Abstract
The heterogeneity in phytoplankton production in the North Atlantic after the spring bloom is poorly understood. We analysed merged microwave and infrared satellite sea surface temperature (SST) data and ocean colour phytoplankton size class biomass, primary production (PP) and new production (ExP) derived from SeaWiFS data, to assess the spatial and temporal frequency of surface thermal fronts and areas of enhanced PP and ExP. Strong and persistent surface thermal fronts occurred at the Reykjanes Ridge (RR) and sub-polar front (SPF), which sustain high PP and ExP and, outside of the spring bloom, account for 9% and 15% of the total production in the North Atlantic. When normalised by area, PP at the SPF is four times higher than the RR. Analysis of 13years of satellite ocean colour data from SeaWiFS, and compared with MODIS-Aqua and MERIS, showed that there was no increase in Chla from 1998 to 2002, which then decreased in all areas from 2002 to 2007 and was most pronounced in the RR. These time series also illustrated that the SPF exhibited the highest PP and the lowest variation in Chla over the ocean colour record. This implies that the SPF provides a high and consistent supply of carbon to the benthos irrespective of fluctuations in the North Atlantic Oscillation.
Highlights
Phytoplankton are responsible for about half of the global primary production (PP) (Longhurst, Sathyendranath, Platt, & Caverhill, 1995) and in the North Atlantic, represent a significant sink for carbon dioxide (Takahashi et al, 2009)
At 52°N, the Reykjanes Ridge (RR) is segregated from the rest of the Mid-Atlantic Ridge (MAR) by the Charlie-Gibbs Fracture Zone (CGFZ), where the North Atlantic Current (NAC) crosses the MAR, which results in an sea surface temperature (SST) front (Fig. 1B)
A 13 year SeaWiFS climatology of PP (Fig. 2A–D) showed that outside of the spring bloom, there was a significantly higher PP over the sub-polar front (SPF) (0.021 GtC) and RR (0.018 GtC), which accounted for 14.8% and 8.6% of the North Atlantic PP respectively, compared to the ICB (0.005 GtC; 2.4%) and CIS (0.012 GtC; 6.9%) (F3,579, P b 0.0001)
Summary
Phytoplankton are responsible for about half of the global primary production (PP) (Longhurst, Sathyendranath, Platt, & Caverhill, 1995) and in the North Atlantic, represent a significant sink for carbon dioxide (Takahashi et al, 2009). North Atlantic sub-polar waters account for ~50% of the global ocean productivity and N 80% of the organic flux to the sea floor (Wassmann, 1990). The magnitude of the spring bloom in this region is one of the largest in the global ocean and is controlled by a combination of physical forcing and biological factors (Koertzinger et al, 2008), with its timing driven strongly by physical forcing (Henson, Dunne, & Sarmiento, 2009). Frontal systems have long been recognised as areas of high phytoplankton biomass and PP (Pingree, Kuo, & Garcia-Soto, 2002). Increases in biomass have been observed along the cyclonic side of the Gulf
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