Characterizing acid episode frequency, duration, and severity in Nova Scotia’s acidified streams

Abstract

Acid episodes are a key factor in determining the state of freshwater ecosystems that have been chronically acidified by acid precipitation. An acid episode can be defined as a rapid drop in freshwater pH to well below thresholds for biological impact (around pH 5.5) that occur during run-off events in acidified catchments. There has been a general trend of recovery from acidification in North America and Europe following policies enforcing a decrease in emissions that cause acid precipitation. South Western Nova Scotia (SWNS) is an exception to this trend with pH levels not showing signs recovery despite decreased acid deposition. Acid episodes can be a barrier to biological recovery even if annual mean pH levels are increasing and have been identified as a threat to aquatic biota in Nova Scotia, in particular local Atlantic Salmon populations. However, there have been no recent studies on episodic acidification in the streams of this highly acid sensitive region. This study uses high frequency measurements of stream pH and water level to determine the annual and seasonal frequency, duration, and severity of acid episodes in four SWNS streams. The aim is to determine what catchment characteristics may impact acid episode behavior and find seasonal episode trends. Episode frequency was found to be greater than in previous studies in SWNS, and in other regions, with up to 15 episodes below pH 5.5 for greater than 24 hours occurring per year. Seasonally, summer episodes in the streams are frequent but short whereas spring episodes are less frequent but both spring and winter episodes have longer durations. This seasonal trend has implications for salmon health due to the spring life stage of salmon being most acid sensitive. Results show that smaller catchments may have a stronger relationship between stage and episode pH response during run-off as well more frequent episodes with more severe drops in pH. Further studies with a larger spatial sampling are needed to determine the impact of other catchment characteristics on episodic acidification.