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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.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.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 reach‐scale metabolism, we found that seasonal variation also influenced the relative contributions of autotrophic and heterotrophic biota in assimilatory uptake.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. [ABSTRACT FROM AUTHOR]

Along the river network, water, sediment, and nutrients are transported, cycled, and altered by coupled hydrological and biogeochemical processes. Our current understanding of the rates and processes controlling the cycling and removal of dissolved inorganic nutrients in river networks is limited due to a lack of empirical measurements in large, (nonwadeable), rivers. The goal of this paper was to develop a coupled hydrological and biogeochemical process model to simulate nutrient uptake at the network scale during summer base flow conditions. The model was parameterized with literature values from headwater streams, and empirical measurements made in 15 rivers with varying hydrological, biological, and topographic characteristics, to simulate nutrient uptake at the network scale. We applied the coupled model to 15 catchments describing patterns in uptake for three different solutes to determine the role of rivers in network-scale nutrient cycling. Model simulation results, constrained by empirical data, suggested that rivers contributed proportionally more to nutrient removal than headwater streams given the fraction of their length represented in a network. In addition, variability of nutrient removal patterns among catchments was varied among solutes, and as expected, was influenced by nutrient concentration and discharge. Net ammonium uptake was not significantly correlated with any environmental descriptor. In contrast, net daily nitrate removal was linked to suspended chlorophyll a (an indicator of primary producers) and land use characteristics. Finally, suspended sediment characteristics and agricultural land use were correlated with net daily removal of soluble reactive phosphorus, likely reflecting abiotic sorption dynamics. Rivers are understudied relative to streams, and our model suggests that rivers can contribute more to network-scale nutrient removal than would be expected based upon their representative fraction of network channel length. [ABSTRACT FROM AUTHOR]

Anthropogenic N loadings and perturbations of macroconsumer communities impair ecological and economic services provided by streams. Organisms are adapted to natural disturbances, such as flooding and desiccation, but how anthropogenic and natural disturbances interact is poorly understood. We used large outdoor mesocosms to study the effect of , a common prairie headwater-stream minnow, and NH4++ additions (to simulate fish excretion) on the recovery of ecosystem structure and function following a flood, highlighting the potential for (and other macroconsumers) to affect denitrification. and NH4++ treatments differentially affected particulate organic matter size and filamentous algal structure. Ecosystem structure responded differently to mesocosm treatment over time, a result suggesting that grazers or NH4++-N availability may be especially important during early recovery periods. The presence of did not influence denitrification, but NH4++ additions altered the response of denitrifiers to nutrient and energy amendments, and denitrification rates decreased following the recovery of mesocosms. Temporal changes in denitrification probably were caused by increasing hyporheic dissolved O2 concentrations, which led to potentially fewer anoxic microsites for production of denitrification enzymes. Our study shows that grazers affect the recovery of ecosystem structure, but denitrification in the context of these prairie-stream mesocosms appears to be unaffected by . [ABSTRACT FROM AUTHOR]

Climate change, recurrent droughts, and increasing urban water demands have limited water availability in urban landscapes. Water quantity challenges have led to irrigation restrictions and turfgrass removal programs. An experiment was conducted at the University of Florida, West Florida Research and Education Center, Jay, FL, to evaluate the effect of turfgrass conversion to other landscape types on nutrient leaching and groundwater recharge. In April 2021, all surface vegetation was removed from existing turfgrass plots using a sod harvester. Thereafter, plots were planted or covered with three landscape types: a pollinator landscape with flowering forbs (Mimosa sp., Coreopsis sp., and Phyla sp.) + turfgrass (Eremochloa ophiuroides); a nitrogen (N)‐efficient lawn (Arachis glabrata + Paspalum notatum); and a low‐input landscape with unplanted woodchip mulch. Undisturbed turfgrass (E. ophiuroides) served as a control. For 2 years, leachate samples were collected weekly from previously installed 168‐L drainage lysimeters for NO3‐N and NH4‐N load determination. Temporal changes in landscape composition, groundwater recharge, water use, and soil bulk density were also quantified. While the mulch leached 44.7 kg ha−1 NO3‐N year−1, this landscape still offers positive attributes, including erosion protection and water conservation. Conversely, the pollinator landscape minimized nitrogen leaching (8.3 kg ha−1 NO3‐N year−1) due to their relatively greater water use rates (3.56 mm day−1). The turfgrass and nitrogen‐efficient lawn returned ∼35% of the water inputs as groundwater recharge while maintaining relatively low nitrogen leaching (3.6 and 2.7 kg ha−1 NO3‐N year−1, respectively), making these landscapes efficient for protecting both water quality and quantity. [ABSTRACT FROM AUTHOR]