Characterization of MODIS-derived euphotic zone depth: Results for the China Sea

Euphotic zone depth (Zeu) products from ocean color measurements are now produced from MODIS ocean color measurements, one of which is based on inherent optical properties (IOP-approach) and the other is based on chlorophyll-a concentration (Chl-approach). For the first time, the quality of these satellite Zeu products is assessed with extensive field-measured Zeu (in the China Sea), where 78% of the measurements were made on the continental shelf (≤ 200 m). For the data with matching location and time window, we have found that the overall average difference (ε) between satellite and in situ Zeu is 21.8% (n = 218, Zeu ranges from 4 to 93 m) with a root mean square error in log scale (RMSE) of 0.118 by the IOP-approach, while it is 49.9% (RMSE = 0.205) by the Chl-approach. These results suggest that 1) MODIS Zeu products for waters in the China Sea are robust, even in shelf waters; and 2) Zeu produced with IOPs are more reliable than those produced with empirically derived Chl. Spatial and seasonal variations of Zeu in the China Sea are briefly described with Zeu products generated by the IOP-approach. These results will facilitate further research on carbon cycling and environmental changes on both local and global scales.