Influence of Vertical Attenuation of Photosynthetically Active Radiation on the Growth of Submerged Aquatic Vegetation in Tropical Reservoir.

Abstract:

Reservoir construction cause many changes in lotic ecosystems and can favor the aquatic macrophyte growth. Nova Avanhandava Reservoir (São Paulo, Brazil), is fully inhabited by submerged aquatic vegetation (SAV) which may cause serious problems to hydropower and irrigation systems. The goal of this study was to assess the radiation availability in the water column in Nova Avanhandava and analyze its influence on SAV development and growth. The measurements were carried out between 28^th and 30^th June, 2013, at 19 sampling stations. Water samples for analytical determination of the suspended solids and chlorophyll-a concentration were collected. Hyperspectral downwelling irradiance (E_d) data, at several depths, were measured using the TriOS/RAMSES optical sensor. Depths and SAV heights were collected through hydroacoustic measurements by transects using the scientific digital sonar BioSonics DT-X (Echosounder). The E_d data were normalized, than calculated the K_d PAR – downwelling diffuse attenuation coefficient of photosynthetically active radiation, and the euphotic zone depth (Z_EZ). Results showed that SAV height values were lower at upstream (around P01) with highest TSS (Total Suspended Solids) and K_d, compared to areas downstream (around P19) (Figure). The maximum depth of SAV development, average SAV height, maximum SAV height, variability, and depth of occurrence of SAV were primarily influenced by K_d. A linear inverse relationship between the average SAV height and the K_d PAR (R² = 0.56, p<0.001), and between maximum SAV height and K_d PAR (R²=0.5, p<0.001) demonstrated that the SAV heigth can be estimated by K_d PAR with significant accuracy. Therefore, studies on subaquatic radiation availability measured by the vertical attenuation of E_d PAR in the water column and the optically active components can be effiectivly linked to SAV growth and occurrence and aid in understanding SAV in tropical reservoirs, contributing to its management.