ALBERT

Description: Because zooplankton feces represent a potentially important transport pathway of surface-derived organic carbon in the ocean, we must understand the patterns of fecal pellet abundance and carbon mobilization over a variety of spatial and temporal scales. To assess depth-specific water column variations of fecal pellets on a seasonal scale, vertical fluxes of zooplankton fecal pellets were quantified and their contribution to mass and particulate carbon were computed during 1990 at 200, 500, 1000, and 2000 m depths in the open northwestern Mediterranean Sea as part of the French-JGOFS DYFAMED Program. Depth-averaged daily fecal pellet flux was temporally variable, ranging from 3.04 * 10**4 pellets m**2/d in May to a low of 6.98 * 10**2 pellets m**2/d in September. The peak flux accounted for 50% of the integrated annual flux of fecal pellets and 62% of pellet carbon during only two months in mid-spring (April and May). Highest numerical fluxes were encountered at 1000 m, suggesting fecal pellet generation well below the euphotic zone. However, there was a trend toward lower pellet carbon with increasing depth, suggesting bacterial degradation or in situ repackaging as pellets sink through the water column. At 500 m, both the lowest pellet numerical abundance and carbon flux were evident during the spring peak. Combined with data indicating that numerical and carbon fluxes are dominated at 500 m by a distinct type of pellet found uniquely at this depth, these trends suggest the presence of an undescribed mid-water macro-zooplankton or micro-nekton community. Fecal pellet carbon flux was highest at 200 m and varied with depth independently of overall particulate carbon, which was greatest at 500 m. Morphologically distinct types of pellets dominated the numerical and carbon fluxes. Small elliptical and spherical pellets accounted for 88% of the numerical flux, while larger cylindrical pellets, although relatively rare (〈10%), accounted for almost 40% of the overall pellet carbon flux. Cylindrical pellets dominated the pellet carbon flux at all depths except 500 m, where a large subtype of elliptical pellet, found only at that depth, was responsible for the majority of pellet carbon flux. Overall during 1990, fecal pellets were responsible for a depth-integrated annual average flux of 1.03 mgC/m**2/d, representing 18% of the total carbon flux. The proportion of vertical carbon flux attributed to fecal pellets varied from 3 to 35%, with higher values occurring during periods when the water column was vertically mixed. Especially during these times, fecal pellets are a critical conveyor of carbon to the deep sea in this region.

Description: Data on large particles (LP; 〉0.15 mm), phytoplankton communities, vertical fluxes, and hydrology were collected between January 1992 and June 1996 in the NW Mediterranean Sea, during DYFAMED, an interdisciplinary program part of JGOFS France. LP concentrations at the study sites were typical for values found in other open-ocean studies. LP temporal evolution showed an annual cycle. Concentrations were the highest in winter/spring (20-120/l, 5-280 mg/m**3) and lowest in summer and autumn (0-20/l, 0.8-60 mg/m**3). We estimated that LP accounted on average, for 2-30% of the total particulate (〉0.7 µm) dry weight (DW). LP temporal evolution between 0-200 m was correlated with total Chl a and fucoxanthin (diatoms), and inversely correlated to zeaxanthin (cyanobacteria and prochlorophytes). Although diatoms were clearly associated to LP, prymnesiophytes were associated to the two highest accumulation of particles 〉1 mm. The DW fraction of particle 〉0.5 mm to total LP increased from 10% in regenerated systems dominated by picoplankton to 50% during spring blooms. LP concentrations in the upper 200 m were correlated to mass flux recorded in sediment trap at 200 and 1000 m. We defined three main periods for LP downward export related to physical stratification. (1) The major LP export occurred in winter and may be affected by deep vertical mixing; (2) in spring, at the onset of the thermal stratification LP downward export decreased, although pulses of phytoplanktonic production may have enhanced LP sedimentation over short time scales; (3) during the summer stratification, the deep water was generally depleted in LP.

Description: The transfer of atmospheric particulate matter through the surface marine layer was studied by comparing atmospheric and marine fluxes. Time series were obtained from the coupling of a coastal atmospheric sampling station (Cap Ferrat, French Riviera) and a marine sampling site (DYFAMED site, central Ligurian Sea). Liquid phase traps were used for measuring total atmospheric fluxes and sediment traps deployed at 200 m depth for measuring marine fluxes. Fluxes of mass, aluminium, and soluble anthropogenic metals (Cd, Pb and Zn) were obtained from both these reservoirs. Physical and biological time series data acquired at the DYFAMED site also were used to describe a three-step seasonal transfer scenario: (i) In summer and autumn, during the period of water stratification, marine fluxes are low and do not account for the transfer of lithogenic material, as revealed by low Al to mass flux ratios and high proportions of organic carbon at 200 m depth. Atmospheric material accumulates along the thermocline, while a series of physico-chemical processes lead to the formation of small (〈=150 µm) non-biogenic organic aggregates. (ii) In winter, the sinking of dense water that occurs in the Ligurian Sea is responsible for a rapid downward transfer of the lithogenic matter accumulated in the surface layer. The fact that soluble trace metals (e.g., cadmium) accumulated in the surface layer are only partially found in sediment traps may indicate that sorption processes play a minor role in the formation of organic aggregates, compared with coagulation processes. (iii) In spring, nutrients brought to surface waters by the winter vertical mixing allow phytoplanktonic blooms, and the transfer of atmospheric matter is then governed by the temporal variations of biological activity. The seasonal variability of the vertical transfer leads to the concept of seasonal variability of elemental residence times in the euphotic layer.