Investigation of the carbonate chemistry of two tide pools on San Juan Island

Open ocean observations have shown that increasing levels of anthropogenically-derived atmospheric CO2 are causing acidification of the world’s oceans. Yet little is known about coastal acidification and very few studies have characterized the carbonate chemistry of coastal zones or intertidal rock pools where many calcifying organisms exist. In addition to various physical parameters (temperature, dissolved oxygen (D.O.), and salinity), we characterized the carbonate chemistry by taking measurements of pH, total alkalinity, and dissolved inorganic carbon (DIC) of two tide pools (0.41 m3 and 0.10 m3) located at Dead Man’s Cove on San Juan Island, Washington, to determine the extent of variation over the course of one tidal cycle and compared those results to local water samples taken from the Strait of Juan de Fuca. Both tidal pools exhibited extreme values in carbonate chemistry over the course of the day (for the 0.41 m3 tide pool: 7.36 – 8.60 pH, 2116.53 – 1669.86 μmol kg-1 total alkalinity, 2171.5 – 1216.1 μmol kg-1 DIC, and 2088.494 – 57.448 μatm pCO2) and calcite and aragonite saturation states (0.623-6.84 Ω Ca, 0.392-4.388 Ω Ar). On separate, consecutive days, the alga Ulva sp. and the mussle Mytilus trossulus were added to the smaller of the tide pools to observe the potential impact of photosynthetic versus respiring biomass on the carbonate chemistry. Adding algae resulted in an expected increase in pH, a decrease in DIC, and an increase in Ω Ca and Ω Ar. Total alkalinity decreased less than expected; perhaps due to the shading of coralline algae by Ulva which may have prevented coralline calcification. pCO2 decreased less than expected on day 2 relative to day 1 in the treatment pool and may have been due to changes in weather relative to the first day. Addition of mussels resulted in an expected increase in DIC, an increase in pCO2 and reductions in Ω Ca and Ω Ar. D.O. did not increase and pH did not decrease as much as expected on day 3 in the treatment pool and these unexpected findings may be due to the fact that mussels were not allowed to acclimate. Excretion of ammonium or addition of organic acids from added mussels may have offset a reduction in total alkalinity. This study demonstrates that many intertidal species are experiencing far greater fluctuations in carbonate chemistry than expected and has significant implications for ocean acidification experiments where intertidal species are used