Short-scale temporal variability of physical, biological and biogeochemical processes in the NW Mediterranean Sea: an introduction

摘要

In the framework of the PROOF-PECHE project (href="http://www.obs-vlfr.fr/proof/vt/op/ec/peche/pec.htm" target="_blank">http://www.obs-vlfr.fr/proof/vt/op/ec/peche/pec.htm) a multi-disciplinaryteam performed experiments and collected samples during the DYNAPROC 2 cruiseaboard the RV Thalassa from September to October in 2004. The cruiseprovided data on the functioning of the pelagic food web by sampling over amonth long period in the NW Mediterranean Sea at a fixed station subject toweak horizontal advection currents during a period of hydrologicalstability. This paper describes the background of the cruise and provides anoverview of the results derived from the campaign which constitute thespecial section. The major objective of the cruise was to assess therelative importance and variability of the pathways of carbon in the openocean. Intensive sampling through 4 periods of 5 days each was accomplishedat a site near the DYFAMED time-series site. The site was near stable interms of hydrodynamics as there was some evidence of an intrusion oflow-salinity coastal water. The cruise yielded a comprehensive data setacquired by sampling over a vertical spatial dimension (0–1000 m) and athigh frequencies (ranging from every 3, 6, 12 and/or 24 h), unique forthe summer to autumn transition in the North Western Mediterranean.Parameters investigated included the biochemical composition of dissolvedorganic matter (lipids), and the structure of bacterial communities,phytoplankton and zooplankton community compositions and abundances, as wellas zooplankton metabolism, and particulate organic carbon fluxes. Nearly allthe parameters described in this section, as well as reports appearingelsewhere, showed time-course variabilities of similar magnitude to thoseknown from a previous study of the spring-summer seasonal transition, aperiod of marked hydrological change, at the same study site. Remarkably,the least variable characteristic of the system appeared to be theidentities of the dominant taxa across several trophic levels (copepods,phytoplankton, ciliates, and bacteria) throughout the study period despitelarge shifts in stock sizes and fluxes. Thus, the studies of DYNAPROC 2documented considerable temporal variability of stocks and rates in a systemwhich was, from a hydrological and taxonomic point of view, relativelystable.