Abstract
We assessed the role of planktonic processes, in comparison to allochthonous input from fish cages and sedimentary loss, in the organic carbon (OC) budget of the water column in a semi-enclosed fish-culturing area (culturing red sea bream Pagrus major and yellow tail Seriola quinqueradiata). The sedimentation rate of particulate organic carbon (POC) at the fish-cage sta- tion was an average of 1.5 times that at non-cage stations. There was no significant difference in photosynthesis or respiration rates between fish-cage and non-cage stations. Annual allochtho- nous OC input in the form of leftover feed and fish feces was estimated to be 5 or 10 times that of autochthonous OC input by planktonic photosynthesis. In contrast, POC derived from phytoplank- ton accounted for a significant part (8 to 61%) of total POC sedimentation. As to sinks of OC in the water column, annual planktonic respiration was twice as high as sedimentary loss at the fish- cage station. The plankton community tended to act as a source of OC in spring and summer and as an OC sink in fall and winter. The present study shows that a significant part of allochthonous and autochthonous OC input is respired by plankton and that the remaining OC input is deposited on the seafloor of fish-culturing areas.
Highlights
In a coastal fish-culturing area, deposition of organic wastes on the seafloor and subsequent oxygen consumption through the mineralization process sometimes result in formation of an anoxic water mass, a decline in benthos density, and occasional mass mortality of cultured fish (Brown et al 1987, Gowen & Bradbury 1987, Wu 1995)
In order to develop an effective countermeasure for organic pollution at fish aquaculture sites, it is necessary to investigate the importance of planktonic processes as a source or sink of organic carbon (OC) relative to organic wastes from fish cages and sedimentary loss
The present study aims to investigate the importance of planktonic processes as a source or sink of OC relative to organic wastes from fish cages and sedimentary loss, in order to develop an effective countermeasure for organic pollution at fish aquaculture sites
Summary
In a coastal fish-culturing area, deposition of organic wastes on the seafloor and subsequent oxygen consumption through the mineralization process sometimes result in formation of an anoxic water mass, a decline in benthos density, and occasional mass mortality of cultured fish (Brown et al 1987, Gowen & Bradbury 1987, Wu 1995). Phytoplankton abundance often increases as a result of nitrogen and phosphorus release from fish cages (Eloranta & Palomaeki 1986, Gowen & Bradbury 1987); it is possible that phytoplankton photosynthesis is another important source of organic matter in fish-. In order to develop an effective countermeasure for organic pollution at fish aquaculture sites, it is necessary to investigate the importance of planktonic processes as a source or sink of organic carbon (OC) relative to organic wastes from fish cages and sedimentary loss. The present study aims to investigate the importance of planktonic processes as a source or sink of OC relative to organic wastes from fish cages and sedimentary loss, in order to develop an effective countermeasure for organic pollution at fish aquaculture sites. The causes of seasonal and regional variations in the relative importance of planktonic metabolism on the OC budget are discussed
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