Hydromorphological mechanisms leading to hypoxia off the Changjiang estuary

Abstract

Based on the seasonal surveying data and long-term data, the annual changes in the geographical locations, occurrence frequency, affected areas and the minimum oxygen level as well as the formation mechanism of the summer hypoxia off the Changjiang estuary are summarized and discussed in this paper. The historical data indicates that there were episodes of hypoxia in the past 50 years but not every year, and the event of summer hypoxia could be traced back to as early as late 1950s off the Changjiang estuary. The minimum oxygen levels in the hypoxia zone did not show any decline in the past 50 years, but all the events with large size of affected area (>5000km(2)) were observed after the late 1990s, suggesting an enlarging trend. The author argues that the development of summer hypoxia off the Changjiang estuary was related not only to stratification and input of suspended particulate matter, but also to the inflow of Taiwan warm current water as well as the bottom topography.

Geochemical forms and seasonal variations of phosphorus in surface sediments of the East China Sea shelf

“…The discharge of Changjiang is highly seasonal with its peak in summer and 75% of the river runoff occurring during the flood/rainy season between May and October (Rabouille et al, 2008). The hypoxia adjacent to the Changjiang Estuary, which is one of the largest coastal low-oxygen areas in the world , always occurs in summer and its zone has increased in recent decades (Rabouille et al, 2008;Wang, 2009). Wang et al (2012) also noted that the increasing trend of hypoxia would continue in next two decades.…”

“…Besides the consumption of oxygen from the organic matter decomposition, physical forcings (stratification, circulation pattern in the region and residence time of the water in the mixing zone) which limit oxygenation of bottom waters were also important factors in the formation of hypoxic off the Changjiang Estuary (Wei et al, 2007;Rabouille et al, 2008;Wang, 2009). In late spring, the combination of high CDW runoff, TWC and strong solar radiation made a strong stratification in the northern ECSS (Wang, 2009). The decomposition of organic matter consumed oxygen, while oxygen could not be replenished due to the strong stratification in late spring.…”

“…The decomposition of organic matter consumed oxygen, while oxygen could not be replenished due to the strong stratification in late spring. Then, the hypoxic condition was formed in late spring, and enlarged in summer due to stronger stratification (Wang, 2009). Among the conditions which resulted in hypoxia, the decomposition of organic matter in the surface sediment was obviously the direct reason for the formation of the hypoxic condition adjacent to the Changjiang Estuary.…”

“…The ECS has obvious seasonal variation characteristics, including the seasonal variations of freshwater discharge from coastal rivers, hypoxia outbreak time, nutrient concentrations, chlorophyll a (Chl a) concentrations, the primary production and so on (Chen et al, 2001(Chen et al, , 2004Rabouille et al, 2008;Wang, 2009;Li et al, 2014). As P has close relations to these parameters, there may be some seasonal variations for P in the surface sediments of the ECS.…”

Monitoring the occurrence of seasonal low‐oxygen events off the Changjiang Estuary through integration of remote sensing, buoy observations, and modeling

“…The hypoxic condition off the Changjiang Estuary (CE) has been detected since the 1950s. The lowest DO concentrations in this region have continually decreased, and the summertime hypoxic zone has continually spread since 2003 [Chen et al, 2007;Wei et al, 2007;Wang, 2009;Ning et al, 2011]. Early hydrological surveys off the CE found that hypoxia occurred in a region approximately 100 km east of the river mouth along 123 E and at a depth of 35 m [Tian et al, 1993].…”