Nitrogen Transformations Within the Trophogenic Zone of Lakes

Abstract

The uptake rates for ammonia and nitrate were measured in limnocorrals and in the adjacent water in the Bay of Quinte, Ontario. Even though the net change of nitrate exceeds that for ammonia over the summer, it is ammonia that is the most significant source of N for the plankton. The uptake rates were light dependent; in the dark uptake of nitrate was reduced 60–95% and ammonia by 40–70%. The uptake of N is not correlated to net production but to gross production. These rates were integrated with sedimentation, N release by zooplankton excretion, and N₂-fixation. Although N₂-fixation provides only a small amount of the daily N requirement compared with ammonia uptake, ammonia is rapidly recycled and the N₂-fixation is the vital process controlling the amount of this essential nutrient in systems where it is in short supply (corral II). Zooplankton grazing and sedimentation (decomposition) can account for much of the ammonia regeneration, but direct release of ammonia from the plankton appeared to be important. Key words: ammonia, nitrate, ¹⁵N kinetics, zooplankton grazing, nitrogen fixation