Different scales determine the occurrence of aquatic macrophyte species in a tropical stream

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.摘要 水生大型植物（aquatic macrophyte）的物种分布通常与水体化学特征密切相关。然而，流水生态系统中影响水生植物出现的其他环境因素——如与周边景观相关联的动态生境——长期以来被低估。本研究旨在评估热带河流流域内，可能影响真水生植物（euhydrophyte）类水生大型植物物种出现的景观特征。 本研究调查了以下7个物种的分布情况：Egeria densa、Cabomba furcata、Potamogeton pusillus、Potamogeton polygonus、Utricularia foliosa、Pistia stratiotes及Salvinia molesta。同时，沿巴西圣保罗州伊坦亨河流域的25.5千米河段，我们测定了三个空间尺度下的环境变量：本地（湖沼学，limnological）尺度、河道尺度及河岸景观尺度。 研究结果显示，本地（湖沼学）及河道特征是决定水生大型植物物种分布的关键变量，而景观尺度对物种组成的影响极小。其中，河道水深与岸坡坡度是解释其中4个物种分布的重要非生物因子，但对P. pusillus、P. polygonus及U. foliosa的分布无显著解释力。 本研究结果凸显了河道地貌（channel morphology）在解析热带河流水生植物分布及群落组成方面的重要性。