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On the Response of pH to Inorganic Nutrient Enrichment in Well-Mixed Coastal Marine Waters
Citation:
Nixon, S., Autumn J. Oczkowski, M. Pilson, L. Fields, C. Oviatt, AND C. Hunt. On the Response of pH to Inorganic Nutrient Enrichment in Well-Mixed Coastal Marine Waters. Estuaries and Coasts. Estuarine Research Federation, Port Republic, MD, 38(1):232-241, (2015).
Impact/Purpose:
This manuscript directly addresses the potential impact that increasing levels of atmospheric carbon dioxide may have on the pH of estuaries (i.e., estuarine acidifiction). It also looks at how high pH excursions associated with excess production attributed to elevated nutrient loads may impact coastal biogeochemistry and ecology.
Description:
Recent concerns about declining pH in the surface ocean in response to anthropogenic increases of CO2 in the atmosphere have raised the question of how this declining baseline of oceanic pH might interact with the much larger diel and seasonal variations of pH in coastal marine ecosystems. Factors such as nutrient enrichment, which can amplify both production and respiration, have the potential to reduce or exacerbate the impacts of ocean acidification in coastal waters. Here we present results from a multi-year experiment in which replicate phytoplankton based mesocosms with a 5 m deep well-mixed water column (salinity = 27-31) and intact benthic community were exposed to a gradient in daily inorganic N, P, and Si addition. These show that the response of water column pH to nutrient enrichment was asymmetric, with increased pH in fertilized systems during the autotrophic winter-spring period and no significant decline in pH during the heterotrophic summer-fall period. This behavior is due to the strong non-linear relationship between pH and pCO2 as free CO2 is near depletion. Increasing pH as a consequence of nutrient enrichment may have consequences for phytoplankton community structure, some species of submersed aquatic vegetation, the cycling of Si, the partitioning and behavior of some metals, and perhaps other ecological processes. The pH response of systems with strong vertical stratification and the spatial separation of net production and respiration may be quite different.
