# Data-Free Area Detection and Evaluation for Marine Satellite Data Products
## 4 Discussion
### Spatial and Temporal Distribution of Marine Coverage Objects Contained in Data-Free Areas
Figure 13: Schematic diagram of the month-by-month change in marine objects in the data-free areas: (a) GOCI, (b) MODIS and (c) OLCI. As a surprising result of the year-round products, GOCI has missing information for all areas in the Yangtze River Delta region, whereas MODIS has missing information only to a lesser extent. OLCI is missing only the nearshore portion (Figure 16), retaining the majority of the muddy water pixels. The largest turbidity zone in China exists in the Yangtze River Delta. The average depth of this shallow coastal zone is only 5-10 m. The high contribution of suspended particles is the main reason for the formation of turbidity zones, particularly the transport of Yangtze River currents, on the one hand, and the accumulation and resuspension of sediments in the Yellow Sea and East Sea due to the regional circulation of water masses on the other hand [24]. A large number of suspended particles accumulate in abundance to form the largest turbidity zone in China. However, suspended particles in the red and near-infrared wavelengths have a higher reflectivity than clean water, and they manifest near-cloud characteristics. These scenarios increase the difficulty of cloud detection and cause large losses among the products. Except for the Yangtze River Delta region, the three kinds of data have frequent missing information for Liaodong Bay, Bohai Bay and Laizhou Bay (Figure 17a,d). GOCI has missing turbid and clean water information in these areas. MODIS is dominated by turbid water with less missing information compared with GOCI. The missing information in OLCI is negligible for the aforementioned three bay waters, and the quality of the product in this region is much better than those of MODIS and GOCI. The reason for the difference  
Figure 14: Schematic diagram of the missing position of altocumulus translucidus: (**a**,**d**,**g**) the true color images at the spatial locations of the altocumulus translucidus for GOCI, MODIS and OLCI data, respectively; (**b**,**e**,**h**) the condition of the data-free area; (**c**,**f**,**i**) the identification status of the marine objects in the data-free area. is that, in 2019, the Bohai Sea did not meet the first-category seawater quality standards in the ocean area of 12,740 square kilometers, accounting for about one-seventh of the entire Bohai Sea. The heavy eutrophic sea area was mainly concentrated in the denser population and more complex marine ecological environment of the three bays. The high planktonic biomass and heavy eutrophication conditions caused by anthropogenic nutrient inputs and fishing activities in coastal waters [66] can serve as the main reasons for the lack of product information in the waters of those areas. The Bohai Sea is the highest latitude sea area in China. Owing to its special geographical location and the presence of cold air, the area suffers from different degrees of sea ice disasters in winter. The main geographic location of disasters is Liaodong Bay, and the ice period of sea ice is one to three months (December to February), which is annual sea ice [67]. The physicochemical properties of sea ice differ from those of clean water, but it is similar to clouds in remote sensing images with respect to color, temperature and highly reflective spectral properties [68; 69]. Therefore, the product corresponding to the location of the sea ice hazard also suffers from missing information, i.e., the products suffer from missing information at that specific geographical location during the annual ice season. The identification of sea ice is much more difficult because of the similarity between sea ice and clouds in various aspects, and the identification effect is usually unsatisfactory.