Phosphorus Release by Fish in Relation to External and Internal Load in a Eutrophic Lake

The aim of this study was to compare the rate of phosphorus release from fish with estimates of external and internal load in the eutrophic Finjasjon, Sweden. Phosphorus release from roach and bream, when fed either benthic (Chironomidae) or pelagic (Daphnia) prey, was measured in laboratory tanks. These results were transformed to whole lake conditions by using calculated fish consumption rates of both benthic and pelagic prey based on field data for the community structure, growth rate, and diet of the fish. Phosphorus (P) release from the fish was on the average 0.53 mg P m-2 d-1, which was of the same magnitude (110% of external load) as the external load and 42% of the internal load. Most of the P released from the fish was of pelagic origin, being recycled within the water column. Some 18% of the released P was due to benthic feeding, mainly by large bream, thus representing P translocated from the sediment to the water. This last component of the P release constituted 27% of the external and 10% of the internal load. The P release from fish was in a form directly available to phytoplankton, suggesting that P release from fish may be an important nutrient source for algae.