Limitation of water hyacinth by nitrogen in subtropical lakes of the Paraná floodplain (Argentina)

Low (0.1–0.7 µ mol liter −1 ) dissolved inorganic nitrogen concentrations are common in the floodplain lakes of the Paraná River, where the water hyacinth ( Eichhornia crassipes ) is the major primary producer. We tested the hypothesis that N supply could limit production of E. crassipes during the growth season by adding NH 4 + or PO 4 3− during 1 month to large in situ enclosures. Those enclosures amended with N produced twice as much biomass as the controls and P‐treated enclosures. The low and limiting N content (1.3% AFDW) of the control and P‐treated plants provide a first diagnostic value for N deficiency in the field for E. crassipes . N limitation in these lakes may be attributable to high P supply from Andean rivers, high losses of macrophyte N during floods, and the unstable nature of floodplain lakes.