Verification of pH fluctuations in Narragansett Bay

In order to understand potential changes in pH driven by increasing atmospheric carbon dioxide, we first need to understand what controls pH and its variability in estuaries today. We measured total alkalinity, dissolved inorganic carbon, pH, temperature and salinity of samples taken hourly for 24 hours once a month at three sites in Narragansett Bay (2/2010 to 4/2011) to understand the controls on daily and seasonal pH variation: GSO, Greenwich Bay (GB), and Potter Cove (PC). We also measured in situ pH (pHe ) and temperature every five minutes at the same sites and during the same time periods. Our calculations of pH (pHc) from total alkalinity, dissolved inorganic carbon, salinity and temperature measurements indicate daily pH variation of 0.10 to 0.62. The largest pHc range for GSO was on April 1st 2010, which had range of pHc of 8.36 to 7.94. The pHe range on the same day was from 8.19 to 8.01. The largest pHc range for PC was on March 8th 2011, which had range of pHc of 8.68 to 8.16. The pHe range on the same day was from 9.02 to 8.46. The largest pHc range for GB was on May 13th 2010, which had range of pHc of 8.04 to 7.42. The pHe range on the same day was from 7.91 to 7.52. We propagate errors in our calculations and use a conservative mixing model to determine if this variation in pH is valid or an artifact of error. The variations in pH are real and are not an artifact because the observed daily range in pH is greater than the pH range due to total error. We compared pH determined from dissolved inorganic carbon and total alkalinity measurements to pH determined from a conservative mixing model. The comparisons show that daily pH variation is not completely explained by the mixing of waters with different salinity, alkalinity, and dissolved inorganic carbon. Short-term pH change that cannot be explained by the model and have carbon dioxide and dissolved oxygen deviation from equilibrium are driven by biological activity, primarily photosynthesis and respiration. The fractional departure of dissolved carbon dioxide (([CO2*] -[CO2 *]sat)/[CO2*], CO2* = dissolved and hydrated CO2) and dissolved oxygen (([O2] - [O2]sat)/[O2]) are anti-correlated, but not clearly linked to chlorophyll concentration. The mixing efficiency of the estuary provides a physical explanation as to why pH below equilibrium concentrations of CO2 co-varies with low dissolved oxygen concentrations