The impacts of harmful algal blooms (HABs) on economies, public health, ecosystems, and aquaculture across the globe have all increased in recent decades, and this has been acutely the case in China. Here, we review the history of HABs and HABs research in China, as well as recent trends in HABs and future prospects of HAB science in China. The most updated analyses demonstrated that the number of HAB events, the number of HAB species, the aerial coverage of HABs, and the impacts of HABs in Chinese waters during the 21^st century were all higher than that during the last two decades of the 20^th century. The increase in the number of HABs in China has been significantly correlated with the increased discharge of ammonium and total phosphorus into coastal waters (p < 0.01 for both). Notable newly recognized events this century have included chronic HABs caused by Prorocentrum donghaiense and Karenia mikimotoi, a paralytic shellfish poisoning event caused by Gymnodinium catenatum that sickened 80 people, brown tides caused by Aureococcus anophagefferens, green tides caused by Ulva prolifera, golden tides caused by Sargassum horneri, and the disruption of a nuclear power plant caused by a bloom of Phaeocystis globosa. A series of key discoveries regarding HABs has been made this century including documentation of nearly all known HAB toxins in Chinese waters, discovery of novel cyst-formation and/or life stages of multiple HABs-causing species, identification of the chemical and physical oceanographic drivers of multiple HABs including those formed by P. donghaiense, K. mikimotoi, and U. prolifera, and the successful mitigation of HABs via the use of modified clay approaches. Future research prospects highlighted include the use of macroalgae as a means to prevent, mitigate, and control (PCM) HABs and the process by which multi-disciplinary studies involving molecular approaches (omics), remote in situ detection, artificial intelligence, and mega-data analyses might be used to develop refined and realistic HAB forecasting platforms. Collectively, this review demonstrates the significant evolution of HAB science since the 20^th century in China and demonstrates that while HABs in China are complex and widespread, recent and on-going discoveries make the development of detailed understanding and effective measures to mitigate the negative effects of HABs a hopeful outcome in the coming years.

The Optimum Multiparameter (OMP) method was used to determine the relative contribution of water masses in the East China Sea (ECS) based on their measured temperature, salinity, and dissolved inorganic iodine species (DIIS, iodate and iodide) from recent oceanographic cruises in June 2014, 2015, and 2016. DIIS (as quasi‐conservative parameters) were added first into the OMP analysis, which enabled us to quantitatively estimate the water composition in the extremely dynamic and productive ECS. The OMP results indicate that seawater in the ECS mainly consisted of Changjiang Diluted Water (CDW), Taiwan Strait Warm Water (TSWW), Kuroshio Surface Water (KSW), and Kuroshio Subsurface Water (KSSW). In June 2014, 2015, and 2016, the KSSW dominated in the deep water, intruded into the ECS shelf from the northeast of Taiwan and bifurcated into a nearshore branch and offshore branch. The nearshore branch could reach north of 29°N off the Zhejiang coast. When comparing the KSSW intrusion in June 2014, 2015, and 2016, the average KSSW contribution of the bottom KSSW area in June 2016 (78.9%) was greater than in the previous 2 years (67.9% and 64.5%). In the northern transects (north of 28°N), the KSSW intrusion in June 2016 also contained the greatest average KSSW contribution and the maximum coverage area. Thus, the KSSW intrusion was strongest in June 2016 and may be comparable in June 2014 and 2015. This interannual variability of the KSSW intrusion is likely related to the El Niño‐Southern Oscillation (ENSO) rather than the Pacific Decadal Oscillation (PDO).

In spring 2014 (May–June), in the southern East China Sea (ECS) and east of Taiwan, nutrient and isotope samples were collected and analyzed. Also, dissolved oxygen (DO), chlorophyll‐a, and physical parameters were determined. The Kuroshio subsurface water intruded into the ECS and separated into two branches on the continental shelf: the nearshore Kuroshio branch current (NKBC) and the offshore Kuroshio branch current (OKBC). Nitrate concentration in the Kuroshio subsurface water slightly decreased east of Taiwan by assimilation and was supplied by upwelling currents northeast of Taiwan. The variations of DO, nitrate, and nitrogen isotopes in the bottom water showed that continuous nitrification occurred in the NKBC after intrusion into the ECS. This process might contribute to the hypoxia zone near the coast of Zhejiang Province, China. Our results also indicated that internal nitrogen cycles appeared in NKBC since the isotope fractionation ratio of oxygen and nitrogen (¹⁸ɛ:¹⁵ɛ) in was 1.40. The flux and budget were calculated in the NKBC via numerical simulations. It demonstrated that at least ∼0.52 kmol ·s⁻¹ was produced by nitrification from DH9 to DH5 transect, and ∼0.11 kmol ·s⁻¹ was consumed in the DH4 transect. Moreover, according to the Rayleigh model, primary production in most of southern ECS was supported by the intrusion of the Kuroshio subsurface water, causing 5‰ isotope fractionation. In some of nearshore stations which located in the northern investigated area, the assimilated nitrate was contributed from both the NKBC and coastal currents originated from the Changjiang diluted water.