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Abstract

In the two years, 2011-2012, at least five red tides were formed by various species, including Ceratium furca (July 2011). Phaeocystis globosa (November 2011), Noctiluca scintillans (April 2012), Gonyaulax polygramma (May 2012) and Chattonella sp. (August 2012). Of those, the second and third species were for the first time recorded to form red tides in the north of Vietnam while the last two were for the first record in bloom proportion in Vietnamese waters. This is a dramatic increase in frequency, intensity and variety of HAB events, considering that during the last 10 years, only one red tide was observed (2008) after the first event in history of the western coast of the Tonkin Gulf was officially recorded in June 2002. These blooms lead to serious losses in aquaculture. The total loss is estimated to be about 3-5 million USD, including 3,000-5,000 tons of bivalves (Meretrix lyrata) and 20 tons of various species of caged fish such as cobia (Rachycentron canadum), snapper (Lutjanus argentimaculatus and L. erythropterus), grouper (Ephinephelus spp.) and seabass (Lates calcarifer). The increase in red tide frequency corresponded with the increase nutrient level and abundance of phytoplankton. Concentrations of N, P, which are measured in the form of NO2-, NO3-, NH4+ and PO43-, have sharply increased by 2-4 fold while phytoplankton abundance has increased 20-300 times during 2005-2012.