Monitoring Secchi depth of the Yellow Sea and the East China Sea using a semi-analytical algorithm

Secchi depth, an important optical characteristic of water, is a useful index of water quality and is widely used in many environmental studies. The Yellow Sea and the East China Sea are typical case 2 waters, where concentrations of suspended matter, phytoplankton pigments, and colored dissolved organic matter are higher than those in other open oceans. Two cruises were conducted to investigate the water optical characteristics in the Yellow Sea and the East China Sea in May and June, 2009. 62 water sampling stations of Secchi disk depth were measured in situ in day time, and their values were in the range of 0.0112 to 15.6 m with the mean of 6.72 m and a standard deviation of 3.18 m. In this paper, we adapted a quasi-analytical algorithm to estimate the Secchi depth from satellite ocean data in both coastal and oceanic waters. The development of the algorithm is based on the use of in situ measurements and 8-day MODIS-Aqua remote sensing reflectance data with 4 km spatial resolution. More than 39 matchups were compiled for the MODIS sensor by spatial-temporal matching. The comparison between water transparency retrievals from remote sensing data and in situ measurements yields showed that the determination coefficient was 0.60 and a root mean square error of 8.4 m. This study suggests that the quasi-analytical algorithm provide a promising result on in situ data. In the future, maps of ocean transparency for this area will be derived using this algorithm.