Anthropogenic impact on spring bloom dynamics in the Yangtze River Estuary based on SeaWiFS mission (1998–2010) and MODIS (2003–2010) observations

Nutrient output from the Yangtze River to the sea has increased dramatically since the 1960s, and over the past 50 years more than 50,000 reservoirs on the Yangtze River basin have had little impact on water discharge, but have drastically reduced the annual river-to-sea sediment flux, especially after 2000. This can be presumed to have a close link with the 73% (accumulated incidences) of algal bloom reported on China's eastern coast which have taken place in the Yangtze River Estuary (YRE) and its adjacent waters from 2000 to 2009. A conceptual view explains that the algal bloom zone varies between the YRE and mid-shelf waters of the East China Sea, where the optimum balance of light availability and nutrient supply exists. A reduction in turbidity with declining river-to-sea sediment load around the YRE would provide a deeper euphotic layer for the growth of phytoplankton, which is stimulated by eutrophication following increased river-to-sea sewage. Although the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) represents an immature state of ocean colour remote sensing, especially in coastal waters, in general, SeaWiFS serves as a useful tool to highlight the geographical spread and intensity of spring bloom dynamics at the basin scale in the YRE. We suggest that the problem of skin effect bias in spaceborne observation is minimal and that satellite-derived pigment data should be mainly from phytoplankton production when estimating Chl-a during the spring bloom season in the YRE by analysis of bio-optical properties of spring bloom waters. Comparisons between in situ and simultaneous SeaWiFS-derived Chl-a suggest that SeaWiFS standard Chl-a data show comparable results with ship survey data, with a mean ratio (in situ to satellite ratio) of 1.28 ± 0.78 (R 2 = 0.71, n = 14, p < 0.001) in spring blooms. We took the seasonal average Chl-a in our defined multi-year highly productive zone in the YRE (28°30′–32°N, 122°–123°30′°E, A) compared with its neighbouring region B from 1998 to 2010, and determined a linear least-squares fit to the trend line. Further analysis in the form of t-tests was run for four seasonal periods, March–April, May–June (spring bloom season), July–August, and September–October. The SeaWiFS-derived 13-year Chl-a mean slope in the spring bloom season is significantly (P = 0.012) increasing by 0.212 mg m−3 per year. However, such a trend in other seasons is not significant and there is also no significant variation in B over all seasons. Our results show that there is a positive relationship between the annual mean of Chl-a in the spring bloom season in A and annual sewage water discharge at the Yangtze River basin, and similarly a negative relationship between Chl-a and annual river sediment flux was found from 1998 to 2010. Variation in 3-year sediment flux, sewage water discharge, and Chl-a was –25% (±18%), 15% (±5%), and 14% (±6%), respectively. This result supports the findings of previous studies that human activity has a measurable effect on coastal phytoplankton biomass and that the eutrophication effect seems to stimulate increased Chl-a in the spring bloom season, but not on the enhancement of annual Chl-a levels in the YRE.