Response Of Plankton And Periphyton To Pulse Nutrient Addition: An Enclosure Experiment

An in situ enclosure experiment was conducted in a deep subtropical reservoir of Southern China to examine the response of plankton and periphyton to pulse nutrient addition. Four treatments were set as follow: no nutrient addition(C) and three nutrient additions. Each nutrient addition enclosure received the same total amount of nitrate(280 μg/L) and phosphate(40 μg/L), but the nutrients were supplied at three different intervals: one single load(L), a biweekly load(M) and a weekly load(H). Timeweighted averages were calculated from the time data series and the difference in response among treatments was compared with multilevel model.TN(total nitrogen) and TP(total phosphorus) were significantly lower in treatments without nutrient addition than treatments with nutrients addition but difference in TP between treatments with nutrients addition were not significant. TN in L was significantly higher than M and H. Sedimentation of nitrogen and phosphorus were significantly higher in treatments with nutrients addition than treatments without nutrient addition but difference between treatments with nutrients addition was not significant. Retention of phosphorus by periphyton was significantly higher than in treatments with nutrients addition than treatments without nutrients addition. Retention of phosphorus by periphyton was significantly higher than in L than M and H. Nutrients addition enhanced the growth of small phytoplankton(< 20 μm) and phytoplankton biomass were significantly higher than in treatments with nutrients addition than treatments without nutrients addition, and higher in L than M and H. Nutrients addition also enhanced the growth of periphyton, and periphyton biomass were significantly higher than in treatments with nutrients addition than treatments without nutrients addition. In contrast to phytoplankton, periphyton biomass was significantly lower in L than M and H. The enhancement of phytoplankton growth in treatments with nutrients addition also promoted the growth of cladocerans and copepods which resulted in total zooplankton biomass was significantly higher in treatments with nutrients addition than treatments without nutrients addition. Similarly to phytoplankton, zooplankton biomass was significantly higher in L than M and H.Overall, nutrients input enhanced the growth of plankton and periphyton. However, the frequency of nutrient pulses may affect the partition of nutrients among phytoplankton and periphyton. Phytoplankton was likely to retain more nutrients under condition of single nutrient pulse, while periphyton retained more nutrients under condition of high nutrient pulse.