakes Under Different Loading Conditions

Abstract

To develop a better conceptual picture for nitrogen dynamics in lake ecosystems, the seasonal changes in the concentrations of nitrate, ammonia, dissolved organic and particulate nitrogen were monitored in the water contained in three limnocorrals located in the Bay of Quinte, Ontario. Following the decline in summer particulate N, there was an increase in ammonia. This was followed by an increase in nitrate throughout the winter suggesting that nitrification was a significant process. To assess the potential contribution of the sediments, ammonia, nitrate, and dissolved organic N were measured for pore waters and total N, exchangeable ammonia, and nitrate in the solid fraction. The influence of nutrient addition was evaluated by comparing corral I, receiving no nutrient additions, with corral II to which phosphate was added (0.92 g P/m² per yr) and corral III which received the same amount of phosphate plus nitrate at an N:P weight ratio of 13. Increases and decreases in phosphorus, nitrogen, and chlorophyll a seemed to take place in all three corrals at the same time. There was a considerable delay in the response to nutrient enrichment, but the addition of nitrate and phosphate eventually resulted in a chlorophyll a level greater than that for phosphate enrichment alone. The importance of sediments as a net source of N for the planktonic community was insignificant, but the flux rate for nitrogen to and from the sediment was important. During the summer an input–output budget showed that nitrate loading plus N₂ fixation was approximately equal to the net change in the total combined N in the water. This implied that denitrification was not important, but over 12 mo most of the nitrogen that entered the system was lost; hence denitrification must be important at certain times. A lake ecosystem tends to compensate for low or high levels of NO₃ loading. Without the addition of nitrogen the requirement was partly met through nitrogen fixation and this can be enhanced by P enrichment. Key words: ammonia, nitrate, nutrient kinetics, nitrogen fixation, denitrification, sediments