Phytoplankton pigment chemotaxonomy of the northeastern Atlantic

Abstract

Phytoplankton pigment distributions were studied between 62 and 37°N in the northeastern Atlantic as a component of the NERC PRIME programme. At the northern end of the transect, waters were characterised by a surface chlorophyll a (CHL a) maximum of 500– 700 ng l −1 (0– 40 m ). At the southern end, surface waters were virtually devoid of nutrients ( NO 3 −<0.5 μM, NH 4 +<50 nM ), and surface CHL a concentrations were <50 ng l −1 . At 37°N a well-defined deep CHL a maximum (DCM) was recorded between 60 and 100 m (∼350 ng l −1) . Highest concentrations of CHL a (1500 μg l −1) were measured in the surface mixed layer to the south of a well-defined salinity and temperature front at 52.5°N. Overall, 19′-hexanoyloxyfucoxanthin (HEX, up to 1120 ng l −1) was the dominant accessory pigment, although zeaxanthin (ZEA) was the major accessory pigment at 37°N. Conversion of pigment data into quantitative estimates of algal class abundances indicated the composition of phytoplankton population was relatively stable in surface waters between 62 and 52.5°N: Prymnesiophytes were the most abundant class, contributing a mean of 38% of the total CHL a, while cryptophytes (20%), chlorophytes (14%) and diatoms (15%) contributed the bulk of the remaining CHL a. Together, these four classes accounted for 79–92% of the total CHL a in this section of the transect. Immediately south of the front (52–50°N), prymnesiophytes contributed approximately half of the total CHL a, while south of 50°N there was a shift to a population dominated by cyanobacteria and prochlorophytes, which together accounted for a mean of 53% of the measured total CHL a at 37°N. At 37°N the contribution of cyanobacteria to total CHL a declined significantly with depth, and the DCM was dominated by prochlorophytes, prymnesiophytes and cryptophytes. Below the DCM, chrysophytes were the most abundant class of phytoplankton, contributing 30% of the total CHL a, with prymnesiophytes, prochlorophytes and cryptophytes also making significant contributions. The contribution of prymnesiophytes to total CHL a was found to be relatively stable throughout the water column (23%, SD 3%). Although highest concentrations of divinyl chlorophyll a (dvCHL a) were recorded in the DCM, the contribution of dvCHL a to total CHL a (dvCHL a + CHL a) was only 21–26% here compared to up to 48% (mean=33±9.6%) in surface waters. The ratio of [dvCHL a]: prochlorophyte biomass increased from 10 ng μg C −1 in the surface 40 m to 56.9 ng μg C −1 between 50 and 100 m. This corresponded to a rise in cellular dvCHL a from 0.215 to 1.83 fg cell −1 and thus significant 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 prochlorophytes, adapted to lower light levels, while the surface oligotrophic layer was composed of a mixed population of both Atlantic and a higher light adapted Mediterranean strain. Correlation studies indicated the potential of CHL a, fucoxanthin (FUC) and HEX to serve as respective proxy markers of POC and the biominerals silicate (SiO 2) and calcite (CaCO 3) in surface waters ( r=0.49–0.78, p<0.001) and in depth profiles at northern latitudes (59° N, r=0.74 –0.75, p<0.001). However, poor correlations were observed in depth profiles at the southern end of the transect.