By Dr. Scott Noakes The University of Georgia Center for Applied Isotope Studies
Ocean acidification, a decrease in seawater pH (acidity scale) caused by the increase of anthropogenic (human-induced) carbon dioxide (CO2) into the atmosphere, has received considerable attention in recent years. As scientists gain an understanding of the adverse affects of increased CO2 and decreased pH, a major concern of ocean acidification is the impact to organisms which use calcium to build their bones, skeletons or other structural components. Recent research has indicated that the oceans act as a net repository or "sink" for atmospheric CO2, but that this sink is not uniform worldwide. Many coastal regions oscillate between being a CO2 sink and source depending on the time of year. The effects of ocean acidification have yet to be fully understood in coastal regions were biogeochemical processes are often vastly different from regions dominated by upwelling.
Greg McFall finishes installing the new microcat (salinity and temperature probes) on the buoy bridle. (Photo: Sarah Fangman)
The coastal areas surrounding our continent (continental margins) are known to play a considerable role in determining global carbon cycling; however, little has been done to determine input from the coastal margins towards the total carbon "budget." Insufficient data exists to adequately determine the natural fluctuation ("flux") of air-sea CO2 with any level of confidence. The ability to explain the control mechanisms driving the variability of coastal partial pressure of CO2 (pCO2 or the concentration of CO2 in seawater) and pH is limited. Understanding these control mechanisms and how they affect pCO2 and pH is essential to predicting future changes in CO2 flux, pH and carbonate saturation in our oceans. The primary reason that scientists are not able to determine the control mechanisms for coastal ocean regions is the absence of long-term high-resolution data. The highly dynamic coastal margin, with its combined terrestrial and oceanic input makes understanding this region a necessity for determining what mechanisms control the fluctuation of carbon dioxide.
Gray's Reef National Marine Sanctuary (GRNMS), located in the South Atlantic Bight (SAB) along the southeastern United States is situated in a very unique and dynamic region. It sits along the divide between the inner and middle shelf with water depths in the 20 m range. The water at the sanctuary is primarily controlled by the middle shelf oceanic dynamics, but during heavy rain events, it can be affected by freshwater plumes coming from the numerous rivers along the Georgia and South Carolina coast. Temperature also plays a major role in the SAB pCO2 variability with seasonal changes being apparent. Recent research along the mid-outer shelf has suggested that the SAB is a CO2 net sink and the inner shelf acts as a net source releasing CO2 to the atmosphere. However, many factors such as ocean mixing, freshwater input, and Gulf Stream intrusions offer considerable input to the water chemistry at GRNMS.
Dr. Scott Noakes replacing pCO2 sensor system on the Grays Reef buoy. (Photo: Sarah Fangman)
The NOAA Pacific Marine Environmental Lab (PMEL) and The University of Georgia (UGA) have been involved in monitoring pCO2 offshore Georgia for many years. PMEL established a monitoring station at GRNMS and has successfully collected high-resolution data since 2006. The PMEL station currently collects pCO2 at the air-sea interface and in the atmosphere; surface seawater temperature; and salinity. Surface water samples have also been collected at the site and analyzed for dissolved inorganic carbon (DIC), total alkalinity, (TA), dissolved oxygen (DO), pH and salinity to gain a concept for the TA-salinity relationship. As a result of these research efforts, it has been noticed that there is a distinct relationship between the pCO2 concentrations and water temperature. As the seawater temperature increases, so does the pCO2 (Figure 1). This phenomenon has been replicated every year since data collection began in 2006. The average air pCO2 as measured at GRNMS is approximately 400 micro-atmospheres (µatm). The level is typically exceeded in the water column during the warm summer months forcing CO2 out of the water into the atmosphere. This data demonstrates the cyclical nature of the middle SAB cycling from serving as a place where CO2 is stored to becoming a provider of CO2 to the atmosphere.
Air-sea interface pCO2 data at GRNMS.
Conventional wisdom has always considered the coastal margin water column to be well mixed and that measurements collected at the surface should reflect conditions at the seafloor. However, preliminary data has shown that this is not the case at GRNMS (Figure 2). UGA has experimented with deployment of a high-resolution pCO2 sensor on the seafloor and has documented several events where the pCO2 on the surface and seafloor have not been in agreement. As much as 200 µatm increases in pCO2 have been detected on the seafloor as compared to the surface pCO2. Both instruments were in relatively good agreement before and after these events and were checked by calibration with gas standards. It was previously discussed that temperature played a definite role in the pCO2 concentrations in the SAB. However, it was noted that when the seafloor pCO2 concentrations increased dramatically, the temperature did not change. This is a major discovery indicating that the relationship between pCO2 and temperature did not play a role in generating these spikes and that other physical or chemical processes were the driving force.
Air-sea interface pCO2 (pink) and seafloor pCO2 (blue, left) and temperature (right). Click here for a larger image.
Several questions have come to light as a result of this research. Are the pCO2 spikes a real phenomenon and if so, are there distinctive changes in water chemistry that define the increase in pCO2? How do the benthic (bottom dwelling) organisms react to such rapid changes in pCO2? What physical or chemical process caused these swings in pCO2? It is anticipated that continued CO2 research at Grays Reef will explain how the benthic community will adapt and hopefully thrive as the Atlantic Ocean changes due to anthropogenic pressures.