DISTRIBUTION, SEDIMENTATION AND FATE OF PIGMENT BIOMARKERS FOLLOWING THERMAL STRATIFICATION IN THE WESTERN ALBORAN SEA

A spring investigation of the phytoplankton in the western Alboran Sea (Mediterranean) was undertaken using chlorophyll and carotenoid bioma rkers to characterize the community in the water column and in driftin g sediment traps set at 100 and 200 m. During 2 drifter experiments, c alm and sunny conditions induced a progressive thermal stratification that reduced pigment sedimentation into deeper water and confined the phytoplankton to the surface layer, resulting in an increase in chloro phyll biomass. 19'-Hexanoyloxyfucoxanthin (prymnesiophytes) and chloro phyll b (chlorophytes, prasinophytes, prochlorophytes) were the major accessory pigments, while fucoxanthin, alloxanthin and peridinin indic ated the presence of diatoms, cryptophytes and dinoflagellates, respec tively, The proportional contribution of each algal group to the chlor ophyll a (chl a) biomass, as derived from multiple regression analysis , revealed that prymnesiophytes, cryptophytes and the green algal grou p collectively accounted for at least 75 % in the upper 100 m, emphasi zing the importance of the nanophytoplankton. Phaeopigments, dominated by phaeophorbide at, were the main pigments observed in sediment trap s, although chl a, fucoxanthin and 19'-hexanoyloxyfucoxanthin were det ected in smaller concentrations as well as traces of chlorophyll b (ch l b). In deep water, fucoxanthin and 19'-hexanoyloxyfucoxanthin were t he only accessory pigments present while total phaeopigment/chl a mola r ratios > 1 reflected the active transformation of fine phytogenic ma terial at depth. High particulate organic carbon (POC)/chl a ratios (> 100 in surface water; >1000 in deep water) suggested that phytoplankto n was a relatively small component of the total carbon biomass down th e water column. Using simple budget calculations, we determined that 5 8 to 65 % of the chl a produced in the upper 100 m accumulated in the water column over both experiments. During Expt 1, 29 % of the chl a s edimented out, mostly as phaeopigment, at 100 m (24 %), and 6 % was de graded to colourless residues in the water column. In contrast, only 1 2 % of the chl a sedimented in Expt 2, while 20% was degraded to colou rless residues.