Tidal restrictions in a central Californian estuarine system are associated with higher mean pH, but increased low-pH exposure

Abstract

Coastal acidification is an emerging concern in estuaries impaired by nutrient pollution. In addition to rising levels of atmospheric CO2 which drives ocean acidification, high nutrient inputs to coastal areas can amplify heterotrophic metabolism, raise water column CO2 levels, and exacerbate pH declines. This study focuses on how a third anthropogenic stressor, tidal restriction, shapes effects of coastal acidification. Tidal restrictions associated with installation of gates that reduce tidal flow to a portion of an estuary are a common impact to coastal landscapes and can negatively affect water quality. This study examined pH in locations subject to varying levels of tidal restriction across a series of interconnected central California estuaries, whose waters are nutrient-impaired due to surrounding agriculture, and where 50% of the system is affected by tidal restrictions. Mean and variance of pH differed based on the level of tidal restriction. Sites lacking tidal restrictions had the lowest mean pH (7.98) but the least pH variance (0.07), and the most infrequent exposure to low pH (<7.0) conditions. In contrast, sites with minimal tidal exchange had the most exposure to low pH conditions, although mean pH levels were greater (8.08), because they also saw greater pH variance (0.46). Our results suggest that tidal restrictions alter pH levels and affect the resilience of estuaries to coastal acidification.