Marine Environments and Phytoplankton in the Area of the Locally Based Coastal Fisheries Management Project: Pathew District, Chumphon Province

Abstract

Investigations on marine environments were carried out at 10 sampling stations in the area of the Locally-Based Coastal Fisheries Management Project, Pathew District, Chumphon Province from 2002 to 2006. The depth of the bottom of the area ranged from 3.4 to 19.3 meters. The environmental variables were measured and the data are as follows: transparency 1.2 - 4.3 meters, mean surface temperature 29.44 ± 0.47°C, salinity 31.08 - 33.53 PSU, pH 7.26 - 8.93 and dissolved oxygen 4.83 - 6.25 mg/l. Nutrients, total suspended solids (TSS) and chlorophyll a were analyzed and the results are: average nitrite 0.010 - 0.290 μg-at N/l, average nitrate 0.121 - 13.570 μg-at N/l, average ammonia 2.153 - 22.294 μg-at N/l, average orthophosphate 0.189 - 0.684 μg-at P/l, average TSS 25.18 - 53.75 mg/l. The biological index used was the average chlorophyll a at 0.116 - 2.201 mg/m³. Phytoplankton was investigated and 66 genera in 5 divisions were identified. Diatom was the most common. High density of phytoplankton was recorded in June 2004 and September 2005. In June 2004 the dominant species were Ditylum sole from 2.06 x 10⁴ to 1.79 x 10⁵ cells/l while in September 2005 Chaetoceros spp. was more pronounced from 3.45 x 10⁵ to 1.79 x 10⁶ cells/l. As experienced, this density level may cause the red-tide phenomena. Such phenomenon was observed when Noctiluca scintillans which almost entirely makes up the red tide, was dominant at Station 9 in August 2004. The water quality in the study area is suitable for coastal aquaculture. The physical, chemical and biological indices of Pathew Bay showed that the Bay is at risk from phytoplankton blooms and accumulation of nutrients. Therefore, limitation on aquaculture in terms of area and density of fish stocks should be considered to prevent environmental damage due to plankton blooms and eutrophication when the load of aquaculture waste exceeds the carrying capacity of the Bay.