Will a shift in Arabian Sea plankton threaten fisheries?

Scientists immersed in the field of invertebrate taxonomy may well be aware of the expanding "dead zone" in the Arabian Sea, a spot roughly the size of Texas. It's the effect of a growing number of a certain type of plankton, which is best suited to low-oxygen environments, taking over these waters and starving the area of its oxygen in order to dominate the food chain. This may be good news for the sea plankton, but as new research shows, it may be detrimental to the ecosystem and fisheries.

The science community has long been interested in green Noctiluca scintillans, colloquially known as the "Sea Sparkle" for its bioluminescence exhibited when it is disturbed. The creature produces thick blooms that turn the sea a beautiful emerald green color each winter, and illuminating the waters using the energy it obtains from eating other plankton as well as from the sun (an absorption process that involves microscopic algae that live within the plankton's cells). These blooms could greatly disrupt the marine environment, as the study's lead author, biogeochemist Helga do Rosario Gomes, said in a Columbia University news release.

"These blooms are massive, appear year after year, and could be devastating to the Arabian Sea ecosystem over the long-term," Gomes said.

"These blooms … could be devastating to the Arabian Sea ecosystem over the long-term," said biogeochemist Helga de Rosario Gomes.

Gomes and co-researchers from Columbia University's Lamont-Doherty Earth Observatory have been closely watching the plankton's rapid rise in numbers in the Arabian Sea. In September 2014, they published a study in the journal Nature Communications demonstrating just how the millions of green algae that live in these plankton allow them to take advantage of the massive oxygen-starved dead zone. These scientists believe that an influx of sewage rich in nutrients is flowing in from cities lining the Arabian Sea, further driving the growth of the sea plankton and expanding the dead zone.

The rise of the plankton
In the past, the food chain of the Arabian Sea was supported largely by photosynthetic diatoms, a kind of algae. Zooplankton fed on these diatoms, passing on nutrients when the zooplankton were eaten by fish. However, there was a big change at the beginning of the millennium, when researchers started noticing that there was a significant drop in diatoms and a steep increase in Sea Sparkle blooms, as well as a decrease in oxygen. Just 10 years later, the sea plankton had pushed out the diatoms and taken over the base of the food chain, signifying the beginning of an historic shift of an enormous ecosystem.

The success of this sea plankton relies in part to its lack of preference when it comes to food. The green Noctiluca, which makes its home in tropical environments, will eat just about anything, including both living and dead plankton in addition to diatoms by the mouthful. They can dive down deep to find food and nutrients, and swim up to the surface to absorb light for energy. This versatility is key to the sea plankton's ability to so rapidly and effectively defeat its competitors.

Other changes in the ecosystem
With the growth in green Noctiluca, there have been many other changes that are of importance to the invertebrate taxonomy field. Predators that feed on these sea plankton have also seen a rise in numbers, particularly jellyfish, sea turtles and gelatinoussalps. While many creatures that tend to feed on crustaceans seem to find the plankton too large to eat, salp in particular has seen huge growth thanks to the spike in sea plankton. The Columbia University researchers conducted an experiment in which they placed several salps into buckets of seawater with high amounts of Noctiluca blooms. The results demonstrated that just one salp can consume about two-thirds of a bucket of these sea plankton in only an hour.

Sea salps, which feed on plankton, are increasing in numbers in the Arabian Sea.

As these shifts in the ecosystem occur, scientists are concerned that a new ecosystem will develop that will have a shortage of predator fish that were once abundant here and feed the 120 million people living on the coast of the Arabian Sea. Many fisheries in the area have already been negatively affected – an April 2014 study published in the Cambridge journal Oryx found that 85 percent of fishermen in Tamil Nadu and Maharashtra, states sustained by these predator fish, had significantly smaller catches compared to 12 years ago.

"One obvious factor that must be addressed is the massive flow of sewage into the Arabian Sea from surrounding cities."

What's next?
As environmental consultants have previously pointed out, overfishing may also be to blame for the smaller catches in the past dozen years. However, one obvious factor that must be addressed is the massive flow of sewage into the Arabian Sea from surrounding cities. The study notes that Mumbai's population has increased two-fold on the past 10 years, reaching 21 million. It produces some 11 tons of phosphorus and 63 tons tons of nitrogen every day, all being sent into the sea and providing energy for the green Noctiluca to thrive. And this is only one example of the many booming coastal cities.

"All of these cities are growing so rapidly they don't have the capacity to treat their sewage," study coauthor Joaquim Goes, a biogeochemist at Lamont-Doherty, said in the news release. "The amount of material being discharged is humongous."

With this clear connection between sewage flow and sea plankton growth, the Columbia University researchers suggest further bioassessment of the expanding dead zone in the Arabian Sea, before the ecosystem shift wipes out predator fish altogether.