Aquatic plants exposed to environmental changes exhibit plastic responses, resulting in functional adjustments to reduce stress effects. Lack of light can limit plant development and can affect biomass allocation and reproduction, stressing plants and sometimes halting their growth. Shading techniques have been used to control the excessive growth of weed plants, such as the aquatic fern Salvinia auriculata Aublet that can form dense mats on the water surface, causing problems in water use. We used shading nets in a greenhouse experiment to evaluate the effect of shade on the biomass of S. auriculata, and to determine if the fern changes its allocation of biomass to sexual (sporocarps) or asexual (buds) reproduction under different shade levels (full-sun control, 35% shade, and 70% shade). Under shade conditions, ramet biomass decreased and no sporocarps were produced, although the number of buds increased. Production of structures for sexual reproduction incurs a high energy cost, so S. auriculata invested in bud production (clonal reproduction). The differing energy requirements resulted in a significant trade-off between bud and sporocarp production. In conclusion, our study indicated that shading is not an efficient control technique for S. auriculata since it did not affect the clonal reproduction, a strategy that accelerates colonisation and facilitates rapid spread.

Aquatic plants can use differential allocation (trade-off) of carbon among their structures depending on the nutrition concentration. Given that N and P are limiting in the growth of plants, our questions were: Are the N and P concentrations in S. auriculata related to the biomass allocation to its structures? Is a differential allocation of N and P between floating and submerged leaves? We evaluated the relation between the nutrients and the biomass allocation, and the trade-off among the leaves using the Spearman correlation. Our results showed that N and P concentrations in S. auriculata are related to the biomass allocation to its structures, and that there is no trade-off of these nutrients between "shoot and root". Thus, we can see the importance of N and P concentration in the biomass of S. auriculata, and why this plant is capable to development in different environments as a weedy.Keywords: nutrients, Salviniaceae, trade-off, resources. Alocação de biomassa e o Balanço de nutrientes relacionados à concentração de Nitrogênio e Fósforo em Salvinia auriculata (Salviniaceae) ResumoPlantas aquáticas podem realizar alocação diferencial (trade-off) de carbono entre as suas estruturas dependo da disponibilidade de nutrientes. Considerando que N e P são limitantes para o crescimento de plantas, nossas perguntas foram: As concentrações de N e P em S. auriculata estão relacionadas com a alocação de biomassa para suas estruturas? Existe alocação diferencial de N e P entre os folíolos aéreos e a "raiz"? Avaliamos a relação entre os nutrientes e a biomassa, e o trade-off entre as folhas utilizando correlação de Spearman. Os resultados encontrados mostram que as concentrações de N e P em S. auriculata estão relacionadas com a alocação de biomassa para as suas estruturas, e que não há um trade-off de N e P entre os folíolos aéreos e a "raiz". Dessa maneira, é possível observar a importância de N e P no ganho de biomassa de S. auriculata, e porque a espécie é capaz de se desenvolver em diferentes ambientes como uma planta daninha.

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Freshwater resources faces threats with aquatic plants invasion, considered biological pollution with deep effects on water quality and nutrients cycling due to their rapid growth. Orbital remote sensing has been an effective instrument of monitoring large water bodies. Thus, the aim of this study was to analyze the relation between reflectance and field measurements (biomass and nitrogen concentration) of aquatic plants to develop estimation equations and to test vegetation indices to use in orbital remote sensing. The most common tropical infesting species (Salvinia auriculata, Pistia stratiotes, Eichhornia crassipes and Eichhornia azurea) were collected during a year, measured their spectral response to simulate satellite bands, and the biomass and nitrogen concentration measurements. The bands intervals of Sentinel-2 satellite were choosing to the simulation due to their narrow bands and the RedEdge new band. The obtained field data were correlated with the reflectance obtained from spectroradiometry of each species and the equations showed R² = 0.64 to estimate biomass and R² = 0.60 to estimate nitrogen using the entire spectrum. Several indices described in the literature were tested with different Sentinel-2 bands but with no significant results. The NDVI index showed a separation among species using RedEdge band and can be used to identify the species, but not to estimate their biomass.