Phytoplankton pigment chemotaxonomy of the northeastern Atlantic

Phytoplankton pigment distributions were studied between 62 and 37 degreesN in the northeastern Atlantic as a component of the NERC PRIME programme. A t the northern end of the transect, waters were characterised by a surface chlorophyll a (CHLa) maximum of 500-700 ng l(-1) (0-40 mj. At the southern end, surface waters were virtually devoid of nutrients (NO3- < 0.5 <mu>M, N H4+ < 50 nM), and surface CHLa concentrations were < 50 ng l(-1). At 37 deg reesN a well-defined deep CHLa maximum (DCM) was recorded between 60 and 10 0 m (similar to 350 ng l(-1)). Highest concentrations of CHLa (1500 mug l(- 1)) were measured in the surface mixed layer to the south of a. well-define d salinity and temperature front at 52.5 degreesN. Overall, 19'-hexanoyloxy fucoxanthin (HEX, up to 1120 ng l(-1)) was the dominant accessory pigment, although zeaxanthin (ZEA) was the major accessory pigment at 37 degreesN. C onversion of pigment data into quantitative estimates of algal class abunda nces indicated the composition of phytoplankton population was relatively s table in surface waters between 62 and 52.5 degreesN: Prymnesiophytes were the most abundant class, contributing a mean of 38% of the total CHLa, whil e cryptophytes (20%), chlorophytes (14%) and diatoms (15 %) contributed the bulk of the remaining CHLa. Together, these four classes accounted for 79- 92% of the total CHLa in this section of the transect. Immediately south of the front (52-50 degreesN), prymnesiophytes contributed approximately half of the total CHLa, while south of 50 degreesN there was a shift to a popul ation dominated by cyanobacteria and prochlorophytes, which together accoun ted for a mean of 53% of the measured total CHLa at 37 degreesN. At 37 degreesN the contribution of cyanobacteria to total CHLa declined sig nificantly with depth, and the DCM was dominated by prochlorophytes, prymne siophytes and cryptophytes. Below the DCM, chrysophytes were the most abund ant class of phytoplankton, contributing 30% of the total CHLa, with prymne siophytes, prochlorophytes and cryptophytes also making significant contrib utions. The contribution of prymnesiophytes to total CHLa was found to be r elatively stable throughout the water column (23%, SD 3 %). Although highes t concentrations of divinyl chlorophyll n (dvCHLa) were recorded in the DCM , the contribution of dvCHLa to total CHLa (dvCHLa + CHLa) was only 21-26% here compared to up to 48% (mean = 33 +/- 9.6%) in surface waters. The rati o of [dvCHLa]: prochlorophyte biomass increased from 10 ng mug C-1 in the s urface 40 M to 56.9 ng mug C-1 between 50 and 100 m. This corresponded to a rise in cellular dvCHLa from 0.215 to 1.83 mug cell(-1) and thus significa nt photo-adaptation with depth. Hence, a component of the DCM fluorescence and pigment signals is the result of increased cellular pigmentation rather than from increases in biomass alone. On the basis of inter-pigment ratios , we suggest that the DCM was dominated by an Atlantic strain of prochlorop hytes, adapted to lower light levels, while the surface oligotrophic layer was composed of a mixed population of both Atlantic and a higher light adap ted Mediterranean strain. Correlation studies indicated the potential of CHLa, fucoxanthin (FUC) and HEX to serve as respective proxy markers of POC and the biominerals silicat e (SiO2) and calcite (CaCO3) in surface waters (r = 0.49-0.78, p < 0.001) a nd in depth profiles at northern latitudes (59<degrees>N, r = 0.74-0.75, p < 0.001). However, poor correlations were observed in depth profiles at the southern end of the transect. (C) 2001 Published by Elsevier Science Ltd.