Fate of 15N-enriched cyanobacteria feed for planktivorous fish in an enclosure experiment: a stable isotope tracer study

A tracer experiment using isotope 15N was performed to study the assimilation and retention of nitrogen from feces by two planktivorous fish, silver carp Hypophthalmichthys molitrix and tilapia Oreochromis niloticus, in Lake Taihu (China). Microcystis was enriched with 15N-NH4Cl, lyophilized to produce feed for fish, and traced to establish the fate of feces nitrogen. Samples of organisms and abiotic substances were analyzed for excess 15N and nutrient concentrations in the water column were determined on days 0, 1, 5, 10, 15, and 20. Nutrient concentration analyses indicated that TN and TP were about 4 times higher in the tilapia enclosure than in the silver carp enclosure due to the digestive capacity of Microcystis, which suggests that the ichthyoeutrophication potential of tilapia is greater than that of silver carp. 11.05 % of the 15N was assimilated by the tilapia whereas 3.58 % of the 15N was assimilated by the silver carp, suggesting that tilapia has a higher capacity to assimilate and retain Microcystis nitrogen than silver carp, although the absorptivities of both species were relatively low. At the end of the experiment, 8.48 % of the 15N was detected in sedimentary detritus in the tilapia enclosure, as compared to 6.07 % of the 15N in the silver carp enclosure, which suggests that only a small fraction of the Microcystis-derived nitrogen sank to the bottom. In conclusion, much of the Microcystis-derived nitrogen was neither assimilated by fish nor accumulated on the sediment floor. It presumably floated in the water column, contributing to phytoplankton propagation and thus degrading the water quality.