Reproductive strategy of the floating alga Ulva prolifera in blooms in the Yellow Sea based on a combination of zoid and chromosome analysis.

Green algal blooms have occurred in the Yellow Sea for 13 consecutive years since 2007. However, little is known about the reproductive strategy of the dominant species Ulva prolifera in the field. In particular, it is not clear whether the floating Ulva species are sporophytes or gametophytes, and if their life history is sexual or asexual. In this study, the life history type was determined based on the size, phototactic response, and flagella number for the zoids in at least two successive generations. In addition, chromosome observations were conducted to distinguish the gametophytes and sporophytes in the floating Ulva species. The results showed that the floating Ulva species were all sporophytes with sexual reproductive patterns, thereby indicating that this Ulva species always maintains vegetative growth from April to June during the early stage of the blooms. In addition, we found that the chromosome numbers were 18 for the diploid sporophytes and nine for the haploid male and female gametophytes. These results provide useful information to help understand the explosive growth of these green algal blooms.

Annual patterns of macroalgal blooms in the Yellow Sea during 2007-2017.

The world's largest macroalgal blooms caused by Ulva prolifera have occurred in the Yellow Sea for 11 consecutive years. The area covered by blooms has been approximately 500 km2 in previous years, while in 2017, the maximum area decreased significantly to 312 km2. In this study, we concluded that species competition between Ulva and Sargassum (fast rise of the golden tides), extreme high sea surface temperature and harvest for floating Ulva macroalgae were the three critical factors influencing the sharp reduction in covered area for blooms in 2017. In addition, analysis of annual variations of Pyropia aquaculture area in the Southern Yellow Sea over the past two decades revealed that a great expansion in "Sansha" regions was mainly responsible for the initial blooms in 2007, and that this expansion supported the great biomass of the blooms in following years. Based on these findings, we suggest comprehensive utilization of the macroalgal blooms is a feasible way to control them.

Environmental triggers of a Microcystis (Cyanophyceae) bloom in an artificial lagoon of Hangzhou Bay, China.

The relationship between Microcystis abundance and environmental variables was studied during a Microcystis bloom in the summer months of 2016 in an artificial lagoon of Hangzhou Bay, China. It was determined that Microcystis abundance increased from 0.16 × 104 cell/L to 5.8 × 107 cell/L within 17 days from 28 July to 14 August, contributing to 96.84-99.56% of the total phytoplankton abundance. Then, Microcystis gradually disappeared 57 days afterwards. The results showed that the growth of Microcystis, including the stage of recovery, outbreak, subsidence and disappearance, was significantly correlated with water temperature, salinity, soluble reactive phosphorus (PO4-P), dissolved inorganic nutrients (DIN), silicate (SiO4-Si), the ratio of DIN/SiO4-Si and zooplankton abundance, and the key environmental triggers which promoted the outbreak of Microcystis were water temperature, PO4-P concentration and zooplankton abundance in this artificial lagoon.

Rapid expansion of Ulva blooms in the Yellow Sea, China through sexual reproduction and vegetative growth.

Green algal blooms have occurred in the Yellow Sea for 11 consecutive years since 2007. A "seed bank" comprising micro-propagules including gametes, meiospores, and zygotes, played an important role in the rapid formation of a green tide. In the present study, germination differences among zygotes, meiospores, and gametes were examined. The growth ability and maturation period of alternating generations of sexual Ulva prolifera strains were also assessed. The zygote and meiospore germination rate was 91.67% and 80.29%, respectively, approximately three times greater than that of gametes (30%). In addition, the highest daily growth rate of sporophytes and gametophytes was 266.7% and 288.1%, respectively, and the maturation period of sporophytes and gametophytes was 35.7 and 31.3 days, respectively. These results indicate that sexual reproduction and vegetative growth are mainly responsible for the rapid expansion of macroalgal blooms in the Yellow Sea.

Nutrient bioextraction and microalgae growth inhibition using submerged macrophyte Myriophyllum spicatum in a low salinity area of East China Sea.

Myriophyllum spicatum was cultivated in a low salinity area of Hangzhou Bay (salinity 5.8-6.5), from August to October in 2016, to evaluate the abilities of its nutrient bioextraction and microalgae growth inhibition. During the 72-day cultivation period, M. spicatum had a specific growth rate (SGR) of 6.23%day-1 and increased 20-fold in biomass (wet weight). Tissue C, N and P assimilation quantities of M. spicatum were found to be 3279.39kg, 360.61kg and 26.97kg, respectively. The concentration of NH4-N, NO3-N, NO2-N and PO4-P after M. spicatum cultivation was decreased by 47.92%, 58.28%, 36.40% and 55.57%, respectively. The phytoplankton density was decreased from 1064.60×104cellsL-1 to 12.85×104cellsL-1. These results indicated that cultivation of M. spicatum can help in nutrient bioextraction and microalgae growth inhibition in low salinity marine water bodies.