A regional analysis of new production on the northwest European shelf using oxygen fluxes and a ship-of-opportunity

Abstract

Seasonal new production (g C m −2) estimates obtained from dissolved oxygen and nitrate concentrations in surface waters (5 m depth) along a track between the UK (Portsmouth) and northern Spain (Bilbao) are compared. An oxygen flux method, in combination with a ship of opportunity (SOO), was tested on the northwest European shelf for its value in distinguishing high production in frontal regions. Dissolved oxygen, nitrate and chlorophyll a samples were collected monthly from February to July 2004, alongside continuous autonomous measurements of salinity, temperature and chlorophyll fluorescence. Depth integrated new production estimates for all the individually analysed hydrographic regions of the route were produced. Results from three widely used gas-exchange parameterizations gave seasonal (February–July) new production estimates of 54–68 g C m −2 for the Ushant region of the western English Channel and 31–40 g C m −2 for the shelf slope, averaging 24–31 g C m −2 for the route. This is double the route average obtained using the nitrate assimilation method (17 g C m −2) and within the ranges of previous estimates in the same region. The oxygen flux method gave a fivefold enhancement compared to the nitrate method in the Ushant frontal region and a threefold enhancement in the English Channel and shelf break regions. Determining oxygen fluxes to estimate new production may be more reliable than nitrate assimilation in active tidal or frontal regions of shelves where nitrate may be added to the system post-winter through advection or entrainment.