Changes in light attenuation during filling, stabilization and operation of a reservoir

Abstract

Transformations in light attenuation (Kd) and Secchi disc transparency (SD) in Round Valley Reservoir, New Jersey, are documented over a 17 yr period (1966–1983), which includes initial filling of the reservoir, post-transition phase conditions, and a perturbation associated with the flow-augmentation function of the system. Attenuation was partitioned empirically, according to contributions from chlorophyll a and non-phytoplanktonic components, and mechanistically, according to contributions from the processes of absorption and scattering. Pertinent measurements included downwelling irradiance, SD, and chlorophyll a. Light attenuation levels were high during the first year of filling (1966; 0.90 to 2.45 m⁻¹), but dropped substantially by the second year of filling (0.40 to 0.90 m⁻¹). Further major reductions in K _d were observed following the transition phase (minimum of 0.17 m⁻¹ observed in 1980). Average S _D values for 1966, 1967 and following the transition phase were 1.4, 3.2, and 7.3 m, respectively. Throughout these major changes the relative contributions of absorption and scattering to attenuation remained essentially uniform. A manifestation of this uniformity was the uniformity of the K _d · S _D product (K _d · S _D = 1.86, 95% confidence limits = ± 0.09, n = 31) for the same interval. Chlorophyll a explained a significant portion of the observed variability in K _d over the monitored period. However, different regimes of attenuation were apparent; in particular the non-phytoplanktonic component decreased dramatically by the second year of filling, and decreased again by the end of the transition phase. Refilling of the reservoir following flow-augmentation releases resulted in increases in K _d , associated largely with increases of non-phytoplanktonic attenuating materials. The increase in K _d caused the development of increased density gradients in the metalimnion of the reservoir during mid-summer, and probably thereby reduced vertical mixing. The dynamics in optical characteristics observed for Round Valley are probably typical of many reservoirs.