Characterization of MODIS-derived euphotic zone depth: Results for the China Sea Shaoling Shang a,b, ⁎, Zhongping Lee c , Guomei Wei a a State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen 361005, China b Key Laboratory of Underwater Acoustic Communication and Marine Information Technology (Xiamen University), Ministry of Education of China, Xiamen 361005, China c Geosystems Research Institute, Mississippi State University, MS 39529, USA abstract article info Article history: Received 4 June 2010 Received in revised form 4 August 2010 Accepted 24 August 2010 Keywords: MODIS Remote sensing China Sea Euphotic zone depth Chlorophyll-a Euphotic zone depth (Z eu ) 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 Z eu products is assessed with extensive field-measured Z eu (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 Z eu is 21.8% (n = 218, Z eu 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 Z eu products for waters in the China Sea are robust, even in shelf waters; and 2) Z eu produced with IOPs are more reliable than those produced with empirically derived Chl. Spatial and seasonal variations of Z eu in the China Sea are briefly described with Z eu products generated by the IOP-approach. These results will facilitate further research on carbon cycling and environmental changes on both local and global scales. © 2010 Elsevier Inc. All rights reserved. 1. Introduction Most marine phytoplankton photosynthesize in the euphotic zone (Kirk, 1994). Although the disphotic zone has recently attracted increasing attention (Buesseler et al., 2007), the euphotic zone is nevertheless the most important zone in the context of both ecosystem dynamics (Platt & Sathyendranath, 1988) and in air–sea interaction through transfer of either heat (Sathyendranath et al., 1991) or gases, especially with regards to green house gases such as carbon dioxide (CO 2 )(Takahashi et al., 2002). In practice, the euphotic zone depth (Z eu ) is defined as the depth at which the PAR (photosynthetic available radiation) value is 1% of the surface value (Kirk, 1994). The value of Z eu is an important input parameter for many models that estimate basin-scale primary production (Behren- feld & Falkowski, 1997a,b). Both Secchi disk depth (Preisendorfer, 1986; Tyler, 1968) and Z eu measure water clarity, but Z eu can be measured objectively with advanced electro-optical instruments. Because Z eu is a cumulative measure of biogeochemical properties in the upper-water column, changes in the Z eu depict environmental patterns that might be associated with climate. Thus it is not surprising to see that Z eu is included as a standard ocean color product in the Global Change Observation Mission (Japan, http:// suzaku.eorc.jaxa.jp/GCOM_C ) and that Z eu products for the global ocean are now derived from MODIS (Moderate Resolution Imaging Spectroradiometer) measured water-leaving radiance. Numerous studies have been carried out to evaluate the quality of satellite- derived chlorophyll concentration (Chl) and the diffuse attenuation coefficient (Bailey & Werdell, 2006; Darecki & Stramski, 2004; Melin et al., 2007; Zhang et al., 2006). While it is important to know the quality of satellite-derived products before they can be further applied to the study of biogeochemistry and environmental changes, there hasn't been any published independent research (to our knowledge) to evaluate or characterize the quality of MODIS-derived Z eu . In general, there are two approaches to estimate Z eu from ocean color remote sensing (Lee et al., 2007; Morel et al., 2007). The simple Chl-approach is an empirical method based on Case-1 assumptions (Morel et al., 2007), where Z eu is calculated from concentration of chlorophyll-a (Chl). A different approach (IOP-approach) is centered on waters' inherent optical properties (IOPs) (Lee et al., 2007); Z eu is computed after the absorption and backscattering coefficients at 490 nm are provided. The inputs, either Chl or IOPs, can be obtained from water samples or from the inversion of measured water color (Gordon & Morel, 1983; IOCCG, 2006). Results from both approaches have been tested or validated with ship borne measurements (Lee et al., 2007; Morel et al., 2007), but there hasn't been any test or evaluation of these products when they are derived from satellite data. In this study, using Z eu data measured in the China Sea over the past six years, we assessed the Z eu products (from both Chl-approach and IOP-approach) derived from MODIS measured ocean color. These measurements were mainly made in the shelf water (≤200 m) to Remote Sensing of Environment 115 (2011) 180–186 ⁎ Corresponding author. State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen 361005, China. Tel./fax: + 86 592 2184781. E-mail addresses: slshang@gmail.com, slshang@xmu.edu.cn (S. Shang). 0034-4257/$ – see front matter © 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.rse.2010.08.016 Contents lists available at ScienceDirect Remote Sensing of Environment journal homepage: www.elsevier.com/locate/rse