In search of floating algae and other organisms in global oceans and lakes

摘要

Surface floating macroalgae, microalgae, and other marine and freshwater organisms have been reported in many specific regions around the globe. However, it is technically challenging to identify similar occurrences, or other types of floating organisms or materials, within the vast global oceans and lakes. In this study, we address this challenge through combining global-scale, 375-m resolution false-colored Red-Green-Blue (FRGB) imagery from the Visible Infrared Imaging Radiometer Suite (VIIRS) in NOAA's online Ocean Color Viewer (OCView) for visual inspection and data from several other satellite sensors for spectral diagnostics. In the FRGB imagery, the near-infrared (NIR) band (862 nm) is used as the green channel, which is sensitive to floating algae and organisms/materials on the water surface. Visual inspection of the daily FRGB VIIRS imagery from January 2018 to October 2019 reveals the appearance of various slicks with different colors in many ocean regions and lakes. Combined with spectral diagnostics of the quasi-concurrent Sentinel-3A/3B Ocean and Land Colour Instrument (OLCI) and other higher spatial resolution satellite data as well as knowledge of local oceanography/limnology, most of these elongated or diffuse image features can be identified as Ulva, Sargassum, Noctiluca, Trichodesmium, Microcystis, oil slicks, or pumice rafts. Some of these identified features are found in regions where such occurrences have never been reported before. Some features are of unknown type as they have not been reported previously from remote sensing. In such cases, contemporaneous scientific literature and news reports as well as spectral diagnostics allow for educated inferences to be made. One example is from surface features in the northern Gulf of Maine, Bay of Fundy, and Southwest of Nova Scotia between early June and early July of 2019. Spectral shapes of the FRGB image features indicate transparent materials lacking pigments. Knowledge of local fisheries oceanography and frequent news reports all suggest that these unknown image features may be aggregations or blooms of sea jellies and, to a lesser extent, salps. Another example is from the Great Salt Lake, where image features in the south arm of the lake are speculated to be caused by aggregations of brine shrimp eggs. Once confirmed from field sightings, these findings may represent a milestone in satellite remote sensing because previously remote sensing could only be used to infer oceanographic environments conducive to sea jellies or shrimps. In addition to the findings presented here, the approach in this study may serve as a template to discover various known and unknown types of floating algae and organisms/materials as well as to routinely monitor and track their distributions and movements.