Residents of the Chinese coastal city of Qingdao are familiar with the green tides that usually appear in summer. The algae washes ashore causing the coastline to resemble a sprawling football turf.
However, this year, they have noticed something different: the algae blooms are undergoing a colorful transformation with shades of red and brown mixed into the usual green.
Aerial photographs showed the coast now looks like a giant painted canvas with brush strokes of green, red, and brown on the otherwise blue sea.
Oceanologists said while the predominantly green algae is not toxic, large-scale blooms formed by Karenia mikimotoi algae -- a species of red algae -- are harmful and may threaten marine life.
Algae blooms turn away beach-goers and hurt tourism, a key sector sustaining the economy of Qingdao and nearby coastal cities.
The scourge of green tides in the Yellow Sea has besieged Qingdao every summer over the past decade. The blooms are usually formed by a species of algae called Ulva prolifera.
A recent study confirmed that the increase in green algae was caused by seaweed farming in neighboring Jiangsu Province to the south, said Yu Rencheng, an scientist with the Institute of Oceanology under the Chinese Academy of Sciences.
Known as zicai in Chinese, gim in Korean, and nori in Japanese, Porphyra is an edible seaweed that plays an important part in the diets of East Asian countries.
Farmers on Jiangsu's northern coast use rafts floating in shallow waters to grow Porphyra. In spring, the sunshine and warmer water temperatures are ideal for U. prolifera to grow on the edges of rafts, Yu said.
When farmers harvest Porphyra, the boats and attached U. prolifera are left at sea. The algae grows very fast. It joins together to form huge floating masses which winds and currents push to the north.
Having identified the source of the algae blooms, authorities have introduced a simple and effective intervention. They told Porphyra farmers to bring the algae-covered rafts ashore instead of leaving them in the water after the harvest.
Another scientist with the Institute of Oceanology, Sun Song, who leads the ongoing research, said this year the biomass of green U. prolifera in the Yellow Sea has reduced to almost half the previous size
On the north Jiangsu coastline, the reduction is even more evident, he said. There were only 98,600 tonnes of green algae in wet weight in May, down from 500,000 tonnes measured during the same period last year.
But Yu said this is just part of the equation. The more worrisome part is that the green algae is being replaced by brown tides.
Brown Sargassum algae competes with U. prolifera for survival space. While past algae blooms were a mixture of both substances, the proportion of Sargassum has grown unusually large this year, according to Yu.
The wet weight of Sargassum in north Jiangsu was estimated at 19,700 tonnes this year, compared to just 900 tonnes last year, he said.
Scientists have not yet been able to fully explain the surge in Sargassum, which also affected the Porphyra harvest this year.
As the green-brown blooms have spread to 312 square kilometers of the sea surface and are moving closer to Qingdao and the nearby city of Rizhao, scientists observed another red tide is also approaching.
The red bloom, caused by K. mikimotoi algae, was measured at 50 square kilometers of the sea surface.
"Red tides are rare in the Yellow Sea," said a third oceanologist involved in the research who declined to be named. "It means that the ecosystem of the Yellow Sea is becoming increasingly fragile."
A red tide often signals the deterioration of sea water quality, he said.
In 2006, a red algae bloom on the coast of Zhejiang Province caused 40 million yuan (5.9 million U.S. dollars) in losses for local fisheries.
In 2012, another red tide in adjacent Fujian Province wiped out farmed abalone, resulting in 2 billion yuan in losses.