Riverine macrophytes control seasonal nutrient uptake via both physical and biological pathways

Abstract

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.

Environmental control of macrophyte traits and interactions with metabolism and hydromorphology in a groundwater‐fed river

“…For ER, significant correlations with plant traits were only found in the patchy reach, which might be explained by the homogeneity of the homogeneous reach: ER was more constant in the homogeneous than the patchy reach. Metabolism and biomass had similar seasonal patterns with the highest value in early summer and the lowest in winter and this temporal variation has also been found in other studies with evergreen vegetation (Riis et al, 2019). As macrophytes interact with their environment, we expected correlations between macrophytes and flow velocity, as well as the depth of the fine sediment layer.…”

Data-Driven System Dynamics Model for Simulating Water Quantity and Quality in Peri-Urban Streams

“…The relatively good simulation results for the NO 3 -N concentrations rule out a possible underestimation of the nitrification process, suggesting instead an assimilation preference for NH 4 -N (rather than NO 3 -N) by the aquatic plant biomass. Such an assimilation preference has also been reported in [83,84], for instance, using controlled releases of nutrient pulses and stable isotope analysis, respectively.…”

“…Many streams are heterotrophic ecosystems, i.e., the gross primary production, GPP, from the autotrophic biomass is less than the overall ecosystem respiration, ER, (sum of autotrophic respiration and heterotrophic respiration, HR, from organic matter decomposition). Notably, such conditions have been documented in streams with important macrophyte coverage, enhancing the settling of fine particles fueling heterotrophic respiration and resulting in oxygen consumption (see, for instance, [56,83]). We defined the SOD enhanced,mac corresponding to this enhanced heterotrophic respiration to account for this effect, with temperature and oxygen limitations, as follows: SOD enhanced,mac = P/EM ratio − AR DO DO + k s,o_lim θ SOD T w −20…”

River network alteration of C-N-P dynamics in a mesoscale agricultural catchment

“…Two main processes may be responsible for apparent NO 3 retention in the river network at low flow: autotrophic primary production and heterotrophic denitrification at the water/sediment interface. For macrophytes, NO 3 retention is temporary because they decompose in autumn and release dissolved inorganic N (DIN) (Riis et al, 2019). For suspended algae, apparent NO 3 retention actually represents transformation into organic N, which will transit through the river network.…”