Satellite monitoring of massive green macroalgae bloom (GMB): imaging ability comparison of multi-source data and drifting velocity estimation

Abstract

A massive floating green macroalgae bloom (GMB) has occurred for several years consecutively in the Yellow Sea since 2007. In view of the rapid growth of green macroalgae, early detection of its patches at first appearance by satellite imagery is of importance, and the central issue is the selection of appropriate satellite data. As a first step towards this goal, based on quasi-synchronous satellite images of HJ-1A/B (China Small Satellite Constellation for Environment and Disaster Monitoring and Forecasting) charge-coupled devices (CCDs), Environmental Satellite (ENVISAT) Advanced Synthetic Aperture Radar (ASAR) and TERRA Moderate Resolution Imaging Spectroradiometer (MODIS), GMB monitoring abilities by these data were compared. The average percentage difference (APD) of the GMB areas derived by ASAR and CCD was less than 15%, which may be partly attributed to the inability of synthetic aperture radar (SAR) data to detect macroalgae suspended beneath the sea surface. The macroalgae area extracted by MODIS was over two times of that extracted by CCD, which was mainly explained by the difference in their spatial resolutions (250 vs 30 m). The effects of the configuration of sensor bands and the aerosol optical properties on the comparison result were found to be negligible, and the underlying reason is analysed by atmosphere radiative transfer modelling. With satellite images, the drifting velocity of macroalgae patches was estimated to be about 0.21 m s -1 , which was in agreement with the surface current field numerically simulated by the Hybrid Coordinate Ocean Model (HYCOM). It indicates that numerical modelling can aid in deduction of the situation of the patches when satellite data are not available, and on the other hand, satellite data can be used to estimate sea-surface currents through monitoring the movement of green algae. By a comprehensive comparison of available satellite data in operation, for the early detection of macroalgae patches and warning of a massive bloom, CCD data from the HJ-1A/B constellation was preferred, with 30 m spatial resolution, 700 km swath width and 2 day revisiting period. SAR data may be an effective supplement, which can avoid the effects of bad weather (cloud, fog and haze) on optical satellite monitoring.

Long-term trend of Ulva prolifera blooms in the western Yellow Sea

“…Numerous studies have documented their growth cycles, seasonality, transport patterns, and their linkage with local aquaculture, eutrophication, and ocean circulations (Hu et al, 2010;Liu et al, 2009Liu et al, , 2013Wang et al, 2015). However, to date, although a number of studies used remote sensing to estimate their spatial distributions and temporal changes, nearly all focused on how to effectively detect the presence of the macroalgae (Cui et al, 2012;Hu, 2009;Hu et al, 2010;Keesing et al, 2011;Lee et al, 2011;Garcia et al, 2013;Bao et al, 2015;Xing and Hu, 2016). Several studies used individual images to show the maximum coverage during the bloom season or the maximum impacted water area to show the severity (Keesing et al, 2011;Hu et al, 2010;Garcia et al, 2013;Xu et al, 2014;Xing et al, 2015a,b).…”

World’s Largest Macroalgal Blooms Altered Phytoplankton Biomass in Summer in the Yellow Sea: Satellite Observations

“…Several indices based on these characteristics were used to delineate the U. prolifera macroalgae slicks, for example the NDVI index [3,5,9,16] and the Floating Algae Index [4]. In this study, Level-1 MODIS aqua data of calibrated total radiance at 1-km resolution (downloaded from the National Aeronautics and Space Administration-NASA, https://ladsweb.nascom.nasa.gov) were mapped to an equidistant cylindrical projection using nearest neighbor re-sampling, and then used to calculate the top of atmosphere (TOA) reflectance (R, unitless).…”

Validation of MODIS-based monitoring for a green tide in the Yellow Sea with the aid of unmanned aerial vehicle

“…20 With the resolution of 250 m, the area of MODISderived Ulva patches was overestimated by an order of two times. 28 By contrast, the compared areal extent varied by and order of magnitude indicating that MODIS images would underestimate the distribution of floating Ulva blooms. As shown in Table 1, the detection performance for small patches was low, while it was up to 100% for the station sites with no green algae patches.…”

“…Based on the drift velocity of the green tide, we evaluated the buffer distance using the observation times and the sea surface wind (SSW) speed. According to previous studies, 28,30 the buffer radius is defined by Eq. (2): E Q -T A R G E T ; t e m p : i n t r a l i n k -; e 0 0 2 ; 1 1 6 ; 4 8 0…”