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Abstract

We report over 600 absorption spectra (250–800 nm) of unfiltered surface waters, <250 m depth, collected at daily stations during three Atlantic Meridional Transect cruises (AMT9, 10, 11) between the UK and Uruguay. AMT cruises 9 and 11 were southbound (15 September 1999–13 October 1999 and 12 September 2000–11 October 2000, respectively), while AMT10 was northbound (12 April 2000–8 May 2000). Absorption coefficients at 300 nm, a300, ranged from 0.13 to 1.32 m−1, showed insignificant differences between filtered and unfiltered samples, and were therefore attributed to chromophoric dissolved organic matter (CDOM). A non-linear single exponential regression provided the best fit to our CDOM absorbance spectra and was used to parameterise the spectral slope, S, of the monotonic absorbance decrease with increasing wavelength over the wavelength ranges 290–350 nm (S290–350) and 250–650 nm (S250–650). We observed distinct patterns in the latitudinal and depth distribution of CDOM absorbance characteristics. Distinct subsurface a300 maxima, characterised by lowest spectral slope values (S290–350=0.010 nm−1 and S250–650=0.014 nm−1) were observed in the vicinity of the deep chlorophyll maximum (DCM) in open ocean and upwelling regions, and indicated in-situ production of CDOM. Converserly, the surface a300 minima and surface S290–350 and S250–650 maxima in these regions were attributed to CDOM photo-oxidation. In order to assess the nature of the observed CDOM variability along our AMT transects, we grouped our data into 12 individual, oceanographic provinces, and further into two seasons (spring: April–May, and Autumn: September–October) and three depth zones (surface mixed layer, pycnocline, and below pycnocline). Comparisons between individual provinces, seasons and depth zones indicated that CDOM variability was dominated by regional factors and depth distribution patterns, while seasonal variability was generally less important in our data. Based on depth distribution patterns together with analyses of inter-relations between a300, and S290–350, S250–650 we propose that our data reflect the presence of two CDOM end-members, each characterised by distinct spectral slope factors, S290–350 and S250–650. The first CDOM end-member (S290–350=0.010 and S250–650=0.014 nm−1) was situated in the vicinity of the DCM, and was attributed to CDOM production from phytoplankton-derived organic matter via planktonic foodweb interactions. The second end-member (S290–350=0.028 nm−1 and S250–650=0.029 nm−1) was attributed to microbial CDOM production. We propose that the CDOM distribution along the AMT cruise track is controlled by autochthonous production near the DCM and subsequent photo-oxidation in surface waters.