Primary Studies On The Harmful Effects And Mechanisms Of Ulva Prolifera Green Tide

During the bloom of green alga Ulva prolifera (the so-called"green tide") in 2008, huge amounts of floating green algae accumulated along the coast of Qingdao. The occurrence of green tide was just at the stage of preparation for the 2008 Olympic Sailing Game in Qingdao, which caused wide attention from domestic and abroad. However, there was little knowledge about the mechanism and potential impacts of green tides in China, and it is urgent to carry out relative studies. In this paper, the effects of green alga U. prolifera on different marine organisms were studied, using 5 species of microalgae, the misid shrimp Neomysis awatschensis, scallop Chlamys farreri, oyster Crassostrea gigas, abalone Haliotis discus hannai and sea cucumber Apostichopus japonicus, and potential impacts of green tide on marine ecosystems were also discussed.It was found that algal culture medium of U. prolifera showed varying allelopathic effects on 5 species of microalgae, Alexandrium tamarense, Prorocentrum donghaiense, Heterosigma akashiwo, Skeletonema costatum and Chlorella vulgaris. The growth of H. akashiwo was significantly inhibited, followed by C. vulgaris. Co-culture of U. prolifera and microalgae could significantly inhibit the growth of the microalgae. The combined effects of allelopathy and allelospoly would inhibit the growth of microalgae. It is therefore suggested that the phytoplankton community, as well as the the primary productivity, could be affected during the period of a green tide.The decomposing green alga U. prolifera could release ammonium and phosphate into seawater, which influenced the growth of microalgae. Low concentration of decomposing algal effluent ( 1 g/L) stimulated the growth of H. akashiwo, A. tamarense, P. donghaiense. High concentration of decomposing algal effluent (10 g/L) showed stimulatory effect only to the growth of H. akashiwo, and had inhibiting effects on the growth of other 3 species of microalgae. The inhibitory effect was mainly duo to the high concentration of ammonium presented in decomposing algal effluent, and H. akashiwo had higher resistant capability to ammonium than the other species. It was proposed that the decomposation of accumulated algae could lead to the occurrence of harmful blooms formed by microalgae like H. akashiwo.The experiment on the toxicity of U. prolifera to the misid shrimp Neomysis awatschensis revealed that decomposing algal effluent had acute toxic effects on N. awatschensis. In the 62-d experiment, the misid shrimps exposed to the culture medium of live U. prolifera also showed decreased survival and reproduction. It was suggested that the survival and reproduction of zooplankton community could be affected by the green tide of U. prolifera.The culture medium and decomposing effluent of U. prolifera both significantly inhibited the egg development of the scallop Chlamys farreri. The crude extracts of U. prolifera with three different organic solvents were applied to test their inhibitory effect on the oyster eggs. It was found that the methanol and acetone extracts had strong inhibitory effects. The methanol extract of U. prolifera was further purified by liquid-liquid extraction with ethyl acetate and petroleum ether, and both extracts could significantly inhibit the hatching of oyster eggs. It was suggested that the live U. prolifera could produce polar chemicals which inhibited the fertilization of mollusca eggs.It was found that both the culture medium and decomposing effluent of U. prolifera showed acute toxic effects to abalones and sea cucumbers. The decomposing algal effluent showed a much higher acute toxicity than the algal culture medium. The ammonium and sulfide produced during the decomposation of green algae account for a major proportion of the acute toxicity of decomposing algal effluent. In the semi-enclosed culture ponds of abalones and sea cucumbers, the hypoxia condition caused by the respiration or decomposition of green algae, could further deteriorate water quality, and led to mass mortality of cultured animals.In summary, it can be concluded that: (1) during the period of green tide, the rapid proliferation of of green algae may inhibit the growth of phytoplankton through the effects of both allelopathy and allelopoly. Meanwhile, a green tide last for a long time may threat the survival and reproduction of zooplankton. (2) after the collapse of the green tide, decomposition of the accumulated green algae will consume dissolved oxygen in seawater and release sulfide and ammonium into seawater, which will lead to the toxic effects to marine organisms. In addition, the released ammonium and phosphate may be assimilated by microalgae and lead to red tides. (3) animals cultured along the coast are more vulnerable to the occurrence of green tides, Due to the low exchange rate of sea water in the culture ponds. It can be seen that the occurrence of green tides in the Yellwo Sea may have potential impacts on the structure and function of coastal and marine ecosystems. However, more intensive field investigations and researches are still needed in future to elucidate the impacts of green tide on marine ecosystems, as well as socioeconomic development.