Red Sea MODIS Estimates of Chlorophyll a and Phytoplank ‐ ton Biomass Risks to Saudi Arabian Coastal Desalination

Harmful algal blooms (HABs) and the high biomass associated with them have afflicted marine desalination plants along coastal regions around the world. Few studies of HABs have been conducted in the Red Sea, where desalination plants along the Saudi Arabian Red Sea coast provide drinking water for millions of people. This study was conducted along the Saudi Arabian Red Sea coast from 2014 to 2015 to assess the potential for using Moderate Resolution Imaging Spectroradi‐ ometer (MODIS) remote sensing of chlorophyll a (Chl a) or fluorescence line height (FLH) to iden‐ tify risks for biofouling at these desalination plants. Ship‐based surveys of phytoplankton were con‐ ducted along the Saudi Arabian coastline offshore of desalination plants at Jeddah, Al Shoaibah and Al Qunfudhuh to assess the density of phytoplankton populations and identify any potential HAB species. Ship‐based surveys showed low to moderate concentrations of phytoplankton, averaging from 1800–10,000 cells L−1 at Jeddah, 2000–11,000 cells L−1 at Al Shoaibah and 1000–20,500 cells L−1 at Al Qunfudhuh. Sixteen different species of potentially toxigenic HABs were identified through these surveys. There was a good relationship between ship‐based total phytoplankton counts and monthly averaged coastal MODIS Chl a (R2 = 0.49, root mean square error (RMSE) = 0.27 mg m−3) or FLH (R2 = 0.47, RMSE = 0.04 mW m−2 μm−1 sr−1) values. Monthly average near shore Chl a concen‐ trations obtained using MODIS satellite imagery were much higher in the Red Sea coastal areas at Al Qunfudhuh (maximum of about 1.3 mg m−3) than at Jeddah or Al Shoaibah (maximum of about 0.4 and 0.5 mg m−3, respectively). Chlorophyll a concentrations were generally highest from the months of December to March, producing higher risks of biofouling desalination plants than in other months. Concentrations decreased significantly, on average, from April to September. Long‐ term (2005–2016) monthly averaged MODIS Chl a values were used to delineate four statistically distinct zones of differing HAB biomass across the entire Red Sea. Sinusoidal functions representing monthly variability were fit to satellite Chl a values in each zone (RMSE values from 0.691 to 0.07 mg m−3, from Zone 1 to 4). December to January mean values and annual amplitudes for Chl a in these four sinusoidal functions decreased from Zones 1–4. In general, the greatest risk of HABs to desalination occurs during winter months in Zone 1 (Southern Red Sea), while HAB risks to desal‐ ination plants in winter months are low to moderate in Zone 2 (South Central Red Sea), and negli‐ gible in Zones 3 (North Central) and 4 (Northern). Citation: Gomaa, M.N. ; Mulla, D.J.; Galzki, J.C. ; Sheikho, K.M. ; Alhazmi, N.M. ; Mohamed, H.E. ; Hannachi, I. ; Abouwarda, A.M.; Hassan, E.A. ; Carmichae, W.W. Red Sea MODIS Estimates of Chlorophyll a and Phy‐ toplankton Biomass Risks to Saudi Arabian Coastal Desalination Plants. J. Mar. Sci. Eng. 2020, 9, 11. https://doi.org/10.3390/jmse9010011 Received: 29 November 2020 Accepted: 17 December 2020 Published: 23 December 2020 Publisher’s Note: MDPI stays neu‐ tral with regard to jurisdictional claims in published maps and insti‐ tutional affiliations.