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Malika KHEIREDDINE and David ANTOINE kheireddine@obs-vlfr.fr

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  1. Diurnal variability of particulate matter from observations of beam attenuation and backscattering coefficients in the Northwestern Mediterranean sea (BOUSSOLE site) Malika KHEIREDDINE and David ANTOINE kheireddine@obs-vlfr.fr Laboratoire  d’Océanographie  de  Villefranche (LOV)(UMR7093) France Observatoire  Océanologique  de  Villefranche (OOV) France

  2. Introduction Motivations • The diel variability of optical properties results from the cyclical solar forcing. It is a phenomenon which is observed in situ and can be replicated in laboratory. (Claustre et al., 1999; 2002, Siegel et al., 1988 , Gernez et al., 2011). • Numerous laboratory measurements have shown that the diel variations of cp are mainly caused by changes of the refractive index and cell size. (Stramski and Reynolds, 1993) • The origin of this variability is still badly known. • In contrast to cp diel cycles, diurnal variations in bbp are poorly documented. (Loiselet al., 2011) • Limitations • In situ studies (cruises): often limited to a few days. • Laboratory studies: schematically represent the natural environment. Oceanoptics 2012 October 10th

  3. Introduction • The backscattering coefficient is derivable from satellite ocean color. • Current (GOCI) and future geostationary satellite ocean color instruments will provide new opportunities to infer biogeochemical processes from space with an increased temporal resolution. • A better understanding of bbp diel cycles is of particular interest. Objectives • to analyse and characterize the bbpand cpdielcycles associated to different environmental conditions. • to compare cp and bbp diel cycles. Suggested solution: Continuous measurement, at high-frequency, on an instrumented mooring (BOUSSOLE). Oceanoptics 2012 October 10th

  4. BOUSSOLE project The BOUSSOLE project 1. Mooring site in open ocean, weak ocean currents. 2. Continuous acquisition (15 min day and night) in surface. 3. Optical measurements: cp (660 nm) bbp (442, 555 nm) 4.Physical information (CTD): Temperature (T) Salinity (S) Buoy depth (Zbuoy) 5. Monthly cruises: CTD profiles Discrete sampling (HPLC) Antoine et al., (2006, 2008b). Oceanoptics 2012 October 10th

  5. Results Seasonal variations 5 years from 2006 to 2010 (cp, bbp & [chl]) 4 seasons, correspondingto situations of winter mixing, development of the bloom, collapse of the bloom, andsummer and fall oligotrophy, have been differenciated for eachyear. Oceanoptics 2012 October 10th

  6. Results Seasonal variations 5 years from 2006 to 2010 (cp, bbp & [chl]) Are the diel variations changing with seasons ? Oceanoptics 2012 October 10th

  7. Results Zoom on five days during each season Winter mixing B. Development of the bloom C. Collapse of the bloom D.Oligotrophy • Characterizationof the diel variability: • A diel cycle appears to be a recurrent feature in the cp and bbp signal. • Differences in shape and amplitude at different period of the years are observed. Oceanoptics 2012 October 10th

  8. Results Diurnal variability by season cp • cp starting increase at dawn and decreasing at sunset. • Amplitude varies with seasons: 10 - 20% during mixing, collapse and oligotrophy and 20% - 50% during bloom. cp diel cycles are marked by a significant seasonal variability, which is consistent with the seasonal cycle observed at BOUSSOLE, which results in seasonal changes in nutrient concentrations, phytoplankton composition and size. ΔX(k) = 100 [X(k) / X1 - 1] • X1 = value at sunrise • k= fraction of day Oceanoptics 2012 October 10th

  9. Results Diurnal variability by season bbp • bbp starts increasing at dawn. • bbp starts decreasing few hours before sunset. • Amplitude varies between 5 and 30 % according to the season. In contrast to cp diel cycles, bbp diel cycles are not marked by a significant seasonal variability. ΔX(k) = 100 [X(k) / X1 - 1] • X1 = value at sunrise • k= fraction of day Oceanoptics 2012 October 10th

  10. Results Comparison of cp and bbp cycles cp bbp cp and bbp daily oscillations appear to be slightly shifted in time. Minimum bbp values are usually synchronized to cp whereas maximum bbp are often reached few hours before than those for cp. 2. For each year, cp diel cycles are higher during the mixing, collapse, oligotrophy than bbp diel cycle by a factor up to ~1.5 and by a factor 2 to 5 during the bloom. Oceanoptics 2012 October 10th

  11. Results Comparison of cp and bbp cycles cp bbp Oceanoptics 2012 October 10th

  12. Results The backscattering ratio diel cycles The ratio starts decreasing at dawn and starts increasing, generally, at sunset. It suggests a decrease of the refractive index and/or a decreasing proportion of small particles relatively to large particles in water. In this case, the variability observed could be arise from changes in the shape of the size distribution. = This assumes that the particle scattering coefficient is spectrally flat [bp(λ) = cp (660 nm)]. Oceanoptics 2012 October 10th Twardowskiet al., 2001; Boss et al., 2004

  13. Discussion Origin of the diel variability ? • Nocturnal decrease • Respiration and loss of cellular material (↓n, ↓PSD) • Cellular division (↓PSD, ↑Number) • Grazing (↓ Number) • Diurnal increase • Growth of cells (↑PSD) • Fixation of carbon (↑n) Siegel et al., 1989; Cullen et al., 1992 ; Walsh et al., 1995; Stramski and Reynolds, 1993; Durand and Olson, 1998; Claustre et al., 2002; Durand et al., 2002; … Each of these phenomena implies a change of abundance, size (PSD) and refractive index (n) and thus IOPs. Origin of the differences observed between cp and bbp ? The lower magnitude observed for diel variations of bbp might be related to the high sensitivity of bbp to changes in the small particle abundance (bacteria and detritus). bbp is mostly influenced by submicrometer particles, whereas cp is mainly driven by particles with diameters between 0.5 and 20 μm (Stramski and Kieffer, 1991; Pak et al., 1988). Oceanoptics 2012 October 10th

  14. Discussion Mie computations The Mie theory (Mie, 1908) is only used as a tool for interpretation to parameterize the dependence of cp and bbp on the daily changes in refractive index (n) and size distribution (PSD). • Objectives • Understand the causes of the diurnal variability observed in this study. • To determine which of n or PSD is the main factor controlling the cp and bbpdiel cycles. • Strategy • Bibliography about several studies performed on the diel variability of the refractive index, size and abundance of phytoplankton cells. (Stramski and Reynolds, 1993; Stramskiet al., 1995 ; Durand et Olson, 1998; André et al., 1999; Durand et al., 2002; Claustre et al., 2002(a)) • 2. Establish a representative population of relatively clear oligotrophic water (BOUSSOLE). (viruses, detritus, bacteria, pico-, nano- and micro-phytoplankton) Oceanoptics 2012 October 10th

  15. Discussion Mie computations Base simulations Concentration, particle size distribution, refractive index and wavelength of each group of microorganisms used in Mie computations. cp and bbpdependentonly on the dynamics of phytoplanktoncells (variation of PSD, n & abundance). background component Oceanoptics 2012 October 10th time-varying component (24h)

  16. Discussion Mie computations PSD, n PSD n • Daily changes in cp and bbp can be related to daily changes in size and refractive index. • The main driving factor for cp is PSD and for bbp, n. • It’s necessary to stimulate daily changes in PSD & n to be in agreement with in situ observations. Oceanoptics 2012 October 10th

  17. Conclusion & perspectives Conclusion The cp and bbp time series show clear daily oscillations whatever the season. The characteristics and the shape of the cpdiel cycle vary seasonally. Seasonal differences in cp diel cycles seem to be related to the trophic state of phytoplankton (nutrient availability, population composition, physiological state,…). Differences between cp and bbp diel cycles can be related to the high sensitivity of bbp to changes in the small particle abundance (bacteria, detritus, etc…). Use of bbp cycle to infer biogeochemical properties and carbon fluxes at diurnal scale will be questionable. Perspective • Investigate the impact of diurnal variations of IOPs on the diurnal variability of AOPs (Kd, R, …). Oceanoptics 2012 October 10th

  18. Thanks BOUSSOLE & Co Thank you for your attention! Without them, I wouldn't have been able to present this work. For more information, come to see my poster (tonight, n°111)! Oceanoptics 2012 October 10th

  19. Data selection Dataqualitycontrol Characterizethe zone where the variability is due to unstable physical conditions Data eliminated Oceanoptics 2012 October 10th