Bight of the North Sea is part of the North
Sea continental shelf, delimited to the South by the English Channel,
and, to the North by the thermal stratification of the water column during
summer (around 54°N). Whereas the northern part of the North Sea has
oceanic characteristics, the Southern Bight is dominated by the input from
the English Channel water (with Atlantic Ocean characteristics) and the inputs
from majors European estuaries (Scheldt, Meuse, Rhine, Ems, Elbe, Thames and
Humber) heavily impacted by anthropogenic activities. Human activities have
led to an enrichment of nutrients, inorganic and organic carbon of the coastal
waters of the Southern Bight of the North Sea. Among the consequences of these
anthropogenic inputs, coastal metabolism and air-sea CO2 exchange
have been modified, towards respectively, heterotrophy and emission of CO2
to the atmosphere (Gattuso et al. 1998, Annual Review of Ecological Systems,
29, 405-434). However, it is difficult to assess the balance of these processes
(autotrophy versus heterotrophy). On one hand, nutrients enhance primary
production, consuming carbon dioxide in the water column. Indeed, the high
nutrients discharges to coastal waters are responsible for eutrophication
and even dystrophisation, with the recurrent occurrence of massive spring
blooms of Phaeocystis
sp. from the French to the German coasts. On the other hand, inputs
of organic carbon promote heterotrophic processes, and so increase the level
of partial pressure of carbon dioxide (pCO2) and fluxes to the
atmosphere. It has been shown that outer estuaries (estuarine plumes) are
sources of carbon dioxide to the atmosphere, with fluxes that can represent
up to 50% of the net emission from the inner estuary, as in the case for the
Scheldt estuary (Borges and Frankignoulle, 2002, Biogeochemistry, 59:
41-67; Borges and Frankignoulle, 1999, Journal of Marine Systems, 19:
251-266; Frankignoulle et al., 1998, Science: 282, 434-436).
Objectives of the project :
The main objectives of the project are to asses the potential role of the Southern bight of the North Sea and the heavily polluted estuarine plumes, as sources or sinks of atmospheric carbon dioxide and their trophic status.
The role as a source or a sink of carbon dioxide depends on the air-water gradient of pCO2. The direct field measurement of pCO2 is carriout with an equilibrator adapted for measurement in coastal turbid waters (Frankignoulle et al., 2001, Water Research, 35 (5), 1344-1347). The theoretical approach is then to compute the air-water CO2 fluxes from the air-water gradient of pCO2 and the gas transfer velocity (parameterized as a function of wind speed).
The assessment of the trophic status is carried out from dissolved inorganic carbon field data and a simple mass balance approach. This approach relies on high temporal (more than one year) and spatial coverage of the pCO2 in surface water. The measured pCO2 is the resultant of all biological processes in the water column as it integrates the consumption of CO2 by autotrophy and release of CO2 by heterotrophic processes. Thus, it allows to compute net ecosystem metabolism (taking into account the air-water exchange of CO2 and the thermodynamic effect of temperature variations).
Experimental approaches :
Over the next three years, the sampling strategy is to cover a great part of the Southern bight with special emphasis on the territorial zones of the French, Belgian, Dutch and English coasts which are under the influence of heavily polluted estuaries (Scheldt, Rhine and Thames).
The following parameters will be measured on board of the R.V. Belgica: dissolved oxygen, speciation of the dissolved inorganic carbon by means of continuous measurements of pCO2 and pH and discrete measurements of Total Alkalinity.
The budget of air-water CO2 exchanges will be achieved by the deployment of unattended and automated measuring device that will run during all the cruises of the R.V. Belgica (CUBE).
Cruises on board of the R.V. Belgica:
CANOPY is funded by the OSTC (Federal Office for Scientific, Technical and Cultural Affairs, Research Project EV/12/20C, 2002-2006).