Distribution of macrozooplankton at time series station MIGZOO-1

Vertical distributions and diel migrations of the main species of micronekton, four euphausiids, one mysid, one decapod and three fishes, were described in detail in the 0-1000 m water column on a fixed station in the Northwestern Mediterranean Sea. The euphausiids Euphausia krohni and Thysanopoda aequalis, the decapod Gennadas elegans and, to a lesser extent, the fish Argyropelecus hemigymnus were shown to perform clear diel vertical migrations. Results of horizontal hauls at a given depth around sunrise and sunset showed a marked diurnal symmetry of the migratory cycles, particularly for E.krohni, T.aequalis and G.elegans. The behaviour of the euphausiid Nematoscelis megalops was more complex: it presented a repetitive bimodal day distribution and only part of its population migrated. As very weak or non-migrators we found the euphausiid Stylocheiron longicorne and the bathypelagic mysid Eucopia unguiculata, for which migration concerned only some of the older individuals. The fishes Cyclothone braueri and Cyclothone pygmaea appeared to be non-migrants. As depth increased, C.braueri was replaced by C.pygmaea, with maximum concentrations at 350-550 and 550-700 m depth, respectively.

Distribution of macrozooplankton at time series station MIGZOO-4

A large population of the colonial pelagic tunicate Pyrosoma atlanticum occurred in April 1991 in offshore waters of the Ligurian Sea (Northwestern Mediterranean). The high numbers of colonies caught allowed their vertical distribution and diel migration in the 0-965 m water column to be described as a function of their size. Daytime depths and amplitudes of the migration were correlated with colony size. The amplitude of the migration ranged from 90 m for 3-mm-length colonies to 760 m for 51-mm-length colonies, with a mean amplitude of 410 m for the whole population, all sizes pooled. The results of horizontal hauls at a given depth around sunrise and sunset showed a marked diurnal symmetry of the migratory cycle relative to noon, and that migration of the population was not cohesive. For example, the larger the colonies, the later after sunset they reached the upper layers during their upward migration.

Dissolved organic carbon measured on water bottle samples at time series station DYFAMED

Dissolved organic carbon (DOC) distribution and dynamics are investigated at the DYFAMED site (central Ligurian Sea, NW Mediterranean) in relation to hydrological and biological contexts, using a 4-year time-series dataset (1991-1994). The DYFAMED site is regarded as a one-dimensional station where simple hydrological mechanisms prevail and where the ecosystem is quite well understood. An average vertical profile of DOC concentration ([DOC]) indicates that maximal concentrations and variability are concentrated in the surface layers. For depths >800 m, the annual variations are on average similar to the analytical standard deviation (~2 µM). The "composite" [DOC] distribution (average distribution over a typical year, integrating about 40 monthly profiles) for surface waters (0-200 m) is closely related to hydrological and phytoplanktonic forcings. It exhibits summer DOC accumulation in surface waters, due to spring-summer stratification and successive phytoplanktonic events such as spring and summer blooms, and winter DOC removal to deeper waters, due to intense vertical mixing. The analysis of vertical [DOC] gradient at 100-m depth as a function of the integrated DOC content in the 0-100-m layer makes it possible to objectively distinguish three specific periods: the winter vertical mixing period, the period of stratification and spring phytoplankton bloom, and the period of stratification re-inforcement and summer-fall phytoplankton bloom. We recalculate the vertical DOC fluxes to deep waters using a larger original dataset, after the first direct calculation (Deep-Sea Res. 40 (10) (1993) 1963, 1972) that was reproduced for other oceanic areas. The seasonal variations of the "composite" [DOC] distribution in surface waters are significantly correlated to the apparent oxygen utilization distribution, but the biogeochemical significance of such a correlation is still under examination. The global significance of our local findings is presented and the role of the oceanic DOC in the global carbon cycle is emphasized, especially with respect to several current issues, such as the oceanic "missing sink" and the equivalence between new production and exported production.