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Climate change is predicted to have widespread impacts on freshwater lake and reservoir nutrient budgets by altering both hypolimnetic hypoxia and runoff, which will in turn alter the magnitude of internal and external nutrient loads. To examine the effects of these potential climate scenarios on nitrogen (N) and phosphorus (P) budgets, we conducted a whole-catchment manipulation of hypolimnetic oxygen conditions and external loads to Falling Creek Reservoir (FCR), an old, eutrophic reservoir in a reforested catchment with a history of agricultural land use. Throughout 2 years of monitoring, internal N and P loading during hypoxic conditions dominated the hypolimnetic mass of nutrients in FCR, regardless of changes in external loading. FCR commonly functioned as a net sink of N and P, except during hypoxic conditions, when the reservoir was a net source of ammonium ( $$ {\text{NH}}_{4}^{ + } $$ ) to downstream. We observed extremely high nitrate-nitrite ( $$ {\text{NO}}_{3}^{ - } {-}{\text{NO}}_{2}^{ - } $$ ), soluble reactive P (SRP), total nitrogen (TN), and total phosphorus (TP) retention rates, indicating that the reservoir served as a sink for greater than 70% of $$ {\text{NO}}_{3}^{ - } {-}{\text{NO}}_{2}^{ - } $$ inputs and greater than 30% of SRP, TN, and TP inputs, on average. Our study is notable in the length of time since reforestation (>80 years) that a reservoir is still exhibiting high N and P internal loading during hypoxia, potentially as a result of the considerable store of accumulated nutrients in its sediment from historical agricultural runoff. Our whole-catchment manipulations highlight the importance of understanding how multiple aspects of global change, waterbody and catchment characteristics, and land use history will interact to alter nutrient budgets in the future. [ABSTRACT FROM AUTHOR]

Climate change is expected to cause an increased frequency of extreme events such as heavy floods and major storms. Such stochastic events have an immediate impact on surface water quality, but the long-term effects are largely unknown. In this study, we assess long-term monitoring data from two Swedish headwater catchments affected by extreme weather events. At one site, where nitrogen effects in soil water, groundwater, and stream water were studied after storm-felling and subsequent forest dieback from bark beetle attack, long-term (>5 years) but relatively modest (generally <1 mg L) increases in ammonium (NH-N) and nitrate (NO-N) concentrations were observed in the various aqueous media. At the other site, where effects on benthic fauna were studied in a stream impacted by extreme geophysical disturbances caused by rainstorm-induced flashflood, only short-term (1 year) effects were revealed both regarding diversity and composition of species. [ABSTRACT FROM AUTHOR]