Limitation of Water Hyacinth by Nitrogen in Subtropical Lakes of the Parana Floodplain (Argentina)

Low $(0.1-0.7 \mu mol liter^-1)$ dissolved inorganic nitrogen concentrations are common in the floodplain lakes of the Parana 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 limpitation 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.