The series of reservoirs located in the upper Paraná River and its tributaries retain a large amount of phosphorus (sedimentation), impoverishing downstream waters. Although these nutrients are limiting for primary production, their role on the growth of native aquatic plants is still unknown. The free-floating macrophyte, Eichhornia crassipes, is among the most successful colonizers and fastest-growing macrophytes in the Neotropics. However, in the upper Paraná River floodplain, this plant rarely develops extensive coverage and it is even absent from some environments. We hypothesize that phosphorus concentration is limiting the growth of E. crassipes in certain aquatic habitats of this floodplain, and to test this hypothesis we designed a field mesocosm experiment using nutrient amendments. Twelve closed mesocosms (1,500 L) plus three open units (with ambient lagoon water) were set up in the Garças lagoon, under five nutrient treatments (three replicates each): L (“lagoon”, no addition), C (control, no addition), N (nitrogen addition), P (phosphorus addition), and NP (nitrogen and phosphorus addition). After nutrient amendments, introduced plants were weighed weekly for fresh biomass over a three-week period. Plants in the P and NP treatments had the most rapid increase in biomass (rmANOVA, F12,30 = 7.54, p < 0.05) and the greatest development of sprouts (F12,30 = 3.25, p < 0.05). In addition, phosphorus content in plant tissues was highest in P and NP treatments, while nitrogen content was similar in L, N, and NP treatments. Our results suggest that the growth of water hyacinth in the upper Paraná River floodplain is limited by phosphorus, not nitrogen. Phosphorus removal in upstream reservoirs may limit E. crassipes populations downstream, although more study is required to establish this link.
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Vol. 28 • No. 4