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

Several Italian and Chinese temperate lakes with soluble reactive phosphorus concentrations <0.015 mg L-1 were studied to estimate nitrogen and phosphorus regeneration mediated by microbial decomposition and possible different mechanisms driven by prevailing oligo- or eu-trophic 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 (>20 ?g C L-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 0.059 mg L-1 NH4 and NO3 in Italy; 0.631 and 1.496 mg L-1 NH4 and NO3 in China) led to chlorophyll a peaks of 14.9 and 258.4 µg L-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 0.005 mg L-1 NH4, and NO3, respectively, versus 0.053 and 0.371 mg L-1 in China) induced high LAP, while low phosphorus (33.6 and 46.3 µg L-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.