Little is known about the relative contribution of different stream habitats to reach-scale metabolism. We measured in situ metabolism in sand, gravel, stone, and macrophyte habitats to compare metabolic rates among these habitat types and to compare habitat-weighted measurements with reach-scale measurements. We used open-bottom chambers in sand, gravel, and macrophyte habitats and closed-bottom chambers in stones, and we estimated reach-scale metabolism from 2-station O 2 budgets. Macrophyte habitats had a significantly higher gross primary production (GPP) and community respiration (CR) than stone, gravel, and sand habitats. A large part of this difference was associated with epiphytic biofilm: 28% of net ecosystem production (NEP), 20% of CR, and 24% of GPP. Macrophyte habitats contributed proportionally more to reach-scale metabolism than did the other habitat types. Forty-one percent of reach-scale NEP, 60% of ecosystem respiration (ER), and 50% of GPP were associated with this habitat type even though only 14% of the reach was covered by macrophytes. We found significant linear relationships between GPP and CR and the amount of autotrophic biomass in the streams. The rates reported in 11 literature studies fit into our observed relationships, showing the generality of our findings. The rates we obtained expand the range of reported metabolic values in relation to autotrophic biomass for both low and high biomass. The importance of macrophyte habitat can be ascribed to the macrophytes themselves, the associated epiphytic biofilm, and the fine organic material trapped in the dense stands. We conclude that besides having an effect on the structural elements in streams, macrophytes contribute significantly to stream ecosystem functions.

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1. In low-gradient, macrophyte-rich rivers, we expect that the significant change in macrophyte biomass among seasons will strongly influence both biological activity and hydraulic conditions resulting in significant effects on nutrient dynamics.Understanding seasonal variation will improve modelling of nutrient transport in river networks, including annual estimations of export, which could optimise decision-making and management outcomes.2. We explored the relationships among seasonal differences in reach-scale nutrient uptake, macrophyte abundance, solute transport and transient storage in the River Gudenå (Denmark), a large macrophyte-rich river. We used the minimal pulse addition technique to measure uptake of ammonium, nitrate, soluble reactive phosphorus, as well as reach-scale metabolism, and surface transient storage in spring, summer, and autumn.3. We found that riverine uptake changed among seasons and was linked to macrophyte biomass via both biological activity, reflected in reach-scale metabolism, and through physical processes, as solute transport was influenced by longitudinal dispersion. In this macrophyte-rich river, seasonal changes in macrophyte biomass affected contact time between the water and biota, which influenced ammonium and soluble reactive phosphorus uptake. Using stoichiometric scaling of reachscale metabolism, we found that seasonal variation also influenced the relative contributions of autotrophic and heterotrophic biota in assimilatory uptake. 4. In summary, riverine nutrient uptake was not static, highlighting the importance of seasonality, with significant implications for modelling of nutrient export in river networks. Moreover, current management strategies that remove macrophyte biomass (i.e. weed cutting and dredging) will short-circuit the positive effects of enhanced nutrient uptake resulting from abundant macrophytes in rivers. K E Y W O R D Snitrogen, phosphorus, river, stream, transient storage | 179 RIIS et al.

It is a generally accepted theory that ecological functions are enhanced with increased diversity in plant communities due to species complementarity effects. We tested this theory in a mesocosm study using freshwater submerged plant beds to determine if increasing species number caused overyielding and species complementarity. We applied a maximum of four species in the plant beds corresponding to the typical species number in natural freshwater plant beds. We found no clear effects of species number (1–4) on biomass production and thus no conclusive overyielding and complementarity effect. This may be explained by low species differentiation among the four species in plant traits relevant for resource acquisition in freshwater, or that other species interactions, e.g. allelopathy, were inhibiting overyielding. The existing knowledge on species complementarity in aquatic plant communities is sparse and inconclusive and calls for more research.