Diel variations in particle stocks in the oligotrophic waters of the Ionian Sea (Mediterranean)

In order to assess the factors responsible for the variability in particle attenuation of light at a daily scale in the oligotrophic waters of the Ionian Sea (Eastern Mediterranean), optical and biogeochemical measurements were collected at high spatial and temporal frequency during late summer 1999. Over 5 days, repeated profiles were conducted in situ every 3 h using a spectral absorption–attenuation meter at 9 wavelengths (WETLabs ac-9) in a profiling mode, and were accompanied by a series of discrete biogeochemical measurements including particle size distribution and concentration, and flow cytometric determination of picophytoplankton and heterotrophic bacteria abundances. Temporal variations in the vertical distribution of the particle attenuation coefficient and particle concentration displayed clear daily oscillations, with an increase during daytime counter-balanced by a similar decrease at night. For the first time in in situ conditions, diel changes in the particle size were also evidenced through discrete measurements of the particle size distribution and in situ continuous ac-9 profiles of the spectral characteristics of particle attenuation coefficient. The diel variability in the concentration and size of the particle stock, and its impact on bulk particle attenuation, is discussed in relation to the composition of the autotrophic assemblage, the balance between algal and non-algal particles and the specific rhythms in biological processes. In Ionian Sea waters, heterotrophic bacteria appear to play a major role in the diel cycle in particle attenuation.