Putth Songsangjinda

The Role of Suspended Oyster Culture on Nitrogen Cycle in Hiroshima Bay

The predominance of bivalves affects the cycle of materials in the coastal ecosystem. In the present study, the role of suspended oyster culture on the nitrogen cycle was demonstrated for the northern Hiroshima Bay. The nitrogen cycle was considered as two systems, (1) the primary production (PP) system and (2) the oyster culture (OC) system. The results show that about 26% of N productivity was supplied to process by cultured oysters in the OC system. This process varies seasonally due to the seasonal variations of PON, physiological activities and biomass of oysters. The N processing rates were found to be high in summer and low in winter. The biodeposition and excretion of N in the OC system are 3.0 and 2.1 ton N d−1, while the natural sedimentation rate and N regeneration in the PP system are 8.3 and 18.0 ton N d−1, which indicates that the PP system is a major system regenerating N in the water column. The release of total dissolved N from the bottom to the water column is about 8.3 ton N d−1. The amount of N harvested as oyster product was about 1.3 ton N d−1, which is about 10% of daily N loading in north Hiroshima Bay. According to the N cycle developed in the present study, the results suggest the significant role of suspended oyster culture on the nitrogen cycle in Hiroshima Bay. In addition, our results indicate that oyster production was efficiently harvested, suggesting that oyster culture could probably be used as a tool to remove N from Hiroshima Bay.

Nitrogen dynamics in the settlement ponds of a small-scale recirculating shrimp farm (Penaeus monodon) in rural Thailand

Rural shrimp farmers in Thailand are being encouraged to adopt practices that will reduce the quantity and improve the quality of their effluent. A simple and cheap option for small-scale shrimp farmers is to use settlement ponds to store and remediate discharge water before being re-used. We undertook a detailed study of the settlement ponds in a small-scale commercial black tiger shrimp farm typical of rural Thailand. We found that over a 9-week period, following the harvest of one of the two farm production ponds, total nitrogen (TN) concentrations in the water column were reduced by 30%, with the greatest removal (56%) occurring during the fifth week. There was a 10% increase in dissolved organic nitrogen (DON) concentrations during the trial. Sediments were a source of total ammonia nitrogen (TAN), and the re-mineralisation rate was the highest in the first two settlement ponds. Coconut fronds added to two of the four settlement ponds to increase the surface area available for microbial activity were found to provide a site for microbial re-mineralisation of TAN, the photosynthetic uptake of TAN and oxidised nitrogen (NOx) and nitrification. The water column was a net assimilator of TAN through autotrophic uptake. This study has shown that settlement ponds are capable of reducing water column N concentrations; however, sediment must be managed to reduce re-mineralisation during successive cropping cycles. In addition, coconut fronds were shown to improve N removal, although they should be periodically removed to maintain efficiency.

Uptake and Release of Particulate Materials by Suspended Oyster Culture in Hiroshima Bay : Results From Raft Study

沿岸域で行われている垂下式カキ養殖による粒状物の取り込みと排出の季節変動を定量的に明らかにするため、1995年10月～1996年8月に広島湾の養殖筏を対象に研究を行った。筏(200m2)の上流と下流の粒状物質(PM)、クロロフィルa(Chl. a)および粒状態有機炭素(POC)・窒素(PON)・リン(PP)を測定した。筏下および対照海域(筏から100m)における沈降物の質量(SM)、炭素(SC)・窒素(SN)・リン(SP)量を測定した。筏に流入する海水の上流側と下流側での粒状物質のC:N:P比に違いはなかったが、それらの濃度には有意な違いが見られた。筏下と対照海域でのSMフラックスは大きく異なっていた。これらのことは、垂下式カキ養殖が物質の取り込みと排出という面で大きな役割を果たしていることを示している。さらに、筏での粒状物質の取り込みと排出の季節変動は大きいことがわかった。すなわち、PM、Chl. a、POC、PON、PPの取り込みは、それぞれ1.1-8.5kg DW raft-1 h-1、0.8-2.8g raft-1 h-1、6.7-43、1.3-7.0、0.09.-0.30g atom raft-1 h-1 であった。広島湾の垂下式カキ養殖による POC と Chl. a の取り込み速度は他の海域の高密度天然カキ礁やイガイ床で報告されている値と同程度である。筏下におけるSM、SC、SPの沈降フラックスは約60-1、400g DW raft-1 h-1、1、4-12、0.1-0.6、0.01-0.06g atom raft-1 h-1であった。この研究の結果は、広島湾において粒状物質の水柱からの取り込みと海底に対する負荷という点で、垂下式カキ養殖が果たしている役割が大きいことを示している。