There are over 20 years of history of shellfish and seaweed polyculture in Sungo Bay.There had not been reported the outbreak of red tide in the Bay.However,in May 2011,there occured large scale red tide outbreak,which almost spread over the total bay,and which caused higher mortality of cultivated shellfish.It is very important to understand the cause of red tide and to provide data for future environmental protection and aquaculture management.We just carried out one survey of environment monitoring before the outbreak of the red tide.Therefore,in this paper,the distributions of sea water temperature,salinity and dissolved nutrients were investigated in Sungo Bay in April 2006 and 2011,so as to understand the cause of the outbreak of red tide.The results showed that the concentration of dissolved inorganic nitrogen(DIN),phosphate(PO₄^3-) and silicate(Si) in Sungo Bay of 2011 was 5.6,3.2 and 1.3 times of the same period in 2006, respectively.The mole ratio of DIN/P,Si/P and Si/DIN was (66.33?47.16),(35.12?21.44)and(0.82?0.77),respectively,which was seriously deviated from the Redfield ratio.Dissolved nutrients showed obvious block distribution,their horizontal distribution was characterized by a concentration decrease from the open sea to the inner bay,and there were two sub-peak values near the Yatou River and Bahegang areas,the isolines were almost overlaps with low-salinity isolines.The probable causes of red tide outbreak were discussed from four aspects including nutrients,hydrological characteristics,kelp and temperature.