World Aquaculture Society Meetings


Endang Sri Utami, Sigid Hariyadi1, Hefni Effendi1 and David A. Bengtson

The concentration of oxygen dissolved plays a vital role in biogeochemical cycling and as a sensitive indicator of physical and biogeochemical changes in aquatic ecosystem. Cirata Lake is  a cascade type, has main potential usage as power plant to suplay electricities for Java and Bali Islands, it is also used as aquaculture activities with the floating cage system. Aquacultue in Cirata Lake has been over carriying capacity, it has reached over 50.000 cages but the legal cages are only 12.000 cages. Investigation and water sampling were conducted in four sites which were considered on high activities of floating cages and no activities of floating cages. Weekly data of DO, temperature, pH, nitrate, nitrite, ammonia, orthophosphate, COD and chlorophyll were collected insitu for 6 weeks from April to May 2015. The DO concentration in high cages activities sites is significantly different from no cages activities sites. Site with high cages activities has lower DO concentration than no cages sites. DO concentration in high cages activities fell to hypoxia (< 3 mg l-1 ) at 7 m in the first week and move heading to 4 m at last week with anoxic layer at 20 m. This condition is significantly different from the site with no cages activities, the hypoxia layer move from 8 m and heading to 6 m in the last week, with anoxic layer was found in 30 m. Chlorophyll-a, COD, orthophosphate and pH in the sites with high cages activities are also significantly different from the sites with no cages activities. It means, this high activities of floating cages lead to high of organic matter in the water column and it affects to high of decomposition process that consume a lot of dissolved oxygen concentration. During the sampling time, it was the rainy day on the last three weeks and this condition might affected to the vertical DO profile. DO concentration, chlorophyll a, COD, temperature and pH in Cirata Lake during the observation is also significantly different in different depth and week. Nitrate, nitrite and ammonia in surface are signicantly different from the column layer. Hypoxic conditions in Cirata Lake can exist for periods of the days to weeks or longer, with occasional episodes of anoxia triggering fish kills. This hypoxia and anoxic layers also could be dangerous if it is continued by the long rainy days because it might be followed the upwelling events. As known that the most terrible problem in almost every year in Cirata Lake is upwelling events which is followed by the massive fish kill. This study advance the understanding of the response of oxygen vertical profile to high of floating cage activities, which could strengthen management efforts (decreasing nutrient load and may hypolimnetic oxygenation) to address lake eutrophication and the possibility of massive fish kill. The result of this study  suggest that the reservoir water quality must be monitored more frequently, especially in local rainy season to ensure the sustainability of aquatic ecosystem according to its purpose.

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