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Macrophyte assemblages are composed of species with different life forms and various ecological functions. Our aim was to investigate the potential environmental determinants of changes in the biomass of individual life forms and of the composition of the macrophyte assemblage in terms of life forms diversity. We sampled 23 waterbodies at low and high water levels in the Middle Paraná River floodplain. Macrophyte biomass samples were collected and classified in terms of life forms. We performed a redundancy analysis using the biomass of the various life forms to assess the importance of environmental variables to the composition of macrophyte life forms. Linear regressions were applied to investigate the environmental determinants of the biomasses of individual life forms. The degree of connectivity and the combination of depth, hydrology and nitrate were the main determinants of the composition in terms of life forms. The biomass of each individual life form was explained by different combinations of environmental variables, but the connectivity was the most important one. Our study shows that groups of species with similar life forms respond to environmental factors in particular ways, which might alter the biomass composition of life forms. Given that the ecosystem functioning depends on the functional characteristics of local communities, our findings about the relation between environmental changes and the community composition in terms of life forms (or functional composition) can be a helpful tool for predicting changes on ecosystem processes (such as nutrient cycling) against possible future scenarios.

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Questions Understanding the processes that determine the variation in community composition (β‐diversity) is a major challenge in ecology, evolution, and conservation. Here we assess the importance of abiotic variables associated with local environmental features, hydrogeomorphology, and space to explain β‐diversity patterns in macrophytes by addressing the following questions: (1) Which are the sets of environmental, hydrogeomorphological, and spatial variables that contribute significantly to the spatial variation of macrophytes? (2) Which are their relative contributions to explaining the total β‐diversity, nestedness and turnover of macrophyte assemblages? Study Site Middle Paraná River floodplain, Argentina. Methods We sampled 20 lakes at low and high water levels in the Middle Paraná River floodplain. To investigate the relationship of total β‐diversity, turnover and nestedness with explanatory variables, we used the constrained analysis of principal coordinates (CAP). We used variation partitioning to assess the proportion of the variation related to each subset of variables (environmental, hydrogeomorphological and spatial). Results Among four alternatives considered to explain the spatial structure, the distance between lakes by watercourses best described the spatial organization of β‐diversity. Species turnover was largely explained by hydrogeomorphological variables, followed by spatial descriptors, whereas nestedness was exclusively related to spatial variables. Conclusions In the Middle Paraná River floodplain β‐diversity of macrophyte communities is partly shaped by turnover (mainly driven by the hydrogeomorphology of the floodplain) and nestedness (resulting from dispersal limitation by watercourses), but it also seems to be randomly organized (possibly because of random colonization and extinction, due to disturbances such as floods). Our work indicates that environmental pressures reflect differences between macrophyte communities of lakes with different geomorphological and hydrographical conditions. This suggests that certain ecological processes (such as species sorting) are driven at a higher degree by hydrogeomorphology than by local environment.

Phosphorus (P) inputs are increasing in river-floodplain systems, but the factors which influence the dynamics of this nutrient are not clear. To assess P dynamics in this kind of river, the main channel of the Middle Paraná River, 3 anabranches, 9 secondary channels, and 20 lakes (7 permanently connected and 13 temporarily connected to the fluvial system) were sampled. Multiple linear regressions were applied to explain spatio-temporal patterns of P through commonly measured limnological variables.Particulate P increased during the sediment peak (evaluated through turbidity). Soluble reactive P (SRP) was positively associated with dissolved organic matter (DOM, mainly the chromophoric fraction), which increased during high waters in the fluvial system but was highly variable in each kind of aquatic environment. In temporarily connected lakes, vegetated zones dominated by emergent macrophytes displayed the highest SRP and chromophoric DOM concentrations. The flood and sediment peak positively affected P load in the river due to the increase in dissolved and particulate fractions, respectively. In addition, particle-bound alkaline phosphatase activity was positively associated with SRP concentration and load. These results suggest that the sediment peak incorporates particulate P in the system while the floodplain is a P source during floods through exportation of the dissolved fraction. Dissolved P could be largely exported associated with DOM, which stimulates phosphatase biosynthesis by decreasing P bioavailability. The effect of aquatic macrophytes on P dynamics seems to be influenced by DOM exudation. According to these considerations, DOM should be taken into account to analyse P dynamics in river-floodplain systems. KEYWORDS aquatic macrophytes, chromophoric dissolved organic matter, hydrosedimentological regime, Middle Paraná River system, particle-bound alkaline phosphatase activity, spatio-temporal patterns of phosphorus

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