Vertical flux and microplankton assemblages in the Gulf of Liions during spring 1990

A vertical flux pulse related to spring phytoplankton development was recorded ·by moored sediment traps at 42°N 06°E in the Gulf of Lions. May 1990 trap samples from 200m to 2000m depth were comprised of freshly produced organic matter and selected microplankton species from the overlaying water column. This vertical flux event was transmitted to the deep sea floor with a high particle sinking velocity of >140 m day-1. Maximal vertical fluxes of 35 mg C m-2 day-1 and 1.2 mg chl.a m-2 day-1 recorded during this event are low compared to the exports from collapsing spring blooms at higher latitudes but demonstrate that particle production and degradation within the spring pelagic system were not in balance

The vertical flux of rare earth elements in the northwestern Mediterranean

Rare earth element (REE) composition of sinking particles was examined in time-series sediment trap samples collected from four depths (200, 500, 1000, 2000 m) in the Gulf of Lions. Vertical flux profiles showed the occurrence of a sedimentation pulse which resulted in the rapid sinking of phytoplankton aggregates to 2000 m depth. These particles were characterized by REE patterns similar to those in the upper 200 m indicating that little, if any, additional REE scavenging occurred during the sedimentation event. In contrast, after the sedimentation pulse, particles from deep waters showed an enrichment of light-REE (LREE) relative to heavy-REE (HREE) and a positive Ce anomaly. Comparing REE patterns in particles from the upper water column (200 m) with those from depth (1000, 2000 m) during and following the sedimentation pulse indicates that time is a key factor in determining REE scavenging by sinking particles. This is particularly evident for the preferential scavenging of Ce (IV) which is most pronounced in the finer, slowly sinking, and presumably older particles. These findings are consistent with REE patterns in sea water from the northwestern Mediterranean which show a strong negative Ce anomaly and gradual enrichment of REE with increasing atomic number. The enrichment of LREE relative to HREE in particles from deep waters results either preferential scavenging of LREE on particles, analogous to the enrichment of Ce, or selective dissolution of HREE in association with particle remineralization processes, or both; this aspect of REE behaviour merits further study

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 biomarkers to characterize the community in the water column and in drifting sediment traps set at 100 and 200 m. During 2 drifter experiments, calm 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 chlorophyll biomass. 19'-Hexanoyloxyfucoxanthin (prymnesiophytes) and chlorophyll b (chlorophytes, prasinophytes, prochlorophytes) were the major accessory pigments, while fucoxanthin, alloxanthin and peridinin indicated the presence of diatoms, cryptophytes and dinoflagellates, respectively. The proportional contribution of each algal group to the chlorophyll a (chl a) biomass, as derived from multiple regression analysis, revealed that prymnesiophytes, cryptophytes and the green algal group collectively accounted for at least 75% in the upper 100 m, emphasizing the importance of the nanophytoplankton. Phaeopigments, dominated by phaeophorbide a2, were the main pigments observed in sediment traps, although chl a, fucoxanthin and 19'-hexanoyloxyfucoxanthin were detected in smaller concentrations as well as traces of chlorophyll b (chl b). In deep water, fucoxanthin and 19'-hexanoyloxyfucoxanthin were the only accessory pigments present while total phaeopigment/chl a molar ratios >1 reflected the active transformation of fine phytogenic material at depth. High particulate organic carbon (POC)/chl a ratios (>100 in surface water; >1000 in deep water) suggested that phytoplankton was a relatively small component of the total carbon biomass down the water column. Using simple budget calculations, we determined that 58 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 sedimented out, mostly as phaeopigment, at 100 m (24%), and 6% was degraded to colourless residues in the water column. In contrast, only 12% of the chl a sedimented in Expt 2, while 20% was degraded to colourless residues

Pelagic processes and vertical flux of particles: an overview of a long-term comparative study in the Norwegian Sea and Greenland Sea

Pelagic processes and their relation to vertical flux have been studied in the Norwegian and Greenland Seas since 1986. Results of long-term sediment trap deployments and adjoining process studies are presented, and the underlying methodological and conceptional background is discussed. Recent extension of these investigations at the Barents Sea continental slope are also presented. With similar conditions of input irradiation and nutrient conditions, the Norwegian and Greenland Seas exhibit comparable mean annual rates of new and total production. Major differences can be found between these regions, however, in the hydrographic conditions constraining primary production and in the composition and seasonal development of the plankton. This is reflected in differences in the temporal patterns of vertical particle flux in relation to new production in the euphotic zone, the composition of particles exported and in different processes leading to their modification in the mid-water layers. In the Norwegian Sea heavy grazing pressure during early spring retards the accumulation of phytoplankton stocks and thus a mass sedimentation of diatoms that is often associated with spring blooms. This, in conjunction with the further seasonal development of zooplankton populations, serves to delay the annual peak in sedimentation to summer or autumn. Carbonate sedimentation in the Norwegian Sea, however, is significantly higher than in the Greenland Sea, where physical factors exert a greater control on phytoplankton development and the sedimentation of opal is of greater importance. In addition to these comparative long-term studies a case study has been carried out at the continental slope of the Barents Sea, where an emphasis was laid on the influence of resuspension and across-slope lateral transport with an analysis of suspended and sedimented material