Our shared sea

Mechanisms of ecosystem change in the Western Channel

Dynamics of the CO2 system in the Western English Channel based on bimonthly measurements performed on the Armorique ferry

Research using the FerryBox ocean observing system installed on a cross-channel ferry shows differences in the CO2 uptake and ecosystem productivity between the two hydrographically distinct regions of the western English Channel.

Figure 1 : Map and bathymetry of the study area with the location of 18 stations along the ferry track and location of the fixed stations ASTAN, E1 and L4.

  • How does the research fit into Marinexus?

The regular ferry line operated by the Britanny Ferries company between Roscoff and Plymouth perfectly match with the Marinexus project. The Armorique ferry equipped with a FerryBox system continuously records biogeochemical parameters along the ferry track. The objectives of this research are to study air-sea CO2 fluxes, the main greenhouse gas, in the Western English Channel and more generally the dynamics of the ecosystems from diurnal to inter-annual time scale.

  • What is the problem under investigation?

Continental shelf ecosystems are highly dynamic regarding physical, chemical and biological parameters and play a significant role in biogeochemical cycles despite their relatively moderate size compared to the global oceans (7%). In the context of climate change, with increasing water temperatures, rising CO2 concentration in the atmosphere and oceans, and decreasing ocean pH, long-term high-frequency monitoring of marine ecosystem variability is essential, particularly in coastal ecosystems. In such dynamic ecosystems, important short scale biological events occur and impact the chemical properties of the WEC waters. The FerryBox appears as the perfect tool to monitor biogeochemical parameters with a high spatio-temporal resolution at a lesser cost.

Moreover the WEC is a particularly interesting area to investigate because it hosts two contrasting hydrographical provinces: the southern WEC, where the water column is well-mixed throughout the year due to intense tidal streams, and the northern WEC, where seasonal stratification of the water column occurs from April to September.

  • What are the key findings of the research?

For the first time we investigated the CO2 system dynamics along a latitudinal gradient in the WEC which allowed us to study the air-sea CO2 flux variability in the two main provinces of the WEC. The CO2 system showed different dynamics throughout the year in these 2 provinces with a greater CO2 sink and higher net ecosystem production in the seasonally stratified region compared to the year-round mixed region. The combined approach of a ferry line and fixed stations appears to be a valuable strategy and can provide a robust assessment of air-sea CO2 fluxes in the WEC.

  • Why it is important for society?

It is essential to improve our understanding of air-sea CO2 fluxes in the context of climate change due to anthropogenic CO2 emissions, especially in coastal ecosystems where these measurements are sparse considering the diversity of these ecosystems. It will allow the scientific community to better quantify the role of coastal ecosystems regarding the global carbon cycle. In addition to climate change, high atmospheric CO2 concentrations lead to high oceanic CO2 concentrations and subsequently to ocean acidification. The carbonate chemistry in seawater will be modified and could affect many marine calcifying organisms. Long term deployments of FerryBoxes are critical to assess anthropogenic forcing (e.g. ocean acidification, eutrophication) on coastal ecosystems in the context of climate change.

The research by lead author Pierre Marrec is published in the journal Marine Chemistry. Further details:
 

1.     Summary of the study

In December 2010 we installed an autonomous ocean observing system, a FerryBox, on the regular ferry line between Roscoff (France) and Plymouth (UK) run by the Brittany Ferries company. In this first study we investigated the dynamics of the CO2 system and ancillary biogeochemical parameters across the Western English Channel (WEC) based on bimonthly discrete measurements from December 2010 to December 2011. The WEC hosts two main hydrographical structures: the all-year well-mixed southern WEC and the seasonally stratified northern WEC. We combined the surface ferry measurements with vertical profiles at fixed stations (ASTAN: Roscoff Coastal Observatory SOMLIT and E1: Western Channel Observatory in Plymouth) representative of each hydrographical province to assess the structure of the water column.

Figure 1 : Map and bathymetry of the study area with the location of 18 stations along the ferry track and location of the fixed stations ASTAN, E1 and L4.

 The contrasting hydrographical properties strongly influenced the ecosystem dynamics. The seasonally stratified northern WEC showed enhanced biological activities characterized by an extensive autotrophic phase, which maintained the pCO2 below the atmospheric level until early fall thus representing a sink for atmospheric CO2 at a rate of 1.1 mol C m-2 y-1. The permanently well-mixed southern WEC was characterized by a shorter autotrophic phase due to a delay in spring phytoplankton growth and an early start of the fall heterotrophic phase, resulting in an annual air-sea CO2 flux close to equilibrium at a rate of -0.4 mol C m-2 y-1. Biological production/respiration processes were the main driver of pCO2 variability during this year.

 

Figure 2 : Air-sea CO2 fluxes (in mmol m-2 d-1) between Roscoff (48.73°N) and Plymouth (50.35°N) in 2011 computed from pCO2 air-sea gradient.

Previous studies carried out in the WEC did not discern the two main hydrographical structures which appears essential to resolve the discrepancies of the CO2 system dynamics. Since April 2012 we also have access to high-frequency pCO2 data along the ferry tracks in addition to other biogeochemical parameters (temperature, salinity dissolved O2 and fluorescence) recorded by the FerryBox. The first analysis of these high-frequency data will be published soon and will provide new insight into the short scale processes controlling air-sea CO2 fluxes in the different provinces of the WEC.

Reference : Marrec Pierre, Cariou Thierry, Collin Edouard, Durand Axel, Latimier Marie, Macé Eric, Morin Pascal, Raimund Stephen, Vernet Marc, and Bozec Yann, 2013. Seasonal and latitudinal variability of the CO2 system in the Western English Channel based on Voluntary Observing Ship (VOS) measurements. Marine Chemistry 155, 29-41.

 

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