Recovery from episodic acidification delayed by drought and high sea salt deposition

Abstract

Abstract. For the prediction of episodic acidification large uncertainties are connected to climatic variability and its effect on drought conditions and sea-salt episodes. In this study data on 342 hydrological episodes in 25 Swedish streams, sampled over 10 years, have been analyzed using a recently developed episode model. The results demonstrate that drought is the most important factor modulating the magnitude of the anthropogenic influence on pH and ANC during episodes. These modulating effects are especially pronounced in southern and central Sweden, where the historically high acid deposition has resulted in significant S pools in catchment soils. The results also suggest that the effects of episodic acidification are becoming less severe in many streams, but this amelioration is less clear in coastal streams subject to high levels of sea-salt deposition. Concurrently with the amelioration of the effects of episodic acidification, regional climate models predict that temperatures will increase in Sweden during the coming decades, accompanied by reductions in summer precipitation and more frequent storms during fall and winter in large areas of the country. If these predictions are realized delays in streams' recovery from episodic acidification events can be expected.

Drivers and evolution of episodic acidification at the Bear Brook Watershed in Maine, USA

“…4) played a major role in the episodic response of pH and ANC during both rain and snow melt episodes (Bjärnborg 1983;Molot et al 1989;Evans et al 1995;Wigington et al 1996;Kline et al 2007). A decrease in the BC concentration up to 70% has been observed in some regions during snow melt , but decrease in BC is commonly less responsive during rain events (Laudon et al 2004;Laudon 2008). In contrast, the results from this study suggest that BC dilution is a less important mechanism associated with episodes in EB and WB.…”

“…The important role of SO 2− 4 in WB is in agreement with most other episodic acidification studies. In Canada and Sweden, spring flood ANC poll correlated with winter SO 2− 4 deposition , whereas the summer episodes were strongly related to antecedent wetness conditions (Laudon et al 2004;Laudon 2008). The correlation with winter precipitation SO 2− 4 concentration in these studies has been explained by a strong connection between chemistry of spring runoff and snow melt.…”

“…However, the deposition of marine ions in coastal areas may continuously affect soil and stream water chemistry in some regions (Evans et al 2001;Rustad et al 1994;. For example, in southwestern Sweden, continuous low amounts of marine sea salt, spiked with occasional high doses, have been suggested to retard the episodic recovery in coastal streams and explain why some coastal watersheds with relatively high stream water Cl − concentrations are not recovering as rapidly as streams farther inland (Laudon 2008). Interestingly, no difference in sea salt-driven episodic response was observed between the two catchments despite the large loading of acidity to the WB soils during the treatment period.…”

Three representative UK moorland soils show differences in decadal release of dissolved organic carbon in response to environmental change

“…Al) leading to the subsequent elevated state of DOC solubility. Climatic episodes associated with droughts are more commonly reported for wetland soils, including peatlands (Clark et al, 2005;Eimers et al, 2008;Laudon, 2008). At the extreme scale of wetness, the MH soil shows different dominant processes associated with hydrological and redox controls on peatland stored S. Droughts and lowering of the water table at MH in 1995 and 2003 led to short-term suppression of DOC concentrations (Fig.…”

Experimental acidification of two biogeochemically-distinct neotropical streams: Buffering mechanisms and macroinvertebrate drift

“…Recognizing the negative acidification effects of elevated sulfate ) and nitrate (NO 3 − ) deposition to watersheds led to passage of emission cap legislation in the US and Europe (Stoddard et al, 1999). While recovery has been observed, there are still episodic acidification events in many watersheds (Kowalik et al, 2007;Laudon, 2008;Lawrence et al, 2008;Wigington et al, 1996). Episodic acidification events are defined as periods of rapid and temporary (days to weeks) declines in stream water pH and acid neutralizing capacity (Laudon et al, 2004).…”