¹⁵N tracer application to evaluate nitrogen dynamics of food webs in two subtropical small-scale aquaculture ponds under different managements

Small, semi-intensively managed aquaculture ponds contribute significantly to the food security of smallscale farmers around the world. However, little is known about nutrient flows within natural food webs in such ponds in which fish production depends on the productivity of natural food resources. 15N was applied as ammonium at 1.1 and 0.4% of total nitrogen in a traditionally managed flow-through pond and a semi-intensively managed stagnant pond belonging to small-scale farmers in Northern Vietnam and traced through the natural food resources over 7 days. Small-sized plankton (1–60µm) was the dominant pelagic biomass in both ponds with higher biomass in the stagnant pond. This plankton assimilated major portions of the applied tracer and showed a high sedimentation and turnover rate. High re-activation of settled nutrients into the pelagic food web was observed. The tracer was removed more quickly from the flow-through pond than from the stagnant pond. A steady nutrient supply could increase fish production