A disrupted ecosystem in the Arabian Sea


How can the snow cover of the Himalayas influence the species that thrive in the Arabian Sea? How could changes in wind speed and humidity cause food and national security issues thousands of miles away? Joaquim Goes, Helga do Rosario Gomes and their colleagues from two continents have spent the past two decades trying to decode these puzzles.

The story begins in the early 2000s, when NASA Aquatic satellite Was launched. Goes, an ocean remote sensing specialist, was examining data from SeaWiFS and Aqua. He focused on chlorophyll-a, a pigment used by ocean phytoplankton (and plants around the world) to harness sunlight and turn it into food energy. He was focusing on observations of phytoplankton populations in the Arabian Sea during the summer monsoon, but by chance he looked at the winter data. There was much more chlorophyll-a than one might reasonably expect.

At first Goes thought it was a mistake. But over the next decade, reports of an increase in algae and a decrease in fish catches came from colleagues in South Asia. Goes and Gomes made several sea expeditions and saw it for themselves: the Arabian Sea was teeming with Shimmering Noctiluca, a poorly reported organism in the region in previous winters.

The image above shows a flowering of Shimmering Noctiluca in 2019, as observed by the NOAA-NASA Suomi NPP satellite. Floating microscopic organisms are dinoflagellates living in a symbiotic relationship with green algae cells. Like oceanic phytoplankton, Shimmering Noctiluca can multiply quickly under the right conditions. (Noctiluca often thrive on low oxygen “hypoxic”Waters.) Drifting with the currents, they aggregate into vast masses near the surface. In the process, they can deplete oxygen from the sea, compete with other phytoplankton for nutrients or consume them for food, and suffocate small predators of hypoxic zooplankton.dead zones. ”

“The changes we have observed in the Arabian Sea ecosystem are among the fastest of any ocean body on our planet,” said Goes, a scientist at Lamont-Doherty Earth Observatory. “The habitat of the sea is changing, and that bypasses the food chain.”

How and why Noctiluca flourished in the Arabian Sea is a complicated story of interconnections between Terrestrial systems and the unexpected ripples that spread from global warming.

Throughout human history, the Arabian Sea has been strongly influenced by monsoon winds which change direction seasonally and change the direction of ocean currents. In past winters, air temperatures over the Himalayan Tibetan Plateau and southern Asia would drop significantly and cause dry northeast winds over the Arabian Sea. In turn, the cooling of surface water and changes in density would propagate through the water column, shifting the pycnocline—Where the density of water changes due to salinity and / or temperature — up and down. The depth of this oceanic layer affects how nutrients good from the depths and feed the growth of phytoplankton.

These winter changes in currents and nutrient availability once fueled blooms of diatoms, another type of phytoplankton. Diatoms were a key link in an oceanic food chain that fed the copepods and fish during the winter and, ultimately, the humans who caught these fish.

But with global warming in recent decades, less snow has fallen and accumulated on the Himalayan-Tibetan plateau and more snow and ice has melted. Temperatures in the highlands and lowlands have increased, as has humidity. Over the past two decades, winter winds blowing over the Arabian Sea have become warmer, calmer and wetter. As a result, the seas are less rough and there are fewer nutrients for diatoms and most other phytoplankton.

“With calmer, warmer winds and waters, there is less ventilation and mixing,” said Helga do Rosario Gomes, oceanographer biologist, also at Lamont-Doherty. “This leads to more stratification and less nitrate enrichment from below. In some cases, this causes hypoxia.

These changes were about perfect for Shimmering Noctiluca. Unlike diatoms, Noctiluca can thrive when there are fewer nutrients dissolved in the water. The graphs above show the coincident changes from 1980 to 2018 in the extent of snow cover on the Himalayan-Tibetan plateau, the depth of the mixing layer in the Arabian Sea in winter, and the concentration of chlorophyll-a (an indicator of phytoplankton). The ‘anomaly’ plots show how much each year was above or below the long-term average for each variable. Snow extent and mixing depth have steadily decreased, while winter blooms have increased.

“The changes observed in the Arabian Sea are an example of potential ecosystem changes induced by climate change,” said Laura Lorenzoni, scientist in the Ocean Biology and Biogeochemistry program for NASA. “As the Earth warms, we can expect greater ocean stratification and species migration to the poles. There will also be a greater chance of harmful algal blooms and some more resilient species surpassing others and altering the entire structure of the ecosystem. “

Scientists have modeled and speculated for years that global warming could alter the snow and ice cover on the Himalayas and the Tibetan Plateau and that the effects could spill over to the sea. The belief was that the Arabian Sea would become less productive from December to March. Instead, he became more productive, but for an entirely different set of creatures.

“There are a lot less diatoms now, and therefore there is a clear loss of biodiversity,” Gomes said. “Before, there were more copepods, sardines, jacks, mackerel and pelagic fish. “Plankton and diatoms have been replaced by mats of Shimmering Noctiluca and an overabundance of jellyfish and salps. Finfish have been replaced by turtles, squid, and animals that can survive in low oxygen environments.

In a 2020 research paper, Goes and Gomes used ocean color data from NASA and snow and ice cover data from the National Snow and Ice Data Center to piece the puzzle together. They found that winter chlorophyll-a in the Arabian Sea has been increasing steadily since the 1990s, up to four times higher in some winters. Chlorophyll-a is a key pigment in oceanic phytoplankton, including Shimmering Noctiluca. The map above shows the trend, mostly upward, in the Arabian Sea from 1996 to 2018.

The result is a problem for fishing, especially in an area with a lot of artisanal and subsistence fishing. “We are passing a tipping point,” Goes said. “The food chain has been turned upside down.

The changes are problems for the people of the Middle East, East Africa and South Asia. An estimated 150 million people in the region depend on fishing for their food and economic development. Yet the surplus of jellyfish and salps and the depletion of diatoms have depleted the food supply of edible fish.

“There will be cascading effects that will likely affect food availability for several countries in the region,” Goes said. “Noctiluca blooms, jellyfish and salps also pose huge challenges for desalination plants along the coast that provide fresh water to the coast of Oman. Masses of jellyfish are known to clog seawater intake pipes.

And the change of Noctiluca– dominated waters have an unusual ripple effect on national security. Shimmering Noctiluca are bioluminescent: they glow when stimulated and this is particularly noticeable at night. This trait can be used for follow the movements of the ships which stir up plankton during their cruise. Sailors and pilots were following such sparkling tracks for decades.

“There are many examples of phytoplankton rampaging around the planet,” said Norman Kuring, a NASA scientist. Ocean Biology Group. “The Baltic Sea has a new summer normal of toxic cyanobacteria blooms. Green algae regularly clog the waters around China’s Shandong Peninsula. Sargassum is becoming a real headache in the Caribbean. Lakes in the United States and around the world are becoming increasingly eutrophic. There are disturbing suggestions from respected scientists that our oceans may be heading for a hypoxic future dominated by bacteria. “

NASA Earth Observatory Map and Graphic by Joshua Stevens, using data courtesy of Goes, J., et al. (2020). NASA Image by Norman Kuring /NASA Ocean Color Web, using VIIRS data from National Suomi Partnership in Polar Orbit. Photograph courtesy of Joaquim Goes. Michael Carlowicz story.


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