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Studies of macrophyte productivity in the Amazon region are limited by accessibility and costs; hence,they may suffer from reduced sample size and representation. The present study compares aphenometric (indirect)method and a subsampling (direct) method in terms of accuracy and applicabilityto estimation of aquatic macrophyte biomass in the Amazon. The results show that phenometric modelswere not as effective as selective subsampling for the estimation of macrophyte biomass under thestudied conditions. Phenometric models performed more acceptably for predicting emergent biomass,and less for submerged and total biomass (r2 = 0.77, p < 0.05, RMSE = 200-600 g/m2 dry mass).Improvements in r2 by using species-specific phenometric models were mostly not significant.Phenotypic variation across the studied region was large enough to preclude the generalization ofphenometric relationships into accurate numericmodels, while the direct subsamplingmethod was ableto account for this variation (RMSE < 500 g/m2 dry mass). Subsampling also allowed a significantreduction on the physical effort of biomass sampling, which directly translated into wider and morecomplete sampling. We suggest that direct subsampling presents the best trade-off between accuracyand coverage for macrophyte biomass measurement in the Amazon floodplain.

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