Effects of the decomposing green macroalga Ulva (Enteromorpha) prolifera on the growth of four red-tide species

The green macroalga Ulva (Enteromorpha) prolifera formed large-scale blooms (the so-called “green tide”) from 2007 to 2010 in the Yellow Sea, China. In June 2008, huge amounts of floating U. prolifera accumulated along the coast of Qingdao. At the late stage of the green tide, a large amount of green algae sank to the bottom and decomposed, which led to concerns about derivative ecological and environmental problems, such as red tides. The effects of nutrient recycling from decomposing green tides on the growth of four selected red-tide causative species, i.e. Heterosigma akashiwo, Alexandrium tamarense, Prorocentrum donghaiense and Skeletonema costatum, were studied in the laboratory. It was found that the decomposing green algae released considerable amounts of ammonium and phosphate into the surrounding seawater. The addition of effluent from the decomposing green algae promoted the rapid proliferation of the raphidophyte H. akashiwo, but inhibited the growth of the diatom S. costatum. The growth of two dinoflagellate species A. tamarense and P. donghaiense was triggered by the addition of low-concentration decomposing algal effluent, but inhibited by the high-concentration decomposing algal effluent, probably due to toxicity from high concentrations of ammonium or water-soluble allelochemicals present in the decomposing effluent. We conclude that the decomposition of green algae releases inorganic nutrients, particularly ammonium, into seawater where the nutrients could support red tides once they were taken up by the opportunistic microalgae.

► This paper gives a new viewpoint to the relationship between red tides and green tides.
► We found decomposition of green algae released ammonium and phosphate into seawater.
► Nutrients regenerated could be used by opportunistic microalgae and lead to red tides.