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.