New production in the East China Sea, comparison between well-mixed winter and stratified summer conditions

Abstract

This paper compares the variations of nitrate-based “new” production (NP) in the continental shelf of the East China Sea (ECS) between summer, when seawater stratification was strong, and winter when seawater was well mixed. New production and f-ratio (ratio of nitrate-based new production to primary production) showed no obvious seasonality and were generally low with two exceptions: one in the coastal upwelling off China in summer, and the other in the Kuroshio upwelling off northeastern Taiwan in winter. When not counting the two exceptional areas which were relatively small area-wise, integrated nitrate-based new production (INP) ranged 0.06–0.41 g C m −2 d −1 in summer and 0.08–0.27 g C m −2 d −1 in winter with f-ratios of 0.25–0.42 and 0.18–0.35. In summer, NP in the Changjiang-influenced coastal waters was significantly correlated to a high ambient nitrate concentration, which originated from the Changjiang discharge or from the coastal upwelling. Phosphate from the coastal upwelling alleviated the phosphorus deficiency caused by the river mixing, and the nitrate to phosphate ratio was near the Redfield ratio. Nitrate was readily consumed and converted into phytoplankton biomass. This leads to a low surface nitrate concentration (mostly below the detectable limit of 0.1 μM) and thus resulting in low NP in the surrounding shelf waters. NP in winter, in contrast, was not correlated to the nitrate concentration. Despite the abundant nitrate on the continental shelf (ranging between 0.52 and 14.24 μM), INP remained relatively low (0.01–0.55 g C m −2 d −1). Light limitation hampered nitrate utilization. The deep mixing layer (>euphotic depth), low photosynthesis available radiation, and a significant correlation between new production and stratification index indicated that NP in winter was controlled by irradiation intensity rather than nutrient concentration. The present results strongly suggest that production in the ECS was limited by N deficiency in summer and by light in winter. P deficiency did not seem to play a major limiting role.