Spatial and seasonal variation in diffuse attenuation coefficients of downward irradiance at Ibitinga Reservoir, São Paulo, Brazil

Abstract

Estimates of light availability in the water column require accurate quantification at different depths of the diffuse attenuation coefficient (K _d). This study examines spatial and seasonal variability of K _d derived variables [euphotic zone depth (Z _eu), attenuation depth (Z _att)], their relationship with optically active constituents (OACs), and their impact on the underwater light field at Ibitinga Reservoir (São Paulo, Brazil). Radiometric data were acquired using profiling spectroradiometers operating from 320 to 950 nm in four campaigns (February–July 2014) concurrently with OAC water samplings. Minimum and maximum values of K _dPAR were, respectively, 0.99 and 3.45 m⁻¹ for average ranges of OACs varying from 14.4 to 16.2 mg/l for dissolved total carbon, 11.5–100.5 μg/l for chlorophyll-a, 1.8–14.5 mg/l for total suspended solid, and from 0.73 to 1.71 m⁻¹ for absorption coefficient of colored dissolved organic matter (CDOM) in 440 nm (a _CDOM440). CDOM removed blue wavelengths in the first meter of the water column throughout seasons, while green wavelengths were predominant with increasing depth. Furthermore, this study demonstrated that K _d variability is mainly influenced by the presence of phytoplankton and CDOM in the reservoir, and reveals that the seasonal variability of K _d was much larger than spatial variability.