A few years ago, the sea waters on the western coast of India started glowing an electric blue at night. Photos and videos of the same thronged social media, people visited the beaches at night wanting to see it for themselves — the bioluminescence phenomenon.

The electric blue water may be good on the eyes and photographs, but climate change has a huge role to play in this phenomenon. The plankton Noctiluca scintillans is what causes the bioluminescence, said Mahi Mankeshwar, a researcher collaborating with the Noctiluca lab of Lamont Doherty Earth Observatory at Columbia University.

Her current research is about understanding how the advent of the blooms of Noctiluca scintillans, which belongs to a group of planktons ‘dinoflagellates,’ are affecting the Arabian Sea food web. She uses remote sensing to observe the spread of these blooms and has also been conducting surveys with fisherfolk of Maharashtra to understand their perception of these blooms and the ecological events that follow.

Also read: Why scientists think Russia's war could be killing dolphins in the Black Sea

What is Noctiluca scintillans

This type of Noctiluca scintillans is not widely seen in the Arabian Sea. It has been surfacing over the sea waters on the western coast of India for around a decade.

“Earlier, there would be a red variety of the plankton which would be seen seasonally along the southwest Indian coast. However, the green one has become more prevalent in the last few years,” Mankeshwar said.

Also read: Explained: What are nurdles, the tiny plastic bits polluting oceans?

The green type of Noctiluca harbours within its cell thousands of green cells, with which it has a symbiotic relationship. "With the help of its green symbionts, it can feed like a plant because these cells can photosynthesise. Noctiluca can also feed on other plankton such as diatoms. So, these multiple ways of feeding help it proliferate a lot,” Mankeshwar said.

The plankton does not have a lot of animals preying on it and the ones that do, such as salps and jellyfish, are not palatable for most members of the food chain, she added.

Blooms of Noctiluca can potentially do away with native planktonic species which a variety of fish species may feed on. "And if that is not available what will the fish feed on?" she wondered.

Also read: Loud and clear: How noise pollution is impacting marine life

"Ground truthing the environmental parameters when these blooms occur and monitoring coastal waters in the days and weeks following an intense bloom will give us a better idea of how they modify the coastal ecosystems," Mankeshwar added.

When does it occur?

The Noctiluca bloom is generally seen during the summer monsoon and winter monsoon. It occurs across the coast and is carried by the winds to spread across a larger area, she says. “In 2020, a bloom packet originated off Karnataka and because the winds were moving upwards , the bloom kept growing and moved along the coast up north. That is how we saw bioluminescence all the way till Mumbai. That was a massive bloom,” she said.

Causes and consequences of the Noctiluca bloom

Upwelling and hypoxic water: Upwelling is when the deeper layers of the sea that are more productive, carry planktons to the surface. They may seem like a good thing. However, it is fodder for the Noctiluca scintillans. Meanwhile, the Arabian Sea also has layers of hypoxic waters, which are low in oxygen. “These are also deeper layers. While other planktons cannot survive in low oxygen water, Noctiluca scintillans thrives in it,” Mankeshwar said.

Harbouring ammonia: The Noctiluca scintillans have a tendency to harbour ammonia. “The sewage and fertiliser-filled water led into the sea is low in oxygen and loaded with urea," she said, adding that it is a possibility that this too is helping the planktons proliferate.

Fish kills: This occurs when the fish don’t have enough oxygen in the water, so they die. It is one of the more dire consequences of Noctiluca scinitllans, Mankeshwar said. She explained that when there is a Noctiluca scintillans bloom, the cells will decay at some point. And when they do, they use up all the oxygen in the water, so it creates more of a hypoxic situation. So if there is a dense bloom that lasts for long, one of the effects is fish kills. In 2017, there was a massive fish kill off Maharashtra's coast, in Kihim, Alibaug, among other areas where many fish were dying.

Jellyfish bloom: Blooms of jellyfish are expected to increase, following the blooms of Noctiluca scintillans. Mankeshwar's current research investigates if the recent jellyfish blooms that have been occurring are a consequence of these algal blooms and their effects on the fishing communities in Maharashtra.

Impact of the increasing jellyfish population

Reiterating the same, Swapnil Tandel, the Head Marine Biologist at Numer8 Analytics Pvt Ltd, said in the last few years, fisherfolk have reported an increase in jellyfish catch. “It is very problematic for fishermen because where there are jellyfish, commercial fish will be very less,” he says.

Tandel said the jellyfish population has drastically increased in the past decade. “Jellyfish are venomous. They also feed on other fish. Also, jellyfish are not present in a handful of quantity. There will most likely be a bloom of jellyfish. So when that happens, other fish species are not found around them,” he pointed out.

Tandel himself belongs to the fishing community in Maharashtra. He holds a master’s degree in oceanography and fisheries and has worked in the field from 2013. At present, the company he works at has developed an application — N8 Ofish— that helps the seafood industry, including the fisherfolk, be traceable and sustainable. The app provides fisherfolk with fishing zone algal bloom advisories and hyperlocal weather data for efficient and safer navigation at sea, thereby helping them save time and fuel as well, he said.

Other impact of climate change on commercial fishing

The ocean temperatures have risen due to global warming and because of the same, phytoplankton production in the sea has reduced drastically over the last 2-3 decades, said Tandel. Phytoplankton is food for primary-level fish such as sardine and mackerel. This is one reason why most of the fish catch has reduced, he added.

Also read: Ships are dumping bilge water into European waters up to 3,000 times a year, much more than what’s reported

These fish have also been migrating to cooler waters because of the rising ocean temperatures along the west coast of India. Sardine, mackerel, some prawn fish, among others, have drastically reduced due to the same. "However, the quantity of triggerfish has increased over the last few years in Mumbai and Kerala because the temperature is favourable for them,” Tandel said, adding that the fish migrate towards warmer water.

The west coast of India is also at a high risk of coral bleaching. “If the water level rises, its temperature does too and then the coral reefs bleaching starts,” he said.

Corals across the globe are at the risk of bleaching, with the most recent example of a mass bleaching event occurring at the Great Barrier Reef in Australia in February.

The increasing number of cyclones hitting the Arabian Sea in the past few years is also a result of global warming, he says.

He added that these factors have also decreased the fishing days of fisherfolk. “Earlier the fishing days were around 280-300 in a year. Now due to less fish catch, extreme weather events and extended rainfall, boats are going less to the sea, so fishing days have also reduced,” he said.

(Edited by : Shoma Bhattacharjee)