Phosphorus Stress Of Phytoplankton In Upwelling Area In Southern Taiwan Strait

Two cruises were carried out to study phosphorus stress of phytoplankton in southern Taiwan Strait in summer during 2005-2006, which upwelling induced by the monsoon. By using bulk and single cell alkaline phosphatase activity (APA) assay, phosphorus stress was studied via mapping, algal bloom process and ship-based mesocosm, the main results were as follows:1. The data from mapping showed that phytoplankton suffered more severe phosphorus stress in July 2005 than that in June 2006 although the upwelling was stronger in July 2005 than that in June 2006. The surface APA ranged 11.1~143.4nmol l-1 h-1, with mean of 44.7±41.2 nmol l-1 h-1 in July 2005, while there was higher than 20nmol l-1 h-1 for coastal waters and lower than 10 nmol l-1 h-1 for the bank, and offshore areas in June 2006.2. It obviously declined phosphorus stress of phytoplankton in upwelling area due to nutrient input induced by the upwelling. Then the bloom happened and accompanied with phosphorus declined. When the upwelling weakened and phosphorus absorbed by phytoplankton exceed supplement by upwelling, phytoplankton suffered phosphorus stress again. Then the stress was aggravated and truned into phosphorus limitation. Phytoplankton stayed in the status of phosphorus stress continuously and kept the balance when there was no or weak bloom happened. So the status of phosphorus in June 2006 would not be more severe than that in July 2005 when there had an intense upwelling. The mixture of the upper and bottom waters because of the disturbing by the heavy wind would bring the phosphorus to the upper water and it could decline phosphorus stress in some degree.3. Results of the mesocosm experiment showed that, phosphorus stress was mitigated both in the lower and the higher nutrient supply groups at the beginning of the incubation. In the ending of the experiment, the degree of phosphorus stress was more serious in lower nutrient supply group than that in the higher nutrient supply group. The control group suffered the most severe phosphorus stress.4. There was not significantly relationship between phytoplankton species succession and phosphorus stress in spatial scale in the study area. In temporal scale, there was significantly relationship between dominant species Pseudonitzschia spp. and the ability in synthesizing the AP. It meant that its variation of abundance relied on its ability in synthesizing AP to avoid phosphorus limitation in same degree of status of phosphorus stress compare to other species. It might be the reason for Pseudonitzschia spp. to be dominanted species in the Taiwan Strait...