Longitudinal and seasonal variations of epilimnetic silica in a morphologically complex reservoir and the significance of flow regime and internal processes to their dynamics

Abstract

Spatial and temporal dynamics of silica (SiO 2) were examined in a morphologically complex reservoir, based on data collected between high-flow year and low-flow year. SiO 2 averaged 3.4 mg/L and varied from 0.1 to 9.7 mg/L depending on the year and the location. The paired sample test of SiO 2 showed that in mainstem sites, SiO 2 was significantly (t = 3.577, p < 0.01) greater in the high-flow year than in the low-flow year, and this pattern was similar to that of embayment sites, indicating an importance of flow regime on the silica loading. During the high-flow year, SiO 2 was significantly (t = 3.577, p < 0.01) greater in the mainstems than in the embayments, but during the low-flow year, there was no statistical difference between the two reaches. SiO 2 showed a distinct longitudinal decline from the headwaters to the dam in the high-flow year, and it was modified by the plunging of metalimnetic density current in the mid-lake reach. Seasonal fluctuation of SiO 2 was influenced by internal nutrient cycling and diatom populations. Dominant phytoplankton abundance had an inverse relation between the two algal populations of bluegreens and diatoms during August-December of the low-flow year. In other words, bluegreen algae dominated at the low SiO 2 (< 2.5 mg/L) during the summer period of the low-flow year, whereas diatoms dominated with the increase of SiO 2 in fall overturn. Overall results suggest that increase of silica in this system is primarily regulated by interannual flow regime, but the internal loading during fall overturn and biological up-take by seasonal growth of diatom community were also considered as an important process controlling the input of silica.