Estuaries and Coasts

, Volume 38, Issue 1, pp 232–241 | Cite as

On the Response of pH to Inorganic Nutrient Enrichment in Well-Mixed Coastal Marine Waters

  • Scott W. Nixon
  • Autumn J. OczkowskiEmail author
  • Michael E. Q. Pilson
  • Lindsey Fields
  • Candace A. Oviatt
  • Christopher W. Hunt


Recent concerns about declining pH in the surface ocean in response to anthropogenic increases of carbon dioxide (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. Nutrient enrichment, which can amplify both production and respiration, has 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 nitrogen (N), phosphorous (P), and silica (Si) addition. We show that the response of water column pH to nutrient enrichment was the greatest during the autotrophic winter-spring period, and there was no significant decline in pH across treatments during the heterotrophic summer-fall period. We believe that the differences in response lie in the seasonal cycles of production and respiration, where spring production peaks are large and discrete, and respiration is more temperature-driven but occurs diffusely throughout the year. The observed basification associated with enhanced nutrient inputs may have consequences for phytoplankton community structure, some species of submersed aquatic vegetation, cycling of Si, and perhaps other ecological processes.


Coastal acidification pH Nutrient enrichment Eutrophication Mesocosms Narragansett Bay 



The observations described in this manuscript were discovered by S. Nixon. He began writing the text of the manuscript and, after his untimely death, his coauthors completed the manuscript and published it posthumously in his honor. We would like to thank the scientists and technicians who worked on the MERL mesocosm experiments, and upon whose careful measurements, this manuscript is based. We are grateful to Alana Hanson for some statistical support. The MERL nutrient addition experiment was supported by the Environmental Protection Agency Centers Program, Cooperative Agreements 807795–02 and 810265–01. Support for this work was also partially provided by the NSF Awards 12010 and OCE851447. This is ORD tracking number ORD-003895 of the Atlantic Ecology Division, National Health and Environmental Effects Research Laboratory, Office of Research and Development, US Environmental Protection Agency. Although the research described in this article has been funded in part by the US Environmental Protection Agency, it has not been subjected to Agency review. Therefore, it does not necessarily reflect the views of the Agency.


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Copyright information

© Coastal and Estuarine Research Federation (outside the USA) 2014

Authors and Affiliations

  • Scott W. Nixon
    • 1
  • Autumn J. Oczkowski
    • 2
    Email author
  • Michael E. Q. Pilson
    • 1
  • Lindsey Fields
    • 1
  • Candace A. Oviatt
    • 1
  • Christopher W. Hunt
    • 3
  1. 1.Graduate School of OceanographyUniversity of Rhode IslandNarragansettUSA
  2. 2.Atlantic Ecology DivisionU.S. EPANarragansettUSA
  3. 3.Ocean Process Analysis LaboratoryUniversity of New HampshireDurhamUSA

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