Since the 1800s, carbon dioxide emissions due to human activities have contributed significantly to the amount of carbon in the atmosphere. Approximately a third of this carbon is absorbed by the ocean, through air-sea fluxes at the ocean surface (Sabine, 2004). Increased CO2 has changed the carbon chemistry of the ocean and hence the pH. pH is expected to drop by 0.4 by the year 2100. It is unclear how this lower pH will affect carbon cycling and sequestration with respect to the biological carbon pump. Most studies have focused on open ocean phytoplankton or bacterial communities in large, stationary mesocosms. Few studies have coupled both phytoplankton and bacterial processes and even fewer have investigated coastal communities, where pH and pCO2 can vary drastically. This study focused first on developing and evaluating a mesocosm and alternative method for elevating pCO2. The second goal was to determine how potential changes in phytoplankton DOC release and community structure and the resulting carbon pool may affect bacterial secondary production and ectoenzyme activity in a natural coastal community. Mesocosms aimed to mimic natural pCO2 fluctuations by maintaining CO2 concentration of 1250 ppm in the headspace, as aqueous pCO2 may change with biological processes. Six mesocosms were filled with 40L of water from the Chesapeake Bay (three ambient pCO2 and three 1250 ppm) and monitored over 15 days. Chlorophyll a, DOC, bacterial respiration, bacterial production, and enzyme activity were measured. Bacterial production and respiration were used to calculate bacterial growth efficiency (BGE). Results showed that there was no significant difference between the ambient and elevated groups with respect to chlorophyll a, DOC, BGE and enzymes activity. However, differences in bacterial respiration and bacterial production during the first four days of the experiment may suggest that bacteria require time to acclimate to elevated pCO2. Phytoplankton and bacteria in coastal areas are exposed to a wide range of abiotic factors such as seasonal temperature variations, salinity, mixing, and terrestrial inputs. The pH of the Chesapeake Bay ranges between 7.5 and 8.3, and it is possible that the phytoplankton and bacteria are adapted to cope with a wide range of pH (Wong, 2012). This study suggests that the biological carbon pump may not be significantly altered in our future ocean.
Cooper R., 2012. Ocean acidification: understanding the coastal carbon pump in a high CO2 world. Master of Science Thesis. Virginia Commonwealth University. Thesis (login required).
