On the continuity of quantifying floating algae of the Central West Atlantic between MODIS and VIIRS

Studying abundance and distributions of floating macroalgae such as pelagic <i>Sargassum</i> calls for long-term continuous and consistent observations from multiple satellite sensors. Previous studies mainly relied on observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Medium Resolution Imaging Spectrometer (MERIS). As a follow-on sensor, the Visible Infrared Imager Radiometer Suite (VIIRS) also has the appropriate spectral bands to detect and quantify floating macroalgae. Based on previous works on MODIS, this study presents an improved procedure to extract floating algae pixels from VIIRS Alternative Floating Algae Index (AFAI) imagery, with image filtering used to suppress noise and adjusted thresholds used to mask sun glint, clouds, and cloud shadows. The overall extraction accuracy is about 85%. Simultaneous daily observations from MODIS and VIIRS over the Central West Atlantic (CWA) show consistent spatial patterns, but VIIRS estimations of the algae coverage (in km²) are consistently lower than MODIS (around – 19% mean relative difference or MRD), possibly due to lower sensitivity of the VIIRS near-infrared (NIR) bands than the corresponding MODIS bands. Similarly, at monthly scale VIIRS also shows lower coverage than MODIS, and their difference (around – 29% MRD) is larger than the difference between MODIS-Aqua and MODIS-Terra estimates (around – 14% MRD). Despite these differences, the spatial and temporal patterns between VIIRS and MODIS observed algae distributions match very well at all spatial and temporal scales. These results suggest that VIIRS can provide continuous and consistent observations of floating algae distributions and abundance from MODIS as long as their differences are accounted for, thus assuring continuity in the future. Furthermore, once <i>Sargassum</i> biomass per unit <i>Sargassum</i> area is determined from field measurements, conversion of these area estimates to <i>Sargassum</i> biomass is straightforward.

针对远洋马尾藻（Sargassum）等漂浮大型藻类的丰度与分布开展研究，需要借助多颗卫星传感器进行长期连续且一致的观测。既往研究主要依赖中等分辨率成像光谱辐射计（Moderate Resolution Imaging Spectroradiometer, MODIS）与中等分辨率成像光谱仪（Medium Resolution Imaging Spectrometer, MERIS）的观测数据。作为后续在轨传感器，可见红外成像辐射计套件（Visible Infrared Imager Radiometer Suite, VIIRS）同样配备了适用于检测与量化漂浮大型藻类的光谱波段。本研究基于MODIS相关既往研究，提出了一种从可见红外成像辐射计套件替代漂浮藻类指数（Alternative Floating Algae Index, AFAI）影像中提取漂浮藻类像元的改进方法：通过图像滤波抑制噪声，并调整阈值以掩除太阳耀斑、云层及云影。该方法的整体提取精度约为85%。对中西大西洋（Central West Atlantic, CWA）海域开展的MODIS与VIIRS同步逐日观测显示，二者的藻类空间分布格局具有一致性，但VIIRS估算的藻类覆盖面积（单位：平方千米）始终低于MODIS结果，平均相对差（mean relative difference, MRD）约为-19%；这一差异可能源于VIIRS近红外（near-infrared, NIR）波段的灵敏度低于对应MODIS波段。类似地，在月度尺度上，VIIRS估算的藻类覆盖面积同样低于MODIS，二者的平均相对差约为-29%，这一差值大于MODIS-Aqua与MODIS-Terra两种传感器估算结果之间的差值（约-14% MRD）。尽管存在上述差异，VIIRS与MODIS观测到的藻类分布时空格局在所有时空尺度上均匹配度极高。上述结果表明，只要对二者的系统差异进行校正，VIIRS即可延续MODIS的观测序列，为漂浮藻类的分布与丰度提供长期连续且一致的观测数据，从而保障未来观测的连续性。此外，若通过野外实测确定单位马尾藻面积对应的生物量，即可将上述覆盖面积估算值直接转换为马尾藻生物量。