Abstract
Drought is a global phenomenon, with widespread implications for freshwater ecosystems. While droughts receive much attention at lower latitudes, their effects on northern river networks remain unstudied. We combine a reach-scale manipulation experiment, observations during the extreme 2018 drought, and historical monitoring data to examine the impact of drought in northern boreal streams. Increased water residence time during drought promoted reductions in aerobic metabolism and increased concentrations of reduced solutes in both stream and hyporheic water. Likewise, data during the 2018 drought revealed widespread hypoxic conditions and shifts towards anaerobic metabolism, especially in headwaters. Finally, long-term data confirmed that past summer droughts have led to similar metabolic alterations. Our results highlight the potential for drought to promote biogeochemical shifts that trigger poor water quality conditions in boreal streams. Given projected increases in hydrological extremes at northern latitudes, the consequences of drought for the health of running waters warrant attention.
Highlights
Drought O2 (μ mol L–1) 0 0 b Med = 165.3 Med = 74.6Hyporheic p < 0.0011000 1200 min max dAerobic respiration Med = 402.8 Med = 130.3Whole-stream p < 0.001
We experimentally simulated drought over a 2-week period during summer 2017 by damming a lake outlet that feeds a small stream in the Krycklan Catchment Study (KCS) (Supplementary Fig. 1)
The broader KCS landscape is typical of boreal Fennoscandia, dominated by coniferous forests (Pinus sylvestris and Picea abies), open wetlands with extensive peat accumulation, and several headwater lakes[28] (Supplementary Table 1)
Summary
Within days of inducing drought, we observed a reduction of O2 in both the surface and hyporheic water of the experimental stream (Fig. 2a). In both cases, O2 concentration decreased nonlinearly with greater WRT, highlighting the hydrological dependency of the vertical, lateral, and longitudinal vectors of O2 transport and atmospheric exchange in streams[23]. O2 concentration decreased nonlinearly with greater WRT, highlighting the hydrological dependency of the vertical, lateral, and longitudinal vectors of O2 transport and atmospheric exchange in streams[23] This reduction was more abrupt and persistent in the hyporheic zone, where all observations remained below the critical saturation level of 25% once WRT surpassed 200 min.
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