Temporal and spatial variations of alkaline phosphatase activity related to phosphorus status of phytoplankton in the East China Sea

Abstract

Phosphorus (P) is a potential limiting nutrient for primary production in the East China Sea (ECS). Four cruises over four seasons were conducted during 2009–2011 to evaluate the dynamics of alkaline phosphatase (AP) activity (APA) and the P status of phytoplankton in the ECS. Sampling for bulk and single-cell APA assays was performed across the ECS, which included the Changjiang River diluted water (CDW), the mid-shelf surface water (MSW), and the Kuroshio surface water (KSW) masses. The results showed that the distribution patterns of APA varied between water masses and with season. Higher APA was normally observed in the CDW, which was influenced by the Changjiang River plume. In the CDW, phytoplankton were P-stressed during the late spring with an average bulk APA of 20.4 ± 19.5 nmol h−1 L−1, which strengthened during the late summer (average APA maximizing at 106.2 ± 133.3 nmol h−1 L−1) and remained severe during the late autumn (average APA of 48.7 ± 34.1 nmol h−1 L−1) until reducing during the winter (average APA of 9.1 ± 13.6 nmol h−1 L−1). The variation patterns of APA and the P status of phytoplankton in the MSW were similar but with slight variations. In the KSW, a certain amount of APA occurred during the late spring and late autumn (averages of 18.7 ± 19.8 and 23.1 ± 18.7 nmol h−1 L−1, respectively). Single-cell APA coincided with the bulk APA to exhibit the temporal and spatial P-stress status of the dominant micro-phytoplankton. Phytoplankton were the main AP producers in the CDW, especially during the late summer, while pico-plankton contributed most to AP in the MSW and KSW. Our results suggest that phytoplankton suffer P-stress in most seasons, and emphasize the importance of AP in the recycling of organic P to support primary production in the P-deficient ECS.