Macroalgal blooms of U. prolifera in the southern Yellow Sea (SYS) have become an ecological disaster. In this study, we attempt to explore the influence of environmental factors and human activity on the annual development of U. prolifera during 2011–2016, combining geostationary ocean color imager images and monitored environmental factors. Blooms of U. prolifera were mainly distributed in the central SYS, drifting from the southern Yellow Sea Radial Sand Ridges. Three growth phases were defined (initial phase, outbreak phase and dissipation phase) to facilitate analysis of the relationship between the environmental factors and growth conditions of U. prolifera. Seaweed cultivation, sea surface temperature (SST) and available radiation influenced the algal biomass during the initial phase; the seawater transparency, precipitation and wind activity affected the algae during the outbreak phase; and SST, intense radiation and cleanup operations increased the rate of algal extinction in the dissipation phase.

Compositions, changes and biomass of attached Ulva species on Porphyra rafts along the radial sandbank in the Yellow Sea were investigated, and potential contributions to green tides was analyzed. Ulva prolifera, Ulva flexuosa and Ulva linza were all appeared throughout the investigated period. U. prolifera and U. flexuosa dominated attached Ulva population on Porphyra rafts. Attached Ulva species biomass showed obviously spatial and temporal variations. Temperature, Ulva microscopic propagules and human activities were main factors to influence attached Ulva species biomass. The total attached Ulva species biomass was more than 20,000 fresh weight tons in April, and the green tide causative species U. prolifera accounted 51.03% in April 2013 before green tides occurred. The high biomass of attached Ulva species would contribute most to green tides in the Yellow Sea. But how attached Ulva species on Porphyra rafts contributing to green tides in the Yellow Sea should be further studied.

The bioremediation efficiency of China’s largest scale Porphyra yezoensis cultivation for removing dissolved nutrients and controlling harmful algae was studied in the radial sandbanks waters of Jiangsu Province in the year 2012–2013. Mean nutrient concentration values in the P. yezoensis cultivation area were significantly lower than those in the non-cultivation area, especially during the cultivation season (p<0.05). Tissue nitrogen and phosphorus contents of seaweeds were 5.99–0.80% (dry weight (DW)) and 0.16–0.19% (DW), respectively. Production of P. yezoensis was 58950.87tons DW. Based on these values, 3688.15tons of tissue nitrogen and 105.61tons of tissue phosphorus were removed by harvesting P. yezoensis. The richness index of the red tide species Skeleton emacostatum declined from 0.32 to 0.05 during the P. yezoensis cultivation season. These results indicate that large-scale cultivation of P. yezoensis can be used to efficiently alleviate eutrophication and control harmful algae blooms in open sea.

How aquaculture impacts the coastal phosphorus (P) cycle remains poorly understood. Here we compared different P species from two sedimentary records off Nan'ao Island, South China, with core S1 collected in a large seaweed cultivation area and core S2 in a non-mariculture area. The results showed that the concentration of total P (TP) in sediment cores varied from 143.67 to 400.92 μg/g, and organic P (OP) was the dominant P species. The TOC/OP ratios in the two sediment cores were higher than the Redfield ratio in 26 samples (52%) from core S1 and 39 samples (78%) from core S2, suggesting that terrestrial organic matter was an important carbon source to Shen'ao Bay. The lack of change in Ex-P (exchangeable or loosely sorbed P) and OP in the area around core S1 since the 2000s may be due to the large-scale seaweed cultivation.

This study analyzes fishers', managers' and scientists' opinions on management measures to facilitate the initiation of management processes towards more sustainable small-scale seagrass fisheries in Zanzibar, Tanzania. The results show that most fishers and managers agreed on the need to include seagrasses specifically in future management. There was further agreement on dragnets being the most destructive gears, and the use of dragnets being a major threat to local seagrass ecosystems. Gear restrictions excluding illegal dragnets were the favored management measure among fishers. Differences between fishers and managers were found concerning seaweed farming, eutrophication and erosion being potential threats to seagrass meadows. A majority of the interviewed fishers were willing to participate in monitoring and controls, and most fishers thought that they themselves and their communities would benefit the most from establishing seagrass management. Co-managed gear restrictions and the inclusion of different key actos in the management process including enforcement are promising starting points for management implementation.

Large seaweed cultivation has proven an effective means to inhibit harmful microalgae at experimental scales and battle eutrophication in Chinese coastal waters, but essentially there is a lack of field-scale studies to explore the underlying mechanism. Here we present a 1.5-year-long time series of particulate organic matter (POM) and settling particulate matter (SPM) concentrations from an integrated aquaculture of Gracilaria lemaneiformis off the coast of Nan'ao Island, South China from April 2014 to August 2015. The microscopic examination and geochemical characteristics show that the POM mainly consisted of microalgae. The mean POM concentration increased 99.8%, 71.2%, 45.8% and 111.9% at the four sampling sites during the non-cultivation period, while decreased 25.5%, 17.3%, 12.2% and 20.3%, respectively, during the seaweed cultivation period. These results suggest that the large scale seaweed cultivation can remove excess nutrients and inhibit microalgal growth, thereby contributing to the improvement of coastal marine aquaculture environment.

Seaweeds are important primary producers and bioremediation materials, but its litter produced during growth and harvest is one of the restrictions to the sustainable development of seaweed cultivation. In this study, we conducted field investigation and indoor experiments to analyze the bioaccumulation and release of metals in Gracilaria lemaneiformis during the growth and decaying. The investigation revealed the 3.5 × 10⁵ t (wet weight) G. lemaneiformis from a 1500 ha cultivation area bioaccumulated 1925–2353 kg Zn, 233.5–251 kg Cu, 70.5–80.5 kg Pb and 25.5–47 kg Cd, indicating that G. lemaneiformis is a good metals remover. The growth and decaying period of G. lemaneiformis releases, absorbs or adsorbs metals. It has the function of a “heavy metal pool”, simultaneously accumulate and release metals. G. lemaneiformis has a strong influence on heavy metals cycling in the seaweed cultivation ecosystem and provides a very good sample for biogeochemistry study for the globally seaweed sustainable development.

Levels, composition profiles and sources of 16 polycyclic aromatic hydrocarbons (PAHs) in surface sediments from Nan’ao Island, one of the largest mariculture bases in South China were investigated. The total PAHs concentrations ranged from 265.49 to 951.27ng/g (dry weight) with a mean of 515.90ng/g, and seven possible carcinogenic PAHs accounted for 76.94% of total PAHs. Among various mariculture sections, the highest PAHs level was found in the large seaweed culture; while the lowest concentration was detected in the abandoned culture. The composition of PAHs was characterized by low molecular weight PAHs, and Phe Ant and Pyr were the dominant constituent. PAHs may be mainly originated from petroleum and combustion of biomass and coal. Risk assessments suggested that the probability of adverse effect was lower than 25% and PAHs exposure was moderately carcinogenic.

The large green-tide events that occurred in the Yellow Sea in 2008 (3489km²) and 2009 (4994km²) are shown to be novel events preceded only once by a much smaller event in 2007 (82km²). The blooms originated in the coastal area of Jiangsu province and spread north-east towards the Shandong Peninsula. The blooms grew at different rates and mesoscale variability in surface winds explained the differences in the spatial and temporal patterns of blooms in 2008 and 2009. The 2009 bloom was tracked to its origin immediately offshore of extensive intertidal flats between Yancheng and Nantong where recent rapid expansion of Porphyra aquaculture has occurred. We review published hypotheses which have been advanced to explain the occurrence of blooms and in light of our findings, we conclude that the accumulation and disposal of waste Ulva prolifera from Porphyra aquaculture rafts is the most likely cause of the blooms.