Lateral water exchange between shelf-margin upwelling and Kuroshio waters influences phosphorus stress in microphytoplankton

Phosphorus stress in microphytoplankton was investigated in an upwelling system and an adjacent segment of the Kuroshio Current in the southern East China Sea. In June, August, and September 2007, 3 cruises were conducted, and alkaline phosphatase activity (APA) and the maximum quantum efficiency of photosynthesis (F-v/F-m) were measured along a cross-shelf cruise track from the mid-shelf to the Kuroshio Current. In June and August, values of APA ranged from 1.41 to 9.56 pmol PO43- (mu g chlorophyll [chl] a)(-1) min(-1), and those of F-v/F-m ranged from 0.28 to 0.45 in surface waters. In September, a strong salinity front appeared between the upwelling zone and Kuroshio waters. Low APA values of 1.23 to 8.08 pmol PO43- (mu g chl a)(-1) min(-1) were observed in the upwelling region, and high values of 34.9 to 77.6 pmol PO43-(mu g chl a)(-1) min(-1) were observed in Kuroshio waters. A significant correlation between the APA and F-v/F-m only occurred in September, and a possible event of phosphate stress in Kuroshio waters was supported by high APA values and changes in the microphytoplankton species composition. Our results suggest that phosphorus stress in microphytoplankton is related to the magnitude of horizontal water exchange between the Kuroshio and upwelling waters.