MONITORING

Ocean Acidification and CO₂ Monitoring

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.

Dr. Scott Noakes replacing pCO2 sensor system on the Gray's Reef Data Buoy (Photo: Sarah Fangman, GRNMS).

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.

Greg McFall completes installation of the new microcat (salinity and temperature probes) on the GRNMS buoy bridle (Photo: Sarah Fangman, GRNMS).

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.

Air-sea interface pCO2 at Gray's Reef. Click image for larger view. (Image: Dr. Scott Noakes, UGA).

The NOAA Pacific Marine Environmental Lab (PMEL), The University of Georgia (UGA), and GRNMS 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. 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.

Seafloor observatory housing the high resolution pCO2 sensor and water quality probes. (Photo: Dr. Scott Noakes, UGA).

What makes Gray's Reef unique among other locations offshore Georgia is its vibrant live bottom environment. Unlike the hard corals commonly found in the Florida Keys, much of Gray's Reef is composed of sponges and soft corals. In an effort to understand the environmental pressures occurring at Gray's Reef, UGA is also working to establish a seafloor observatory at the reef. Upon completion, this observatory will house a high resolution pCO₂ sensor, high resolution pH sensor and water quality probes measuring temperature, dissolved oxygen, salinity, chlorophyll and turbidity. Conventional wisdom has always considered the coastal 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 always the case at GRNMS. To date, UGA has documented several events where the pCO₂ on the surface and seafloor have not been in agreement.

Air-sea interface pCO2 (green), seafloor pCO2 (blue) and temperature (red) collected at Gray's Reef during 2011. Click image for larger view. (Image: Dr. Scott Noakes, UGA).

Full understanding of the seafloor data will not be achieved until all sensors have been installed and the data interpreted. The sponges and soft corals are generally more tolerant of environmental change than hard corals, but it is not known how these organisms living at Gray's Reef will be affected by changes in water chemistry due to increased CO₂. With the seafloor observatory in place, it will open the door for scientists to more effectively study the benthic community.

It is anticipated that continued CO₂ research at Gray's Reef will explain how the benthic community will adapt and hopefully thrive as the Atlantic Ocean changes due to anthropogenic pressures.