Response Of Phytoplankton To Change In Phosphorus Loading Induced By Hydrodynamic In Liuxihe Reservoir, South China

To understand the impact of hydrodynamic to the distribution of phosphorus in profile and theresponse of phytoplankton to change in phosphorus loading, an enclosure experiment, from19July to31August2012, and field observation were carried out in Liuxihe reservoir, a deepsubtropical water body in South China, from16April2012to March2013. Three treatmentswere set in the enclosure experiment: artificial disturbance per three days without addingnutrients in the sediment; adding nutrients in the sediment without artificial disturbance; addingnutrients in the sediment with artificial disturbance per three days. In the field observation, wesampled weekly the water at the transition-zone at five depths:0.5m,5m,10m,15m and20mfrom16April to31May and from28September to15November, and monthly in the otherperiods.In the enclosure experiment, the nutrients added in sediments released to upper water and wereabsorbed by phytoplankton, and the artificial disturbance transported nutrients into the up-water.In the treatments of adding nutrients, the phytoplankton biomass and Biomass/TP both increased,the small sized phytoplankton (<30μm) increased and first responded to phosphorus change.In the field observation, the water column was stratified and water level raised rapidly in thewet season. The depth for the maximal phosphorus concentration became deeper with increasingwater level, from5m to10m, then to15m and20m. The decrease in the phosphorusconcentration in the upper water layer became stronger in the period. The rising water levelrestricted the phosphorus releasing from the deeper layers. The depth for maximal phytoplanktonbiomass became deeper: from0.5m to5m, then10m the depth. The maximal depth ofBiomass/TP became deeper: from0.5m to10m, corresponding to the change in phosphorusconcentration. In the period, phytoplankton in the upper water layers was limited by phosphorus,the mass ratio of Biomass/TP increased with water level. The small sized phytoplankton (<30μm)dominated the phytoplankton assemblage and was the first responder for phosphorus change.In the dry season, water level went down and the water was mixed. The vertical profile ofphosphorus and phytoplankton biomass became more homogeneous with low value and themaximal value at20m and10m respectively, even if the water level fluctuated. The Biomass/TP became lower but with great variation. Large sized phytoplankton (>30μm) became moreimportant.In conclusions, in the sub-tropical deep reservoir, the phosphorus is transported from thesurface layers to the deeper in the wet season and from the deeper layers to the surface in the dryseason. The transportation is affected by water mixing regime. The response of phytoplankton tochange in phosphorus loading is very strong in the phosphorus-limited-system. Thephytoplankton biomass and the small sized phytoplankton (<30μm) increased with the increasingphosphorus concentration.