Analysis of a Method to Estimate Chlorophyll-a Concentration from Irradiance Measurements at Varying Depths

Abstract

A model to estimate chlorophyll-a concentration and yellow substance absorption at 440 nm from irradiance measurements made at varying depths is examined. The derivation of the model, requiring irradiance measurements at three wavebands, is presented and tested on data collected in oligotrophic (with low chlorophyll concentrations) waters and in coastal waters (with both high and low chlorophyll concentrations). The results indicate excellent quantitative agreement with chlorophyll-a concentration and yellow substance absorption measurements. A sensitivity analysis of the model shows it to be highly sensitive to pressure sensor precision, the accuracy of the value used for the mean cosine of the downwelling radiance distribution, and the irradiance sensor measurement error. However, provided that these factors are taken into account, accurate estimates of chlorophyll-a concentrations in case I (phytoplankton-dominated) waters using a single (or multiple) irradiance sensor with three wavebands can be derived. This model can be applied to irradiance data from a variety of deployment methods including profilers, bottom-tethered moorings, subsurface drifters, and towed platforms.