Nutrient dynamics across the river-sea interface in the Changjiang (Yangtze River) estuary-East China Sea region

A total of 13 cruises were performed in different seasons between July 2010 and July 2012, and in various subsystems of the Changjiang (Yangtze River) Estuary and the adjacent East China Sea (ECS). Multiple vessels were deployed to conduct transect sampling, simultaneous and consecutive sampling, sampling with buoys, and sampling during phytoplankton bloom events, which allowed us to track detached patches of water masses and to collect discrete and time-series samples in different patches. We found that the river-derived nutrients from the Changjiang could be transported through the western ECS and reach as far as the northeastern ECS during summer, but in the form of isolated patches rather than continuous plumes. The concentrations of NO^-₃; SiO^2-₃, and PO^3-₄decreased with increasing seaward distance of these patches, showing largely nonconservative mixing behavior and the ECS being a sink for nutrients. During winter, conversely, all nutrient species showed mostly conservative behavior, with correlative coefficients (R²) even reaching > 0.97 in the salinity-nutrient correlations for both NO^-₃ and SiO^2-₃ during the western ECS cruise in February 2012 and during the Changjiang mouth cruise in March 2012. In addition, a large variability in nutrient concentrations was observed within the Changjiang freshwaters and could be partly attributed to inputs from local tributaries. This study highlights the importance of simultaneous field sampling with multiple vessels for investigating the biogeochemical behavior of nutrients in highly dynamic ocean margin areas such as the Changjiang Estuary-ECS region.