Light attenuation in a shallow, turbid reservoir, Lake Houston, Texas

Results of measurements of light penetration at sites in Lake Houston near Houston, Texas, indicate that light-extinction coefficients during 1989– 90 range from about 2.49 to 7.93 meters-1 and euphotic zone depth ranges from about 0.61 to 1.85 meters. The coefficients are largest near the inflow site of West Fork San Jacinto River (upstream) and decrease slightly toward the dam (downstream). Total suspended solids and total organic carbon concentrations also are largest at the upstream end. Chlorophyll a+b concentrations are smallest near the dam, increase slightly upstream, and are largest during growing-season months. Color and turbidity show the strongest correlations with light-extinction coefficients in Lake Houston. Dissolved phosphorus and nitrogen concentrations are greater than growth-limiting concentrations during the study period, indicating that nutrient availability did not limit primary productivity or the phytoplankton biomass in Lake Houston.

Light-extinction coefficients in relation to selected water-quality constituents indicate that more than one constituent affects the light-attenuating properties of Lake Houston. Attenuation of light in water depends on total suspended solids for predominant light scattering and on dissolved organic matter (color) and chlorophyll a+b for absorption of light.

A statistical analytical model using Spearman rank order correlation shows that color and turbidity are the most useful water-quality constituents sampled to determine light-attenuating properties of water in Lake Houston. Multiple-regression analysis of measured light-extinction coefficients as the dependent variable and measured color and turbidity as independent variables for water from Lake Houston produced the relation:

light-extinction

coefficient (η) = 2.78 + 0.007 X color

+ 0.036 X turbidity,

with an average error of the computed coefficient to measured value of ±13 percent. The model can be useful in computing the thickness of the euphotic zone to determine primary productivity in the reservoir.