Diel variation in the vertical distribution of particulate matter (> 0.15 mm) in the NW Mediterranean Sea investigated with the Underwater Video Profiler

Day/night variations in the size distribution of the particulate matter > 0 .15 mm (PM) were studied in May 1995 during the DYNAPROC time-series cruise in the northwestern Mediterranean Sea. Data on vertical distributions of P M (>0.15 mm) and zooplankton were collected with the Underwater Video Profi ler (UVP). The comparisons of the UVP data with plankton net data and POC d ata from water bottles indicated that more than 97% of the particles detect ed by the UVP were non-living particles (0.15 mm) and that the PM contribut ed 4-34% of the total dry weight measured on GF/F filters. Comparison of se ven pairs of day and night vertical profiles performed during the cruise sh owed that in the upper 800 m, the mean size and the volume of particles was higher at night than during the day. During the night, the integrated volu me of the Ph I increased on average by 32 +/- 20%. This increase correspond ed to a shift of smaller size classes ( < 0.5 mm) towards the larger ones ( > 0.5 mm). During the day, the pattern was reversed, and the quantity of PM > 0.5 mm decreased. During the study period, the standing stock of PM (60- 800 m) decreased from 7.5 to less than 2gm(-2) but the diel variations pers isted, except for two short periods in the superficial layer following a wi nd event. The cyclic feeding activity induced by the diel vertical migratio n of zooplankton could be the best candidate to explain the observed diel f luctuations in the size classes of PM in the water column. However, our res ults also suggest that in the upper layer additional driving forces such as the increase of the level of turbulence after a wind event or the modifica tion of the zoo- and phytoplankton community can influence the PM temporal evolution. (C) 2000 Elsevier Science Ltd. All rights reserved.