Abstract:
Eutrophication in the upper Gulf of Thailand was assessed by using UNscale TRophic
status IndeX (UNTRIX) based on related indicators inducing Chlorophyll-a, dissolved oxygen,
dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP). The residence
timesof fresh water in August, September, November and December in 2014 and February, April
and June in 2015 were assessed by using a box model developed by the Land-Ocean interactions
in the Coastal Zone project (LOICZ). The results showed that the residence times was long in wet
season and short in dry season, with the longest (39 days) and the shortest (3 days) residence
times in September 2014 and February 2015, respectively. ∆DIN and ∆DIP were -1.05
mmol/m2
/day and+0.17 mmol/m2
/day, respectively, suggesting that DIN was removed while DIP
was added in the upper Gulf of Thailand water. Stoichiometric analysis yielded the values of net
ecosystem metabolism (p-r) and net nitrogen production (nfix-denit), which were equal to
110.88 mmol/m2
/day and+161.98 mmol/m2
/day, respectively, indicating that the upper Gulf of
Thailand was a heterotrophic ecosystem and nitrogen fixation. UNTRIX was significantly
different between stations and cruises (p< 0.05). The distributions of hypertrophic and eutrophic
conditions were found at the river mouths and from the east coast (Chonburi province) to west
coast (Phetchaburi province) of the upper Gulf of Thailand. The result illustrated spatial
distributions of eutrophication, which was controlled by seasonal variations in river discharge and
circulation. Overall analysis of the study suggested that the limiting nutrient in the upper Gulf of
Thailand be DIN.