Optical properties of highly turbid shallow lakes of contrasting turbidity origin: ecological and water management implications

G. L. PÉREZ; L. LAGOMARSINO; H. ZAGARESE

Córdoba

Encuentro; Segunda Reunion de Fotobiólogos Argentinos (GRAFOB II); 2013

GRAFOB

Turbid lakes are optically complex environments, however their optical characteristics have been seldom described with enough detail. Here, we present a comprehensive optical study of three highly turbid shallow lakes. Lakes presented very high concentrations of total suspended solids (TSS) with mean values above 120 mg L-1. Clear differences in chlorophyll a and ash concentrations were observed among them. Lakes showed elevated values of total absorption [at (λ)] and scattering coefficients [b (λ)], which were translated into extremely high diffuse light attenuation coefficients [Kd (PAR)] with mean values above 23 m - 1. Using Kirk ́s model we analyzed the relationships between inherent optical properties (IOPs) and Kd (PAR), an apparent optical property (AOP) of paramount importance in water ecology and water quality applications. Modelled values of Kd (PAR) agreed very well with measured ones (R2 = 0.95), with a relationship close to 1:1 and a low relative root mean square error (RRMSE) of 11.7%. In contrast, relationships between Kd (PAR) and two typical estimators of light penetration, i.e. nephelometric turbidity (Tn) and Secchi disk (ZSD), elicited lower accuracy (R2 = 0.83 and R2 = 0.74 respectively). On the other hand, we described natural phytoplankton absorptive characteristics, key variables for bio-optical models and remote sensing. In studied lakes, spectral composition of PAR was quickly constituted by wavelengths poorly absorbed by phytoplankton, with values of absorption efficiency parameter (Ae) lower than 1. Some of the variation in specific phytoplankton absorption [aph*(λ] was explained by differences in the ratio between unpigmented particulate absorption and phytoplankton absorption (up to R2 = 0.48 to the blue band). Pigments composition and packaging were also important for the variation in aph* (λ) in these highly turbid environments.