Snow loss fuelling harmful algal blooms

Continued loss of snow over the Himalayan-Tibetan Plateau region is fuelling the rapid expansion of destructive algal blooms in the Arabian Sea, new research suggests.

Noctiluca scintillans, a tiny planktonic organism, was virtually unheard of in the area 20 years ago but now forms thick green swirls and filaments that can be seen from space, according to a team led by Columbia University, US. 

Writing in the journal Scientific Reports, they say the organism, also known as sea sparkle because of its striking bioluminescence, has an extraordinary capacity to thrive and force out diatoms, the photosynthesising plankton that have traditionally supported the local food web.

The large blooms occur annually and last for several months, disrupting the base of the region’s marine food chain, threatening fisheries that sustain 150 million people.

And it’s not a problem specific to the Middle East, says lead author Joaquim Goes who, with colleague Helga do Rosario, has been studying the organism for more than 18 years.

“We are seeing Noctiluca in Southeast Asia, off the coasts of Thailand and Vietnam, and as far south as the Seychelles, and everywhere it blooms it is becoming a problem,” he says. “It also harms water quality and causes a lot of fish mortality.” 

In the latest study, the researchers used field data, laboratory experiments and NASA satellite imagery to link the rise of N. scintillans in the Arabian Sea with melting glaciers and a weakened winter monsoon.

Normally, cold winter monsoon winds blowing from the Himalayas cool the surface of the oceans, Goes says. Colder waters sink and are replaced with nutrient-rich waters from below, and during this time phytoplankton – and thus also fish – thrive. 

But with the shrinking of glaciers and snow cover in the Himalayas, the monsoon winds blowing offshore from land are warmer and moister, resulting in diminished convective mixing and decreased fertilisation of the upper layers.

Phytoplankton then struggle but Noctiluca doesn’t, because it doesn’t rely only on sunlight and nutrients, Goes says. It can survive by eating other microorganisms. It also accumulates ammonia, which helps it survive but deters larger grazers.

Only jellyfish and salps seem to find Noctiluca palatable, and that brings problems beyond the environmental damage.

The researchers say desalination plants, oil refineries and natural gas plants in Oman are forced to scale down operations because they are choked by Noctiluca blooms and the jellyfish that swarm to feed on them.