For most of November, come night, and the waters across parts of the west coast gave out a blue shimmery glow that left us in rapture and wonderment. While these eye-catching, glow in the dark waters have spawned a lot of speculation one thing we know for certain is that is the glow is emitted by a planktonic organism known as noctiluca scintillans.

This organism like the fireflies (Kajva in Marathi) that we see in our gardens, contains a light-emitting pigment called luciferin, which reacts when the cells are buffeted and agitated by waves to emits light that makes the waters look an eerie blue.

But did you know that these waters that seem so ethereal and blue at night are really a murky green in the day? Far from a scene out of the Life of Pi, the waters have a greater likeness to the “about to barf” WhatsApp emoji, both in sight and sentiment.

The murky green waters during an intense bloom in the Arabian Sea. Photo credit: Author provided/ helgagomes.com

Over the past few years, these large-scale noctiluca blooms have been occurring with an almost seasonal regularity around this time of the year.

What is the story behind these waters and why are we seeing them now? The answers lie in the fast-changing climate of the adjoining Arabian Sea.

Seas and oceans harbour tiny microscopic cells called plankton and depending on their ability to photosynthesize they are either classified as phytoplankton (oceanic plants) or zooplankton which fed on phytoplankton. These single-celled organisms at the base of the food chain are critical because like plants on land they sustain all marine life, large and small.

Cause of sudden appeareance

So why has noctiluca suddenly made its appearance in the Arabian Sea in the last two decades? And what has triggered a transition from the common phytoplankton known as diatoms to these more invasive and aggressive cells? The answer, amazingly, is tied to the far off snowcapped Himalayan mountains.

A 2020 study has shown that since 2001 the Himalayan glaciers have been melting at an alarming rate due to global warming. This, in turn, has set off a complex interplay of temperature, currents, winds and sea conditions in the Arabian sea that has turned the waters extremely advantageous for the proliferation of noctiluca.

While occasional blooms of a red form of noctiluca have always been seen along the south-west coast of India, the sudden advent of the green type, which contains thousands of tiny green phytoplankton inside its bulbous cell forecast a grim scenario.

Like plants, these green cells can photosynthesize, turning sunlight into food and energy. But like animals, this noctiluca can also graze on other plankton species including the native, diatoms. And no, their buffet does not end there, in extreme conditions noctiluca spits out its green symbionts and cannibalise on them. It is this array of feeding mechanisms that has turned noctiluca, with a cell size of barely a millimetre, into an invader of epic proportions.

Cells of noctiluca scintillans with its green symbionts. Photo credit: Abhishek Jamalabad

The two-way hypoxic street

The armoury of advantages hardly ends there for these glowing creatures. The little green cells within noctiluca have a legacy of 1.3-billion years when the earth had much lower levels of oxygen. This gives them the ability to thrive in hypoxic conditions (low oxygen) that most other organisms cannot withstand. Add to this the fact that the Arabian Sea inherently has low oxygen in its deeper waters which gives noctiluca an upper-hand over other cells and helps it bloom. But not just that, as these dense blooms die off, they end up using the available oxygen, making for a win-win situation for noctiluca but not so much for other members of the food chain.

Consequences and climate change

These blooms that were first observed in the early 2000s was mostly restricted to the open waters of the Arabian Sea. Since then the intensity and size of these floating green mats have steadily increased, making their presence felt along our coastlines. Apart from lighting up our shores, these blooms leave in their wake a series of effects which are cascading swiftly up the trophic chain and making their way right up to us.

Noctiluca being invasive is not the favoured prey species for most marine animals. Jellyfish and salps are known to predate on them and consequently, events of noctiluca blooms are increasingly being followed by bloom events of jellyfish in coastal waters. And these events are alarming because fishermen in coastal waters end up hauling shoals of jellies that hardly have economic value but the process does take up a lot of manpower and fuel.

In Oman, a country especially affected by these blooms, desalination plants have had to shut down because dense cell and jellyfish patches clog up entire pipelines. Fish-die-offs which are an extreme consequence of hypoxia are being reported time and again from the Arabian Sea. These series of events are an example of an ecosystem transitioning under the influence of climate change and which can potentially affect the livelihoods of millions.

So far, the mental image of climate change for most of us has been that of melting glaciers and receding ice-lines. It is time that those images are also accompanied by something that is closer to home and more tropical. Like something that glows in the dark.

Mahi Mankeshwar is an independent researcher interested in understanding the transition of marine communities under a globally changing climate.