Nutrient accumulations in high-saline bottom waters in the eutrophic East China Sea inner shelf

Abstract

Despite decades of mitigation efforts, eutrophication-induced algal blooms and hypoxia have not significantly decreased globally, possibly due to the legacy effects of eutrophication. The legacy effect has been more explored in inland waters and enclosed estuaries than in open coastal waters. Here, we reanalyzed cruise data from the East China Sea inner shelf to explore the effect of eutrophication on nutrient accumulations in high-saline bottom waters. Our dataset showed elevated nitrate (12.75 ± 6.51 μmol L⁻¹) and phosphate (0.85 ± 0.26 μmol L⁻¹) in high-salinity (salinity>34, temperature < 23 °C) bottom waters during the summer of 2006 to 2013. They were higher than those typically observed in the Taiwan Warm Current Bottom Water by approximately 5.45 ± 6.51, and 0.29 ± 0.26 μmol L⁻¹, respectively. Significant correlations of nitrate with apparent oxygen utilization (AOU) and elevated AOU suggested that organic matter decomposition contributed to increased nitrate in bottom waters under eutrophication conditions. Based on an end-member mixing estimation, we found that the organic matter decomposition accounted for 28% to 37% of the nutrient concentrations in the bottom waters, with a standard deviation of 20%. Results from our mass-balance model indicated that 40–74% of regenerated nutrients are flushed out of the model box set within 95% equilibrium time due to the advection of offshore waters, assuming a residence time of 46.0 to 13.9 days. Based on cruise results in June and August 2009, the net accumulation rates of nitrate, phosphate, and AOU in the Taiwan Warm Current Bottom Water were estimated to be 0.046, 0.0036, and 0.44 μmol L⁻¹ d⁻¹, respectively. Such nutrient accumulations in the water column and the residual nutrients in sediment are crucial legacy nutrients, potentially triggering algal blooms. Conversely, the flushing effect suggests a significant transport of nutrients and other chemical elements to the offshore and open ocean.