The emission of anthropogenic carbon dioxide leads to the lowering of seawater pH. Ocean acidification is a major problem for marine calcifying organisms. There is a need to study short- and long-term effects of lowered pH on marine organisms such as oysters. Oysters are an important food source and useful for nutrients recycling in the coastal estuarine environments. The coastal estuarine environment such as mangrove ecosystems connected to the Sargasso Sea, Ferry Reach, Bermuda, has a natural variation of pH according to the changes in tidal regime (thus low and high tide activities). The unique environment serves as a model place to carry out the effect of changing pH on a marine organism such as flat tree oysters inhabiting this coastal ecosystem.For the laboratory experiment, a total of 84 specimens of the flat tree oyster, Isognomon alatus, were randomly collected on 21 January 2009 from rocks exposed at low tide in Mullet Bay, an intertidal mudflat, St. George, Bermuda (latitude: 32° 22' 30'' N, longitude: 64° 41' 35''W). An experiment was performed to test the effect of projected future pH decrease in a seawater flow-through system at Bermuda Institute of Ocean Sciences (BIOS) for a short period (February to April 2009). Physicochemical conditions (seawater temperature, salinity, pH and oxygen concentration) in three control tanks (C1, C2, C3, pH = 8.1 - 8.2) and three acidification tanks (T1, T2, T3, pH = 7.8 - 7.9) used for the culture of the oysters were recorded. Changes in shell morphometrics of the oysters were determined.For the field experiment, 42 specimens of I. alatus were randomly placed in 6 tanks (approx. n = 7 oysters/tank). Two tanks were then positioned along the transect at each station (A, B ,C) in Mangrove Bay, Bermuda. The shell parameters of flat tree oysters and physicochemical conditions were monitored biweekly.

Laboratory experiment:Tygon tubes (1.9 m in length) were placed in the head tanks, which then supplied continuous seawater at a constant flow rate of 60 ml/min into the experimental tanks (n = 6) based on gravitational flow. Both head tanks initially had natural seawater (pH 8.1 - 8.2) running through them. The carbon chemistry of seawater in one of the head tanks was altered by bubbling pure (100%) CO2 gas at a fixed rate. An air-stone ensured small bubbles that enabled rapid solution of the gas into the seawater. The flow rates of air and concentrated CO2 gas were controlled at a rate of 12.0 ml/min monitored using an adjustable Agilent Flowmeter ADM1000. Three tanks were randomly chosen to be held under ambient (control, n = 3) conditions. These were controls in the sense that the seawater chemistry was not manipulated. Three other tanks were used for acidification treatment (n = 3) and adjusted to a pH of 7.8 to 7.9. However, the seawater chemistry of the controls as well as the acidified tanks was influenced by natural variations in the source seawater from Ferry Reach. The daily variation in pH between the control (pH = 8.1 - 8.2) and acidification treatment tanks (pH = 7.8 - 7.9) was ~0.3-0.4 pH units, similar to the expected drop in ocean surface water pH predictions by IPCC BAU IS92a scenario in the year 2100 (Caldeira & Wickett 2005, IPCC 2007).