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 phosphorusconcentrations 20 μg C L−1 h−1) and could favor the release of inorganic phosphorus. High extracellular enzyme activities and phosphorussolubilizing bacteria abundance in sediments accelerated nutrient regeneration. In these conditions, the positive GLU-AP relationship suggested the coupling of carbon and phosphorusregeneration; an efficient phosphorusregeneration 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- mesotrophiclakes, 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 phosphorusregeneration 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 phosphorussolubilizing bacteria found in this compartment.