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We report the complex spatial and temporal dynamics of hyporheic exchange flows (HEFs) and nitrogen exchange in an upwelling reach of a 200 m groundwater‐fed river. We show how research combining hydrological measurement, geophysics and isotopes, together with nutrient speciation techniques provides insight on nitrogen pathways and transformations that could not have been captured otherwise, including a zone of vertical preferential discharge of nitrate from deeper groundwater, and a zone of rapid denitrification linking the floodplain with the riverbed. Nitrate attenuation in the reach is dominated by denitrification but is spatially highly variable. This variability is driven by groundwater flow pathways and landscape setting, which influences hyporheic flow, residence time and nitrate removal. We observed the spatial connectivity of the river to the riparian zone is important because zones of horizontal preferential discharge supply organic matter from the floodplain and create anoxic riverbed conditions with overlapping zones of nitrification potential and denitrification activity that peaked 10–20 cm below the riverbed. Our data also show that temporal variability in water pathways in the reach is driven by changes in stage of the order of tens of centimetres and by strength of water flux, which may influence the depth of delivery of dissolved organic carbon. The temporal variability is sensitive to changes to river flows under UK climate projections that anticipate a 14%–15% increase in regional median winter rainfall and a 14%–19% reduction in summer rainfall. Superimposed on seasonal projections is more intensive storm activity that will likely lead to a more dynamic and inherently complex (hydrologically and biogeochemically) hyporheic zone. We recorded direct evidence of suppression of upwelling groundwater (flow reversal) during rainfall events. Such flow reversal may fuel riverbed sediments whereby delivery of organic carbon to depth, and higher denitrification rates in HEFs might act in concert to make nitrate removal in the riverbed more efficient. [ABSTRACT FROM AUTHOR]

Macroinvertebrate communities in Western European rivers have changed substantially in recent decades. Understanding the causes is challenging because improvements in water quality have coincided with climatic variations over this period. Using data covering >2300 rivers and 21 years (1991-2011) across England and Wales, we analysed family-level distributions and nationwide trends in prevalence (proportion of sampling locations where an organism was present) to diagnose the causes of ecological change. Our aims were to: (i) reveal the taxa driving assemblage-level trends; (ii) identify the main changes in family-level prevalence and distribution patterns; and (iii) test whether changes were accounted for by improving water quality, increasing temperatures or variations in discharge. While previous analyses revealed increasing richness among British river invertebrates, a partial turnover of taxa is now evident. Two distinct components of temporal trend have comprised: (i) overall increases or decreases in taxon prevalence over 21 years, which correlated with pollution sensitivity and discharge; and (ii) short-term variations in prevalence that correlated primarily with temperature and nutrient concentrations. The longer-term changes in prevalence were reflected in expansions or contractions in families' distributions linked to water quality, with little evidence of shifts consistent with increasing temperatures. Although these monitoring data had limitations (e.g., family-level data, few headwaters), they provide no clear evidence of long-term climate effects on invertebrates; the one feature consistent with climate warming - a small northward expansion of the range of many taxa - was accounted for by large improvements in water quality in northern England. Nevertheless, changes linked to discharge and temperature over the shorter-term (<2 years) point to the climatic sensitivity of invertebrate communities. It is therefore likely that any long-term climatic changes since 1990 have been outweighed by the strength and geographical extent of the recovery from poor water quality. [ABSTRACT FROM AUTHOR]