Implications of evolution of circulation patterns on nutrients regime from 2008–2018, in Xiangxi Bay of the Three Gorges Reservoir, China

Abstract

A 2-D CE-QUAL-W2 hydrodynamic and water quality model was established from 2008 to 2018 in Xiangxi Bay (XXB). The recently built upstream Xiluodu, Wudongde, Xiangjiaba and Beihtan dams have altered the ecohydrology of mainstream and tributaries, so nutrients regime during pre and post dam stages are comparatively investigated here. Although total phosphorus (TP) increased at the lower reach, eutrophication remained TP limited with few exceptions. Maximum supply from main stream, potential of water exchange on surface corresponding to increased overflow intrusion facilitated increased average total nitrogen (TN) at euphotic zone in XXB by 0.1, 0.3 and 0.6 mg/L respectively since 2014. Eutrophication and algal growth in XXB remained TP limited with few exceptions at the upper reach. Average TN increased by 0.1, 0.3 and 0.5 mg/L at the upper, middle and lower reaches respectively in Late March since 2014 compared to those in early years. It increased by 0.2, 0.4 and 0.6 mg/L at the upper, middle and lower reaches in Early April since 2014. It increased by 0.3, 0.6, and 0.9 mg/L at the upper, middle and lower reaches in mid April since 2014. It increased slightly at the upper reach, 0.6 mg/L at the middle reach and 0.9 mg/L at the lower reach during late April. It increased slightly at the upper reach and by 0.3 mg/L at the middle reach and by 0.5 mg/L at the lower reaches corresponding to either of the patterns. During mid May, it remained the same at the upper reach, decreased by 0.2 mg/L at the middle reach and 0.5 mg/L at the lower reach. During late May, it remained the same at the upper reach, decreased by 0.4 mg/L at the middle reach and 0.6 mg/L at the lower reaches. These statistics indicate average an increase in TN in spring except in late May .TN concentration was beyond danger limit sustaining and prolonging eutrophication status in spring. As overflow, interflow and upper interflow patterns are prevalent in random order so nutrients could not be predicted theoretically, however nutrients management from non-point sources and the main supplier Three Gorges Reservoir is advisable. HIGHLIGHTS Continuous and increased overflow intrusion from the Three Gorges Reservoir (TGR) caused increase in total nitrogen (TN) concentration at upper, middle and lower reaches since 2014.; TN and total phosphorus (TP) concentration were beyond pollution indices.; TN/TP ratio declares eutrophication in XXB as TP limited in spring.; All non-points sources inputs containing TN should be controlled to inhibit algal growth.; As overflow is prevalent in random order so nutrients are not predictable.;

Keywords

• ce-qual-w2
• density currents
• eutrophication level
• nutrient distribution
• three gorges reservoir