Changes in concentrations of oxygen, dissolved nitrogen, phosphate, and silicate in the southern Yellow Sea, 1980–2012: Sources and seaward gradients

Abstract

To investigate the influence of nutrients on the occurrence of the world's largest trans-regional green tide bloom along the coast of the southern Yellow Sea, spatial and temporal variation in dissolved inorganic nutrients, dissolved oxygen and the dissolved inorganic nitrogen/phosphorus (N/P ratio) in the southern Yellow Sea from the 1980s to 2012 were analyzed on the basis of our field data and historical data. The transport and concentration of nutrients and oxygen may be significantly affected by the complex hydrographic conditions that were found in the southern Yellow Sea year-round. The input from diverse nutrient sources, especially from the Yangtze river plume, the rising nutrient loading from rivers and mariculture along the Subei coast, could contribute to sustaining green algal blooms. The highest chlorophyll-a concentration appeared in 2012 of the study area. The horizontal distribution of nitrate, phosphate and silicate generally decreased from the nearshore to offshore waters from the 1980s to the present. A zone of high nutrient concentrations was generally found in the southwestern southern Yellow Sea (121–123°E, 32–33°N) over the past three decades, which may be primarily induced by the input of the Yangtze river plume into this area. The lowest nutrient concentrations were found in the mid-north of the southern Yellow Sea (122–124°E, 34–36°N). A zone of high DO concentrations was identified in the northeastern region of the study area (122–123.5°E, 35–36°N). Though ammonium values fluctuated during 1984–2012, nitrate and DIN concentrations increased across the 30 years of sampling, implying the intense influence of terrestrial discharge to the southern Yellow Sea. And the high nitrogen levels may be one of the most important contributors to green algal blooms in this area. Phosphate concentrations fluctuated from the 1980s to 2005 and have since decreased. A large upward spike in the N/P ratio was apparent in 2008 and since, and the DO concentration decreased sharply in the same year. Concentrations of dissolved inorganic nutrients and oxygen were generally high in winter, lower in summer.