Title: Contribution of Submarine Groundwater Discharge (SGD) to the marine carbonate biogeochemistry of western Irish coastal seas
Researchers: Mariateresa Guerra, Dr Carlos Rocha
Over the past 200 years, due to the burning of fossil fuel, human activities have altered the global carbon cycle, which has led to an increase of CO2 in the atmosphere (40% from about 280 ppm in the pre industrial area to nearly 380 ppm in 2010) (Scott C Doney, 2010). The consequent absorption of atmospheric CO2 by the ocean has led to an alteration of marine carbonate chemistry and a 30% increase in H+ concentration in sea water and decreasing pH, called ocean acidification (Fabry et al., 2005). The pre-industrial ocean pH was slightly alkaline with a pH value of 8.2 (Caldeira & Wickett, 2003). Following the industrial revolution the ocean pH decreased by 0.1 pH units, from approximately 8.21 to 8.10 (Raven et al., 2005), and is expected to decrease a further 0.3–0.4 pH units (Fabry et al., 2005) by the end of the XXI century if atmospheric CO2 concentrations will reach 800 ppm (Byrne, Mecking, Feely, & Liu, 2010; Caldeira & Wickett, 2005). This scenario is likely if the CO2 emission are not mitigated. Under these circumstances, reasonable projections suggest that global mean ocean pH will fall below 7.9 by 2100 (Zeebe and Wolf-Gladrow, 2001). Currently CO2 concentration continues to rise at the rate of 2.73 ppm yr-1(Bopp et al., 2013). In addition fossil fuel combustion and agriculture also increase atmospheric inputs of strong acids (HNO3 and H2SO4) and bases (NH3) which in turn affect the coastal and open ocean carbonate geochemistry. These inputs are particularly important close to major source regions, primarily in the northern hemisphere, and are responsible of reduction in surface seawater alkalinity, pH, and Dissolved Inorganic Carbon (DIC) (S C Doney, Fabry, Feely, & Kleypas, 2009).
This project aims to understand the drivers that influence the carbonate chemistry of coastal areas, like river (surface) and submarine groundwater discharge (SGD) and how they might contribute to modify the carbonate biogeochemistry of coastal seas. Coastal ecosystems may show acidification or basification, depending on the balance between the invasion of coastal waters by anthropogenic CO2, watershed export of alkalinity, organic matter and CO2, and changes in the balance between primary production, respiration and calcification rates in response to changes in nutrient inputs and losses of ecosystem components (Duarte et al., 2013). Due to the prolonged and intensive use of inorganic fertilizer in agriculture, changes in land use, for instance deforestation, and discharge of industrial and municipal waste, have all contributed to the eutrophication of river water and of the coastal ocean, on a global scale. For example, amongst others, the supply of nutrients through SGD is linked to the occurrence of red tides, (C. Hu, Muller-Karger, & Swarzenski, 2006; Kim & Lee, 2009; Lee, Hwang, Kim, Lee, & Oh, 2009; McCoy & Corbett, 2009). SGD is also a major source of carbonates produced by weathering, in the form of DIC (De Weys 2011; Santos and Eyre, 2011; Cyronak et al., 2013; McMahon et al., 2013; Cyronak T. et al., 2014; Santos et al., 2014). Globally, SGD has been linked to severe degradation of water quality and alteration of the marine food web and community structure (Rabouille, Mackenzie, & Ver, 2001).
Since coastal marine systems are considered the most ecologically and socio-economically vital ecosystems on the planet (Bopp et al., 2013; Costanza et al., 1998; Gazeau et al., 2007), the safeguard of ecosystem services, as aquaculture, is necessary. A consequence in the increasing amount of CO2 dissolved in the water column and associated pH reduction, results in a decrease in the metabolic activities of calcified organisms, from zooplankton to commercial value species like mussels (Mytilus edulis) (Hinga, 2002). In order to predict the trend in coastal area is important considering all the drivers that can affect the coastal carbonate chemistry. A biogeochemical approach is fundamental to avoid overlooking of important drivers such as SGD, since it is an important factor to take into account in order to understand the carbon cycle in coastal areas. In this sense, this review also wants to emphasize the complexity of studies of this type in coastal areas, highlighting the need for a multi-disciplinary approach. In producing estimates of the impact of ocean acidification on aquaculture species, the scientific community have employed until now assumptions that are valid only for the open ocean, and are not applicable on the coast; this should increase the interest on the coast especially in terms of future economic lose. In this project different investigation sites, characterized by contrasting catchment hydrogeology, will be studied. Kinvara Bay, which is located in a lowland but is fed by allogenic sources, is a limestone area, inserted into a karstic catchment area. The surrounding lands are characterised by an almost complete lack of surface drainage, abundant seasonal lakes (turloughs), and extensive underground drainage systems (Wilson et al. 2016; McCormack et al. 2014). Limestone areas cover most of the Ireland territory (Clare & Mcnamara 2009) and this makes Kinvara Bay representative of many Irish coastal spots. Conversely, Killary Harbour is characterized by shale, sandstone, conglomerate (Brendan F. Keegan 1986). The choice of these two contrasting areas characterized by environments with different hydrogeochemical settings, allows the comparison between groundwater and river effects in terms of TA and DIC values in the associated coastal systems.
Ocean acidification, pH fluctuations and shifting in carbonate parameters are identified as stressful conditions for all marine species but especially for calcified organisms (Gattuso et al. 1998; Riebesell & Zondervan 2000; Riebesell 2004; Gazeau et al. 2007; Rodolfo-Metalpa et al. 2011; Kroeker et al. 2013). Considering the lack of knowledge regarding the variations in coastal area carbonate parameters, and the drivers identified as important source to buffer or to acidify the system, some research questions arise:
What are the pH dynamics in coastal areas characterized by contrasting origin of freshwater inputs?
Which is the TA/DIC relationship in estuarine areas characterized by contrasting water-flows input?
How do SGD and rivers influence the DIC and TA relationship in the water column over diurnal and seasonal time-scales?
Which is the contribution of Non-carbonate (NC)-alkalinity to TA in estuarine areas dominated by contrasting water-flows inputs?