In Depth: Why ‘Green Tides’ Keep Rolling Into East China

At noon on July 9, Dujiadao Island in Shandong, China, experienced a severe green tide event, with Ulva prolifera algae covering the shore. The algae formed thick layers, suffocating local aquaculture. Pan Fei, a resident and aquaculture farmer, hired workers to remove the algae from his ponds, but the blooms returned with the tides, resulting in the death of fish and crabs due to oxygen depletion. Pan lost significant investment: 100,000 yuan ($13,800) worth of sea cucumber seedlings, which perished in the hostile conditions [para. 1][para. 2][para. 3]. The 2024 yellow sea algae bloom covered about 49,400 square kilometers, down from a record 61,900 square kilometers in 2021—almost ten times the area of Shanghai and the largest globally recorded green tide [para. 5]. Pan attributed the problem to seaweed farming in Jiangsu province, whose farming bamboo poles frequently wash up on Shandong beaches [para. 6]. While authorities have ordered a reduction in seaweed farming, these measures have had only temporary effects; for example, after an initial drop in bloom size, the worst-ever bloom occurred in 2021. The frequency of massive blooms has increased since prevention efforts began [para. 7][para. 8].

Government measures, focusing on removing algae rather than addressing the influx of excessive nutrients fueling these events, appear to be ineffective. Nutrient runoff from agriculture and urban areas continues to drive recurring blooms, undermining local aquaculture and tourism [para. 9][para. 31][para. 38]. In Weihai and nearby areas, farmers have suffered major losses: Pan rented two ponds covering over 11.5 acres, investing heavily in seedlings for a product that grows slowly but fetches high prices. More than 1,000 sea cucumber farmers in Weihai contribute to 15% of China's production, but sea cucumbers and other marine life are highly vulnerable to oxygen-depleting green tides [para. 13][para. 14][para. 16][para. 17][para. 18].

Tourism is also heavily impacted. In Rushan, a popular 20-kilometer beach was blanketed by algae, deterring tourists and causing economic loss. Tourism is a major source of local GDP: in 2024, Weihai's tourism revenue was 70 billion yuan (18.8% of GDP), Yantai’s was 113.9 billion yuan (also 18.8% of GDP), and Qingdao’s was 10.6% [para. 26][para. 27][para. 29]. Annual green tides cost Qingdao’s coastal tourism industry between 1.2 billion and 2.1 billion yuan, or up to 1.5% of tourism revenue [para. 30].

The origin of the blooms was traced to Jiangsu's intertidal zone, where nori (laver) farming dominates (over 90% of China's output). Algae attach to bamboo poles and nets used for farming [para. 36][para. 39]. The government introduced restrictions in 2016 and 2019, causing a drop in area farmed and industry losses exceeding 10 billion yuan [para. 43][para. 44][para. 45]. Jiangsu produces 85% of China’s striped laver, supporting 200,000 fishermen. However, raft farming has not been proven as the root cause, as similar practices abroad do not cause green tides. Meanwhile, excess nutrients—largely from agricultural and municipal sources—persist, suggesting current control measures are misdirected [para. 47][para. 49][para. 51].

Scientists argue for reexamining Ulva's ecological role, suggesting that in healthy systems, it could help regulate excess nutrients. Offshore, its decay is harmless, but mass beachings necessitate costly cleanup: Qingdao spent 305 million yuan on 13,400 cleanup vessels in 2022 alone [para. 55][para. 57]. Some see opportunity in converting algae into food, fertilizer, or biofuel, and major companies have processed millions of tons, generating billions in revenue [para. 60][para. 62]. Experts call for more supportive policies to expand algae recycling, turning this recurring 'green nuisance' into a resource [para. 63][para. 64].