Reducing snow layers feeds harmful algal blooms in the Arabian Sea

An exceptionally resistant organism, almost unknown in the Arabian Sea 20 years ago, has been proliferating and spreading at an alarming rate, forming swirling and thick, smelly green filaments that are visible even from space. This unusual organism is Noctiluca scintillans, a millimeter-sized planktonic organism with an extraordinary ability to survive, thrive, and expel diatoms, the photosynthetic plankton that has traditionally supported the Arabian Sea food web. Noctiluca is not a preferred food for larger organisms, so these large blooms, which recur annually and last for several months, are disrupting the base of the region’s marine food chain, threatening the fisheries that support 150 million people. and possibly exacerbating the increase in criminal piracy. in the region.

New research published this week in Scientific reports from nature describes how continued loss of snow over the Himalayan-Tibetan plateau region is driving the expansion of this destructive algal bloom. Led by Joaquim I. Coming from Columbia University’s Lamont-Doherty Earth Observatory, the study uses field data, laboratory experiments, and decades of NASA satellite imagery to link the emergence of Noctiluca in the Arabian Sea to glaciers. melted and a weakened winter monsoon.

Normally, the cold winter monsoon winds that blow from the Himalayas cool the surface of the oceans. These cooler waters sink and are replaced by nutrient rich waters from below. This convective mix is ​​no different than putting an ice cube in a hot cup of coffee. During this time, phytoplankton, the main producers of the food chain, thrive in the nutrient-rich, sunlit upper layers, and the surrounding countries see an abundance of fish that feed directly or indirectly on phytoplankton. But with shrinking glaciers and snow cover in the Himalayas, monsoon winds blowing offshore from land are warmer and wetter, resulting in decreased convective mixing and less layer fertilization superior.

In this scenario, phytoplankton like diatoms are at a disadvantage, but not Noctiluca. Unlike diatoms, Noctiluca (also known as sea spark) does not depend only on sunlight and nutrients; It can also survive by eating other microorganisms. Noctiluca houses thousands of photosynthetic endosymbionts within its bulbous, transparent, greenhouse cell. Green endosymbionts provide it with energy from photosynthesis, while its tail-like flagella allow it to take any microscopic plankton from the surrounding water as an additional food source.

This dual mode of energy acquisition gives you a great advantage to flourish and disrupt the classic Arabian Sea food chain. The second advantage of Noctiluca is that its endosymbionts accumulate a lot of ammonia in the cell, which makes the organism unpleasant for larger herbivores. As a third advantage, accumulated ammonia is also a nitrogen nutrient reservoir for endosymbionts, making them less vulnerable to decreased nutrient inputs from a weakened convective mixture.

Noctiluca blooms first appeared in the late 1990s. The large size of their blooms, which occur annually, threaten the already vulnerable food chain of the Arabian Sea because its symbiotes not only compete with phytoplankton for nutrients that are replenished annually, they also feed on phytoplankton. However, only jellyfish and salps seem to find Noctiluca palatable. In Oman, desalination plants, oil refineries and natural gas plants are forced to curtail operations because they are clogged by Noctiluca flowers and jellyfish that swarm to feed on them. The resulting pressure on seafood supplies and economic security may also have fueled the increase in piracy in countries like Yemen and Somalia.

“This is probably one of the most dramatic changes we have seen related to climate change,” said Goes who, along with Lamont researcher Helga do Rosario Gomes, has been studying the rapid growth of this 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 flourishes it is becoming a problem. It also hurts water quality and causes high mortality from fishes”.

The study provides compelling new evidence for the cascading impacts of global warming on Indian monsoons, with socioeconomic implications for large populations on the Indian subcontinent and the Middle East.

“Most studies related to climate change and ocean biology focus on polar and temperate waters, and changes in the tropics go unnoticed,” Goes said.

The study highlights how tropical oceans are disproportionately affected, losing their biodiversity and changing faster than conventional model predictions. This may herald dire long-term consequences for countries in the region already affected by the socio-economic problems of war, poverty and loss of livelihoods, Goes said.