Low oxygen triggers massive bloom outbreak in Arabian Sea

Scientists studying massive outbreaks of a new type of phytoplankton Noctiluca scintillans in the Arabian Sea report that the unnatural blooming event could be a result of oxygen-deficient waters and the plankton’s ability to survive in such adverse conditions¹.

As to why the water is turning oxygen-deficient or hypoxic, the scientists have no clear answers but they suspect that rampant discharge of untreated sewage and other forms of polluted water into the sea could be the reason.

The team of scientists from Columbia University’s Lamont Doherty Earth Observatory and Goa-based National Institute of Oceanography (NIO) and Goa University has been studying the flourishing Noctiluca population since 2009. They say the main drivers of the outbreak are the low oxygen waters and Noctiluca 's ability to photosynthesize food efficiently with the help of an organism that lives within its body – the 'endosymbiont' Pedinomonas noctilucae.

Menacing Noctiluca blooms have been appearing every year from January to March in the Arabian Sea with predictable regularity. First noticed in the early 2000s, the blooms have been spreading at a fast rate. Noctiluca is a large plankton which gets its green colour and unusual photosynthetic abilities from its endosymbiont.

The scientists sifted through historical data to find that Noctiluca was not prevalent in the Arabian Sea before the 2000s and common diatoms were the dominant phytoplankton during winter months. The appearance of Noctiluca in early 2000s coincided with oxygen deficiency in the Arabian Sea waters.

Between 1965 and late 1990s, oxygen saturation was always close to 100% or higher in this region. Since early 2000s, the saturation has fallen to below 70%.The scientists also found that Noctiluca 's ability to photosynthesize increased by 25-300% in hypoxic waters, in contrast to diatoms that showed a 3-fold decrease in photosynthetic ability in such waters. Apart from deriving its nutrition from photosynthesis, Noctiluca also feeds on the diatoms, directly competing with other predators of diatoms, the scientists report.

Globally, various such examples of change in the composition of marine habitats and species have been studied but they are mostly in the coasts. The “special cause for concern” in the Arabian Sea is that changes in its biota are occurring offshore and on a basin-wide scale, the scientists point out.

The biggest impact of the emergence of Noctiluca is a threat to the regional fishery-based livelihoods. The blooms are disrupting the traditional food chain. Earlier, diatoms were eaten by zooplanktons, which in turn were the food for small fish. Now, diatoms are being eaten by Noctiluca, which becomes the food for salps or jelly fish, explains S G Prabhu Matondkar, Emeritus Scientist at NIO, Goa.

Small fish prefer zooplanktons over salps or jelly fish, so this change affects their diet and yield. These small fish are a staple coastal food as also a means of livelihood for the local fishing community. A coastal population of 120 million people could be threatened if the region’s fishery collapses, the researchers estimate.