Different pathways of nitrogen and phosphorus regeneration mediated by extracellular enzymes in temperate lakes under various trophic state.

Abstract

Several Italian and Chinese temperate lakes with soluble reactive phosphorus concentrations < 0.015mgL-1 were studied to estimate nitrogen and phosphorus regeneration mediated by microbial decomposition and possible different mechanisms driven by prevailing oligo- or eutrophic conditions. Leucine aminopeptidase (LAP), beta-glucosidase (GLU) and alkaline phosphatase (AP), algal, and bacterial biomass were related to trophic and environmental variables. In the eutrophic lakes, high algal and particulate organic carbon concentrations stimulated bacterial respiration (> 20μgCL-1h-1) and could favor the release of inorganic phosphorus. High extracellular enzyme activities and phosphorus solubilizing bacteria abundance in sediments accelerated nutrient regeneration. In these conditions, the positive GLU-AP relationship suggested the coupling of carbon and phosphorus regeneration; an efficient phosphorus regeneration and high nitrogen levels (up to 0.067 and 0.059mgL-1 NH4 and NO3 in Italy; 0.631 and 1.496mgL-1 NH4 and NO3 in China) led to chlorophyll a peaks of 14.9 and 258.4μgL-1 in Italy and China, respectively, and a typical algal composition. Conversely, in the oligo-mesotrophic lakes, very low nitrogen levels (in Italy, 0.001 and 0.005mgL-1 NH4 and NO3, respectively, versus 0.053 and 0.371mgL-1 in China) induced high LAP, while low phosphorus (33.6 and 46.3μgL-1 total P in Italy and China, respectively) led to high AP. In these lakes, nitrogen and phosphorus regeneration were coupled, as shown by positive LAP-AP relationship; however, the nutrient demand could not be completely met without the supply from sediments, due to low enzymatic activity and phosphorus solubilizing bacteria found in this compartment.