ABSTRACT Distributions of aquatic macrophyte species are commonly associated with water chemistry characteristics. However, other environmental factors that can lead to the occurrence of aquatic plants in lotic ecosystems, such as dynamic habitats linked to the surrounding landscape, have been underestimated. This study aims to evaluate landscape features that may influence the occurrence of species of euhydrophyte aquatic macrophytes in a tropical river basin. We assessed the occurrence of the following seven species: Egeria densa, Cabomba furcata, Potamogeton pusillus, Potamogeton polygonus, Utricularia foliosa, Pistia stratiotes and Salvinia molesta. We also measured environmental variables related to three spatial scales, local (limnological), channel and riparian landscape, along 25.5 Km of the Itanhaém River basin (São Paulo, Brazil). We found that local (limnological) and channel characteristics were important variables in determining the occurrence of aquatic macrophyte species while the landscape scale had little influence on species composition. Channel depth and margin slope were especially relevant abiotic variables in explaining the occurrence of four of the species but not P. pusillus, P. polygonus and U. foliosa. Our results highlight the importance of channel morphology for understanding aquatic plant occurrence and community composition in tropical rivers.

A temperatura e fotoperíodo são variáveis abióticas que influenciam diretamente a produção primária das macrófitas aquáticas, determinam a distribuição geográfica das espécies e afetam a estrutura das comunidades. O objetivo desse trabalho foi avaliar a influência de diferentes condições de temperatura e fotoperíodo no crescimento de Pistia stratiotes. Indivíduos de P. stratiotes foram acondicionados em câmaras climáticas, com controle de temperatura e fotoperíodo. Utilizou-se oito tratamentos, 1 (15 ºC/8 h de fotoperíodo); 2 (15 ºC/12 h); 3 (25 ºC/8 h); 4 (25 ºC/12 h); 5 (30 ºC/8 h) e 6 (30 ºC/12 h). O maior ganho de biomassa ocorreu no tratamento 4, que apresentou diferenças significativas (p<0,05) em relação aos tratamentos 1, 5 e 6. Nos tratamentos 1, 5 e 6, a biomassa final foi inferior à biomassa inicial de P. stratiotes. A maior redução de biomassa ocorreu no tratamento 6, que foi significativamente inferior aos tratamentos 2, 3 e 4. Observou-se nos tratamentos com temperatura de 25 ºC a maior produção de brotos, enquanto que, nos tratamentos com temperatura de 15 ºC e 30 ºC, essa produção foi muito reduzida. Concluiu-se que temperatura e fotoperíodo influenciam o crescimento de Pistia stratiotes e que esta espécie apresentou maior crescimento quando submetida à temperatura de 25 ºC e fotoperíodo de 12 horas.

Temperature and photoperiod are abiotic variables that directly influence aquatic macrophyte production. They also determine the geographic distribution of the species and affect community structure. The aim of this study was to evaluate the growth of Pistia stratiotes in different temperature and photoperiod regimes. P. stratiotes plants were conditioned in growth chambers with controlled temperature and photoperiod. Six treatments were used: 1 (15 ºC/8 h photoperiod); 2 (15 ºC/12 h); 3 (25 ºC/8 h); 4 (25 ºC/12 h); 5 (30 ºC/8 h) and 6 (30 ºC/12 h). The highest biomass gain occurred with treatment number 4 which showed significant differences (p<0.05) compared to treatments number 1, 5 and 6. In treatments 1, 5 and 6 the final biomass was lower than the initial biomass of P. stratiotes. The greatest biomass reduction occurred in treatment number 6 which was significantly lower than in treatments 2, 3 and 4. The highest sprout production was observed in treatments at 25 ºC, while in treatments at 15 ºC and 30 ºC sprouting was greatly reduced. We conclude that temperature and photoperiod influence P. stratiotes growth and higher growth rates occurred at 25 ºC and 12-hour photoperiod.