Primary production at time series station DYFAMED

Phytoplankton carbon assimilation has been measured near monthly using the 14C method at DYFAMED France JGOFS time-series station from 1993 to 1999. Data were obtained using the "LET GO" technique, which allowed in situ injection of bicarbonate and incubation in enclosures at 10 depths. Incubation duration was 4 h around noon, from which daily production was estimated. The seasonal variation of the depth-integrated carbon assimilation exhibits a marked cycle. Maximum values reach 1.8 g C/m**2/d in March or April; constant lower values were observed from August to January, in the range 100-300 mg C/m**2/d. The annual primary production vary in the range 86-232 g C/m**2/yr, in the upper range of older estimations. Primary production normalized to chlorophyll a shows maximum values in the period of oligotrophy. This increase of carbon assimilation rate per unit of chlorophyll a appears as linked to the period of phosphorus-limited ecosystem, and vertical distribution of taxonomic pigments suggests a possible role of cyanobacteria. Potential export production has been estimated from primary production data and Fp ratio based on pigments concentrations. These estimates (which imply biological steady state conditions) vary in a wide range, from 19 to 71 g C/m**2/yr. There is a decoupling between years with high potential export production and years with high measured particulate fluxes, which highlights the question of balance by resupply of the limiting nutrients and the role of dissolved organic carbon. A possible shift of primary production towards a more regeneration-dominated system is suggested for recent years.

The C37 alkenone record of sea water of time series station DYNAPROC

The concentrations of suspended particulate pigments, C37-C38 alkenones, total organic carbon and nitrogen in the Ligurian Sea (northwestern Mediterranean) have been studied at 5 and 30 m depth during well defined thermocline conditions. An accurate description of the short term changes of these compounds has been achieved by means of four 36-h sampling cycles each encompassing consecutive filtration periods of 4 h. During sampling the thermocline changes were followed closely by simultaneous measurements of water column temperature, salinity and other physical parameters. The analysis of the collected samples indicates that the Haptophyte pigments and alkenones are essentially synthesized at the levels of highest primary production and therefore the C37 alkenone record reflects the seawater temperature at this depth level. The study also shows that part of these alkenones are distributed throughout the water column in association to the suspended particles. This process results in C37 alkenone distributions that, due to their high resistance to chemical and microbial degradation, record the temperature of the highest primary productivity layers even at shallow (e.g., 5 m depth) or deep (e.g., 1100 m depth) waters.

Phytol and phytyldiol concentrations at DYFAMED time series station and sediment trap

Particulate samples from the water column were collected monthly from depths of 5-150 m, between May 1996 and March 1997, in the northwestern Mediterranean Sea (Ligurian Sea) as part of the DYFAMED project within the French JGOFS program. These samples were analyzed by gas chromatography-electron impact mass spectrometry for their phytol and 3-methylidene-3,7,11-trimethylhexadecan-1,2-diol (phytyldiol) content. The corresponding Chlorophyll Phytyl side chain Photodegradation Index, molar ratio of phytyldiol to phytol, was calculated and the mean amount of chlorophyll photodegraded within the euphotic zone estimated. Seasonal differences in the chlorophyll photodegradation process appear in the one-year study. The chlorophyll appeared more photodegraded in the surface water (generally more than 40% photodegraded at 5-10 m) than at the deep chlorophyll maximum (DCM) (40-50 m) observed in the summer stratified waters (about 20% photodegraded). This difference was attributed to the healthy state of the phytoplankton community (coincidence with the highest primary production levels) and to the lower intensity of irradiance at the DCM level. On the other hand, the bulk of the detrital chlorophyll (chlorophyll associated with phytodetritus, phaeopigments) undergoes photodegradation before it sinks out of the photic zone. However, in January (winter mixed water) the pigments exported towards the sea floor were less photodegraded. This is thought to result from a shorter period of residence of the pigments in the photic zone due to vertical convection and grazing activity of macrozooplankton (salps), which are producers of rapid sinking fecal pellets.

Atmospheric and seawater carbon dioxide at time series station DYFAMED

A time series of fCO2, SST, and fluorescence data was collected between 1995 and 1997 by a CARIOCA buoy moored at the DyFAMed station (Dynamique des Flux Atmospheriques en Mediterranée) located in the northwestern Mediterranean Sea. On seasonal timescales, the spring phytoplankton bloom decreases the surface water fCO2 to approximately 290 µatm, followed by summer heating and a strong increase in fCO2 to a maximum of approximately 510 µatm. While the DELTA fCO2 shows strong variations on seasonal timescales, the annual average air-sea disequilibrium is only 2 µatm. Temperature-normalized fCO2 shows a continued decrease in dissolved CO2 throughout the summer and fall at a rate of approximately 0.6 µatm/d. The calculated annual air-sea CO2 transfer rate is -0.10 to -0.15 moles CO2 m-2 y-1, with these low values reflecting the relatively weak wind speed regime and small annual air-sea fCO2 disequilibrium. Extrapolating this rate over the whole Mediterranean Sea would lead to a flux of approximately -3 * 10**12 to -4.5 * 10**12 grams C/y, in good agreement with other estimates. An analysis of the effects of sampling frequency on annual air-sea CO2 flux estimates showed that monthly sampling is adequate to resolve the annual CO2 flux to within approximately ±10 - 18% at this site. Annual flux estimates made using temperature-derived fCO2 based on the measured fCO2-SST correlations are in agreement with measurement-based calculations to within ± 7-10% (depending on the gas transfer parameterization used), and suggest that annual CO2 flux estimates may be reasonably well predicted in this region from satellite or model-derived SST and wind speed information.

Zooplankton biovolume time series at station STARESO (Bay of Calvi, Corsica)

A long-term time series of subsurface zooplankton is performed in the oligotrophic Bay of Calvi (Corsica, Ligurian Sea, NW Mediterranean) from 2003 onwards. It is carried out from the marine station STARESO and is based on weekly measurement of zooplankton biovolumes. The main objectives are (i) to determine the seasonal cycle and inter-annual variability of the organisms, (ii) to study the dynamics of the populations, and (iii) to understand zooplankton interactions with other co-sampled hydrographic, meteorological and phytoplankton variables at the site.