Ecological studies on benthic polychaetes with respect to oranic enrichment condition in Kung Krabaen, Thailand

Abstract

Ecological studies of benthic polychaetes with respect to organic enrichment condition in Kung Krabaen Bay, Chanthaburi Province, Thailand provided another alternative for impact assessment of organic enrichment due to shrimp farm effluents in this area. This study would focus on the change in the benthic communities particularly species composition, abundance and biomass of polychaetes assemblage due to enriched organic materials in Kung Krabaen Bay. The study also provided the scenarios of the Kung Krabaen Bay in term of shrimp farm development and hydrological characteristics in particular the water, salt and nutrients models prior to the operation of seawater irrigation system and the first operational phase of the system in 1999. The results showed the positive correlation between the number the of shrimp ponds operated and the nutrient loading in drainage canals and the bay during the same period in particular the inorganic and organic nitrogen. According to the nutrient budget model revealed the level of eutrophication in the Kung Krabaen Bay system depended on the level of shrimp farm activities. The period before the seawater irrigation installation Kung Krabaen Bay was autotrophic especially in dry season. After the irrigation system installation, Kung Krabaen Bay is net heterotrophic in both seasons. The degree of eutrophication in the bay was reduced by the water circulation enforced by tidal actions and the geographic location flourished by mangrove forests, seagrass beds and open bay. However the high organic content found in the drainage canals and gradually low toward the bay. High content of hydrogen sulfide also associated with high mud content inside the drainage canals and in some areas of seagrass beds. Polychaetes of 78 species from 27 families was the dominant group in the benthic communities of Kung Krabaen Bay. The gradients in organic content showed relationship with the polychaetes species. There were two distinctive polychaetes assemblages found in Kung Krabaen Bay. The first group was the benthic community associated with high organic content consisted of Prionospio (Minusplo) japonica Mediomastus sp.A and Glycined sp.A as the major species. They were common in mod and very fine sediment in the drainage canals and the mouth of canals. The latter group, consisted of Lumbrineris sp.B mediomastus sp.A and Sigmabra cf, tentaculate, associated with low organic content sediment in for moderated organic enrichment area. It can be concluded the that Kung Krabaen Bay was in normal condition with the exception of organic enrichment in the drainage canals. The dominance of polychaetes in the benthic communities reflected the organic enrichment condition in the bottom sediment. Thus the monitoring program on the environmental impacts in the bay should included the water quality and changes in the benthic communities.