3. Eutrophication
Eutrophication of coastal waters has only recently become apparent as a problem in Thailand. In the Gulf of Thailand, the species found to bloom most frequently are the blue-green algae Trichodesmium erythraem, and Noctilluca sp. The relationship between these blooms and the nutrient enrichment of coastal waters (due mainly to the disposal of untreated sewage) is probably inescapable, but firm evidence is elusive. A widespread bloom in the Eastern coast of Thailand was recorded in 1983, and caused losses to local fish farming facilities ( Suvapeepun et al., 1984). A red tide also occurred on the west coast of the Upper Gulf at about this time, and paralytic shellfish poisoning (PSP) was recorded for the first time in Thailand as a consequence. The responsible organism was identified as the dinoflagellate Gonyaulax sp. According to Suvapepan (1995), 43 major red tides were recorded in the Gulf during 1988–1995. 21 red tides were caused by Trichodesmium sp., 17 were caused by Noctiluca sp. and the rest by diatoms.
The areas effected by phytoplankton blooms were nauseabond and discolouration of the water was usually observed. Red tides could cause mass mortalities in nearby shrimp and shellfish farms. For example, major shrimp farming areas in Samut Songkarm and Samut Sakorn provinces were severely affected in 1977 resulting in a sharp decline in output per hectare (Rientrairut, 1983). Green mussel larvae were also severely affected by red tides as they were unable to settle on the wooden poles during the outbreaks. This caused heavy losses to the shellfish industry during the outbreaks.