Drivers of pH Variability in Coastal Ecosystems

A synthesis of long-term
changes in pH of coastal ecosystems shows
that, in contrast to the uniform trends of open-ocean acidification
(−0.0004 to −0.0026 pH units yr–1)
driven by increased atmospheric CO2, coastal ecosystems
display a much broader range of trends (−0.023 to 0.023 pH
units yr–1) and are as likely to show long-term
increase as decline in pH. The majority of the 83 investigated coastal
ecosystems displayed nonlinear trends, with seasonal and interannual
variations exceeding 1 pH unit for some sites. The high pH variability
of coastal ecosystems is primarily driven by inputs from land. These
include freshwater inputs that typically dilute the alkalinity of
seawater thereby resulting in reduced buffering, nutrients enhancing
productivity and pH, as well as organic matter supporting excess respiration
driving acidification. For some coastal ecosystems, upwelling of nutrient-rich
and corrosive water may also contribute to variability in pH. Metabolic
control of pH was the main factor governing variability for the majority
of coastal sites, displaying larger variations in coastal ecosystems
with low alkalinity buffering. pH variability was particularly pronounced
in coastal ecosystems with strong decoupling of production and respiration
processes, seasonally or through stratification. Our analysis demonstrate
that coastal pH can be managed by controlling inputs of nutrients,
organic matter, and alkalinity. In well-mixed coastal waters, increasing
productivity can improve resistance to ocean acidification, whereas
increasing productivity enhances acidification in bottom waters of
stratified coastal ecosystems. Environmental management should consider
the balance between the negative consequences of eutrophication versus
those of acidification, to maintain biodiversity and ecosystem services
of our coastal ecosystems.