INSIGHT Green Tide in the Sea of Oman


June 24 2020

Green Tide in the Sea of Oman

Khalid Al-Hashmi

Have you ever wondered why the waves splashing on Oman's beaches are often green, red or luminous? You may have heard that it is algae in the water, but where does this come from? What causes it and what effects does it have on the ecosystem? This insight is from biological oceanographic researcher Khalid Al Hashmi, who works at Sultan Qaboos University, and specialises in Marine phytoplankton productivity, harmful algal blooms and microalgae culture.

Algal blooms are reportedly increasing both in frequency and magnitude in oceanic and coastal waters. Initially, blooms were believed to be restricted mostly to temperate waters, but since the 1990s a similar trend has been observed in tropical and subtropical regions. Coastal ecosystems are becoming more vulnerable to harmful algal blooms (HABs), especially in enclosed coastal embayments, as a result of increased nutrient enrichment caused by urbanization, tourism, industrial wastes, desalination plants, and agricultural activities.

Green Noctiluca bloom

Natural processes, such as water circulation, upwelling relaxation, and cyst formation, are considered important factors contributing to formation of algal blooms. While some algae are known to produce toxins that can be accumulated by filter-feeding organisms making them hazardous for humans, blooms of the other (nontoxic) species can result in high fish mortalities caused by development of low oxygen condition or gill clogging and damage due to mucus secretion and asphyxiation .

Physical-biological interactions in the coastal ecosystems of Oman are driven mainly by seasonally reversing monsoon winds. During the Southwest Monsoon (SWM), southwesterly winds persist during July–September. The winds then become moderate during October–December in a period known as the Fall Intermonsoon (FIM), when shallow mixed layers that are depleted in nutrients occur. During January–March, the Northeast Monsoon (NEM) is characterized by northeasterly winds, less intense than the SWM winds, but sufficient to cause deep mixing. Mixing provides nutrients to upper layers making the productivity of the Arabian Sea relatively high in this season. Later, during April–June, wind speeds are reduced, leading to calm seas and a Spring Intermonsoon season (SIM) of low phytoplankton abundance.


Diagram of Eutrophication;   Red Noctiluca, Al-Bustan

As a result of global warming, snow cover over the Himalayan-Tibetan plateau region has steadily declined.  This loss of snow decreases the strength of the winds and thus destabilizes monsoon mixing and causing stratification of the upper water column of the Arabian Sea. Given that the Arabian Sea has also experienced an intense loss of inorganic nitrate, changes that appear to create a favourable environment to the common Green tide organisms Noctiluca scintillans.

Noctiluca has inflated body (balloon-like) with relatively large size (range from 200-2000 micrometres) and its known for luminescent qualities. Blooms of Noctiluca scintillans are common yearly events in the Sea of Oman as well as in the Arabian Sea. Unlike the red N. scintillans that is found in the temperate coastal waters, in the Arabian Sea, N. scintillans appears green because of the green photosynthetic symbiont that lives in their vacuoles. Blooms develop twice per year—late SWM (September– October) and NEM (January–February)—and cell concentrations are generally higher during the NEM than the SWM, with some interannual variation.

Noctiluca with green symbionts

Noctiluca is less harmful than most harmful algal species, and any fish killed are usually due to low oxygen during the decomposition of the bloom. Noctiluca has relatively high intercellular ammonium which is released to the water when bloom decomposes, leading to increase of the ammonium concentration in the ambient waters  but only in the upper very thin surface layer which is not toxic for most organisms. This gives the unpleasant odour during the bloom. It’s worth mentioning that prior to the mid-1990s, the red Noctiluca was abundant frequent blooms in the Sea of Oman and western Arabian Sea.  

I am a biological oceanographic researcher at Sultan Qaboos University. I earned BSc from James Cook University, Australia, MSc and PhD from Sultan Qaboos University. My work interest is Marine phytoplankton productivity, harmful algal blooms’ causes and effects and microalgae culture for biodiesel and protein production. I've participated in Oceanographic Cruises; International ARABESQUE Oceanographic Cruise in the Arabian Sea (lead by British scientist, 1994), ROPME cruise in the sea of Oman and the Arabian Gulf and in the French Hallaniyat Expedition, Arabian Sea.  I strengthened relations with international instituted to enhance research in the field of Oceanography. These collaborations have resulted in several joint research and publications in pioneer international oceanographic journals, like Frontiers in Marine Science and Marine Pollution Bulletin as well as book chapters. I published phytoplankton taxonomy book for the coastal water of Oman.

Khalid Al-Hashmi

Sultan Qaboos University, Department of Marine Science and Fisheries, Sultanate of Oman

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