Remotely Sensed Water Quality in Qatari Coastal Waters Between 2002 and 2022

Abstract

Over the past two decades, Qatar has undergone significant economic growth and development, yet little information is available on long-term trends in seawater quality around the Qatar Peninsula. This study analyzed spatiotemporal variations of remotely sensed optical water quality (OWQ) parameters in Qatari coastal waters between 2002 and 2022. These OWQ parameters, including chlorophyll-a concentration (Chla), turbidity (Turb), and Secchi disk depth (SDD), along with sea surface temperature, were derived from Moderate-resolution Imaging Spectroradiometer (MODIS)/Aquaobservations after applying an optically shallow-water mask. Additionally, changes in floating algae scum density, an indicator of harmful algal blooms (HABs), were derived from MultiSpectral Instrument (MSI)observations. Strong nearshore–offshore gradients were generally observed for all OWQ parameters (multiannual mean Chla ∼ 0.6–3 mg m−3; Turb ∼ 0.2–3 FNU; and SDD ∼ 5–12 m). SDD was typically greatest in late spring and summer when Chla and Turb were relatively low. OWQ variability in the main territorial sea was mainly driven by suspended sediments, while in the broader Exclusive Economic Zonewas driven by algal blooms. HABs dominated by Margalefidinium polykrikoides, Noctiluca scintillans, and Trichodesmium spp. were frequently observed in deeper (>20 m) waters. Despite Qatar's massive economic development in recent years, declines in Chla and Turb and increased SDD were observed. Qatari coastal waters, however, are warming at a rate of 0.64 °C/decade, ∼2–3 times faster than neighboring Red Sea and Northern Arabian Sea waters, and ∼8 times faster than the global oceans. This thermal stress may pose future challenges for marine ecosystems and the services they provide.

Keywords

• Long-term trend
• Qatari coastal waters
• remote sensing
• water quality