Biological and physical controls on multidecadal acidification in a eutrophic estuary

Estuaries support many ecologically and economically important resources
that are especially vulnerable to ocean acidification from rising
anthropogenic CO2. However, complex local processes in estuaries
complicate and may disguise long-term pH trends. For example, terrestrial
nutrient runoff and marine coastal upwelling may exacerbate pH variability
and declines. We investigated eutrophication impacts on acidification in a
central California estuary, Elkhorn Slough, which receives high nutrient
loads from intensive surrounding agriculture and upwelling of the
California Current System. We examined drivers of acidification including
nutrients, ecosystem metabolism, and upwelling by modelling pH trends over
20 years using a Generalized Additive Mixed Model at four sites from the
National Estuarine Research Reserve Systemwide Monitoring Program and
collected additional water samples to calculate aragonite saturation. Our
models revealed acidification trends over two decades which were more
pronounced near the marine inlet. Near the marine inlet, high nutrient
levels and lower buffering are associated with the greatest rate of
acidification in the estuary, which was four times greater than the trend
from anthropogenic CO2 alone. However, tidally restricted areas
experienced a different acidification pattern. A tidally restricted site
recorded higher mean pH and aragonite saturation and increased pH levels
associated with stronger upwelling conditions supplying marine-sourced
nutrients. Therefore, variable ecosystem metabolism and tidal cycles are
threats to acidity in this location. The effects of enhanced seasonal
cycles or long-term trends in different zones of the estuary have
implications for monitoring with a temporal frequency and scale to capture
coastal acidification risks in estuaries.