The type of N-supply (ammonia or nitrate) determines the performance of seaweed biofilters integrated with intensive fish culture

Submerged macrophytes can efficiently remove nutrients from low-level effluents and from aquatic environments, while generating income. We studied the removal of nutrients from an intensive fishculture system by seaweed (Ulva lactuca L). The system was compartmentalized, so that the fish and the seaweed grew in separate tanks between which the water recirculated. An additional seaweed biofilter tank polished the system's outflow. Overall water residence time was about 4 days, but water recirculation brought it to about 8 h in the recirculating biofilters. Fish excretions supplied NH₄⁺-N (NH₃+NH₄⁺) to the system. ToxN (NO₂^- and NO₃^-) accumulated up to a total of 200 μM, and it greatly reduced the N-removal capacity of the seaweed relative to NH₄⁺-N. Of the total dissolved N (TDN), NH₄⁺-N was taken up by the seaweed much more readily than ToxN. Furthermore, NH₄⁺-N inhibited the uptake of ToxN, so that the latter was taken up by the seaweed only when NH₄⁺-N concentrations dropped below 1-2 μM. This happened only in the polishing biofilter. NH₄⁺-N removal efficiency in the recirculating biofilter was 74% and in the polishing tank, due to much lower supply fluxes, 85%. The polishing biofilter also removed over 16% of the ToxN input while the recirculating biofilter did not remove ToxN at all. The supply fluxes of N required by seaweed for equal aerial rate of N-removal were about triple for ToxN than for NH₄⁺-N. The N content in the seaweed was about a third higher for the recirculation biofilter than for the polishing biofilter. Curtailing nitrification, for instance by limiting and disturbing the surface area available for bacterial colonization, can significantly increase the aerial rate of N removal by seaweed and other harvestable macrophytes from various aquatic environments and therefore greatly improve their economic viability.