Seasonal dynamics of the carbonate system in the Western English Channel
Highlights
► We examine carbonate system data from a coastal observatory (English Channel). ► pCO2 seasonality was driven by thermal and biological controls (net autotrophy). ► Near shore samples were influenced by freshwater inputs. ► Monthly mean pH varied seasonally by up to 0.15 units at the surface. ► The study area is a small annual sink for atmospheric CO2.
Introduction
Carbon dioxide is a long-lived, climatically-active gas in the atmosphere. Anthropogenic activities have led to an increase of atmospheric CO2 concentration from 280 ppmv before the industrial revolution, to approximately 385 ppmv at present. The marine environment has acted as a net sink for rising CO2 over this period, absorbing approximately one quarter of human-related emissions (Gruber et al., 2009, Sabine et al., 2004, Sarmiento et al., 2010, Sweeney et al., 2007). Ongoing research efforts over the past few decades have therefore focused on understanding and quantifying carbon fluxes in the ocean and CO2 exchange with the atmosphere. More recently, the scientific community has been investigating the effects of progressive seawater acidification as a result of increased atmospheric CO2 uptake by the oceans.
Temperate coastal seas play an important role in linking the terrestrial, marine and atmospheric carbon cycles (Gattuso et al., 1998). Nutrient inputs from land, sediments and the open ocean fuel primary production and atmospheric CO2 uptake in surface waters, while remineralisation of organic matter leads to release of CO2 (Chen and Borges, 2009, Wollast, 1998). In addition to these biological sinks and sources, the effect of annual heating and cooling of surface waters on CO2 solubility, give rise to further exchange of CO2 between the atmosphere and coastal seas. The relative, seasonal imbalance between these sources and sinks typically results in a seasonal pattern of CO2 exchange, with net influx from the atmosphere in spring and net release in the autumn (Borges and Frankignoulle, 2003, Frankignoulle and Borges, 2001). In seasonally stratified shelf seas, phytoplankton blooms, particularly in spring and late summer/early autumn, cause drawdown of atmospheric CO2, while respiration in summer and the breakdown of stratification in autumn cause the release of CO2 from deeper waters (DeGrandpre et al., 2002, Omar et al., 2010, Schiettecatte et al., 2007, Thomas et al., 2005). Nevertheless, productivity in such waters can be highly dynamic and ‘patchy’ (Jönsson et al., 2011). In coastal waters, the respiration of organic inputs from rivers may drive net heterotrophy (Smith and Hollibaugh, 1993) and thereby CO2 emission to the atmosphere, though the quantification of this flux is fraught with uncertainty due to large spatial and temporal heterogeneity of CO2 distribution (Borges et al., 2006). Overall, temperate, shelf seas are a sink for atmospheric CO2, while estuaries and near-shore waters acts as a source over an annual cycle (Borges et al., 2005, Borges et al., 2006, Cai et al., 2006, Chen and Borges, 2009, Frankignoulle and Borges, 2001, Laruelle et al., 2010). Sustained, multi-annual observations are required in order to capture the variability of fluxes in highly dynamic coastal seas, where episodic events can lead to productivity pulses and associated CO2 exchange. The paucity of such observations limits our understanding of C-cycling in coastal seas and by extension their role in global climate (Chen and Borges, 2009).
Coastal seas generally show high pH variability over an annual cycle owing to the relative magnitude of CO2 fluxes. The dissolution of rising atmospheric CO2 in seawater with subsequent formation and dissociation of carbonic acid has led to a decrease in pH of –0.1 units since 1800. This “Ocean Acidification” (OA) is projected to continue into the 21st century leading to regional pH reduction of up to 0.7 units (Caldeira and Wickett, 2003). Ocean acidification has potential consequences for many biological processes and the material fluxes that these mediate. Such processes include calcification, organic matter production, microbial growth, nitrification and bioturbation of sediments (Beman et al., 2011, Delille et al., 2005, Grossart et al., 2006, Huesemann et al., 2002, Kitidis et al., 2011, Riebesell et al., 2007, Widdicombe et al., 2011). The extent to which seawater can buffer changes in atmospheric CO2 is measured by the Revelle factor (RF; Eq. (1)), the ratio of the fractional change in pCO2 (δpCO2/pCO2) over the fractional change in DIC (δDIC/DIC) (Egleston et al., 2010, Revelle and Suess, 1957).
The values for RF are typically in the range of 8–15, with lower values suggesting a higher capacity for a body of water to absorb CO2. Of particular interest to calcification is the effect that OA has on the saturation state of calcite-minerals: calcite and aragonite. Thereby, Ωcalcite and Ωaraconite are defined as the ratio of Ca2+ and bicarbonate ions (CO3−) over the solubility product (ksp) of calcite and aragonite, respectively (Eq. (2)).Ω values higher and lower than 1 suggest favourable conditions for carbonate formation and dissolution, respectively. In addition, other pH-sensitive physical and chemical processes are expected to be affected by increasing acidification. For example, a shift from ammonia to ammonium ion in seawater, under lower pH, would lead to a drawdown of atmospheric ammonia following Le Chatelier’s principle (Jacobson, 2005, Wyatt et al., 2010). In this instance, ammonia is expected to fuel primary production and atmospheric CO2 drawdown in a negative feedback to rising CO2. Nevertheless, increased primary productivity and CO2 uptake due to eutrophication may outweigh the negative effect of OA on the carbonate system of coastal marine waters over the next century (Borges and Gypens, 2010). In order to understand the impact of ocean acidification on coastal ecosystems it is therefore necessary to quantify their seasonal pH range. Our study was part of a baseline study to establish pH variability in UK waters, funded by the UK Department for the Environment Food and Rural Affairs in recognition of this knowledge-gap (DEFRA-pH programme).
Here, we examine seasonal changes in the carbonate system from an extensive biogeochemical dataset spanning four years at a long-term oceanographic observatory in the Western English Channel. We use direct observations of the carbonate system (DIC, TA and pCO2) from two offshore stations to describe pH seasonality and derive sea-air fluxes of CO2 for our study area.
Section snippets
Sampling sites
Samples were collected from 2007 to 2010 as part of the Western Channel Observatory (WCO) oceanographic time-series. Marine laboratories in Plymouth have been collecting samples at a series of stations within the WCO for over a century (Smyth et al., 2010, Southward et al., 2005). Here, we focus on two of these stations, L4 (50.25°N, 4.22°W; depth 50 m) and E1 (50.03°N, 4.37°W; depth 75 m), which are located approximately 12 km and 38 km offshore of Plymouth in the English Channel (Fig. 1). The L4
Surface time-series data
Our carbonate system data, spanning a period of nearly four years, were distinguished by two temporally overlapping measurement phases. pCO2 was measured during the first phase, while DIC and TA measurements were made in the second phase. Our TA data were strongly correlated with sample salinity (Pearson R2=0.58, n=136, p<0.001) (Fig. 2). We therefore used the regression equation between TA and Salinity () to calculate TA for the first phase of our dataset and
Causes of pCO2 seasonality
The strong seasonal trends in surface pCO2 and n-pCO2 observed in this study demonstrate the physical and biological controls of C-cycling in the Western English Channel (WEC) in agreement with previous work (Borges and Frankignoulle, 2003, Dumousseaud et al., 2010, Litt et al., 2010, Padin et al., 2007). The annual cycle of surface water CO2 dynamics is dominated by undersaturation due to cooling in late winter, biological productivity in the spring–summer and oversaturation in the autumn
Acknowledgements
We would like to thank the crew of the RV Plymouth Quest for their assistance during fieldwork. We would also like to thank two anonymous reviewers for their comprehensive and constructive comments on this paper. This work was funded by the UK Natural Environment Research Council (NERC) through the CARBON-OPS (NE/E002021/1) and UK Ocean Acidification Area A (NE/H017054/1) projects, the Centre for Observation of Air-Sea Interactions and Fluxes, the UK National Centre for Earth Observation and
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