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 (SiO2) were examined in a morphologically complex reservoir, based on data collected between high-flow year and low-flow year. SiO2 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 SiO2 showed that in mainstem sites, SiO2 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, SiO2 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. SiO2 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 SiO2 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 SiO2 (< 2.5 mg/L) during the summer period of the low-flow year, whereas diatoms dominated with the increase of SiO2 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.