Abstract:
Impacts of organic enrichment from shrimp farm effluent on benthic communities in Chanthaburi Estuary were investigated during November 1998 to September 1999. Benthic macrofauna, sediment properties and water quality were sampled on bimonthly basis in 10 stations along the Chanthaburi River. It can be concluded from the monitoring of sediment properties and water quality that the Chanthaburi estuary had received impacts from shrimp farming activities by the increases of the total organic content and nutrients in sediment which as reflected in the acidic soils. The water quality were within the Thailand National Water Quality Classification for aquaculture purposes. However during the intensive shrimp farming season, the water quality declined reflecting in the increases in the biochemical demand oxygen, ammonia and phosphate. A total of 41 macrofaunal species were collected with polyches as the the dominant group. Species composition of benthic macrofauna were different according to location, sediment properties and water quality. The densities of these benthos in the Chanthaburi estuary were recorded in the wide range of 31.99-4,200.10 individuals per squarementer, comprising of polychaetes in the range of 63.38-95.54 in percentage. The biomass recorded were also in the wide range of 0.03-425.69 grams per squarementer. The highest density and biomass were recorded from the head of estuary where there were few shrimp farms. Seasonal variations in the density and biomass of benthic macrofaun were pronounced. During the intensive shrimp aarming activities in the wet season, the density of benthos was higher than the dry season. However the highest biomass was recorded in the dry season due to the abundance of gastropods and bivalves. This study revealed that salinity, organic carbon, organic nitrogen, ammonia and available phosphorus in sediment were the major factors determining the species composition and distribution of the benthic communities in the Chanthaburi estuary. The two polychaetes, Nereis sp. and Parheteromastus sp. were proposed as the indicator species of fluctuating salinity and also the organic riched sediments of high organic carbon, nitrogen compound and available phosphorus content.