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 Laboratoire d’Océanographie de Villefranche (LOV)(UMR7093) France Observatoire Océanologique de Villefranche (OOV) France

Introduction 1.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). 2.Numerous laboratory measurements have shown that the diel variations of c p are mainly caused by changes of the refractive index and cell size. (Stramski and Reynolds, 1993) 3.The origin of this variability is still badly known. 4.In contrast to c p diel cycles, diurnal variations in b bp are poorly documented. (Loisel et al., 2011) Limitations In situ studies (cruises): often limited to a few days. Laboratory studies: schematically represent the natural environment. Limitations In situ studies (cruises): often limited to a few days. Laboratory studies: schematically represent the natural environment. Motivations Ocean optics 2012 October 10 th

Introduction 5.The backscattering coefficient is derivable from satellite ocean color. 6.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 b bp diel cycles is of particular interest. to analyse and characterize the b bp and c p diel cycles associated to different environmental conditions. to compare c p and b bp diel cycles. Objectives Suggested solution: Continuous measurement, at high-frequency, on an instrumented mooring (BOUSSOLE). Ocean optics 2012 October 10 th

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

5 years from 2006 to 2010 (c p, b bp & [chl]) Seasonal variations 4 seasons, corresponding to situations of winter mixing, development of the bloom, collapse of the bloom, and summer and fall oligotrophy, have been differenciated for each year. Results Ocean optics 2012 October 10 th

5 years from 2006 to 2010 (c p, b bp & [chl]) Seasonal variations Are the diel variations changing with seasons ? Results Ocean optics 2012 October 10 th

A.Winter mixing B. Development of the bloom C. Collapse of the bloom D. Oligotrophy Zoom on five days during each season Characterization of the diel variability: 1.A diel cycle appears to be a recurrent feature in the c p and b bp signal. 2.Differences in shape and amplitude at different period of the years are observed. Results Ocean optics 2012 October 10 th

Diurnal variability by season ΔX(k) = 100 [X(k) / X1 - 1] X1 = value at sunrise k= fraction of day cpcp c p starting increase at dawn and decreasing at sunset. Amplitude varies with seasons: % during mixing, collapse and oligotrophy and 20% - 50% during bloom. c p 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. Results Ocean optics 2012 October 10 th

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

cpcp b bp Comparison of c p and b bp cycles Results Ocean optics 2012 October 10 th 1.c p and b bp daily oscillations appear to be slightly shifted in time. Minimum b bp values are usually synchronized to c p whereas maximum b bp are often reached few hours before than those for c p. 2. For each year, c p diel cycles are higher during the mixing, collapse, oligotrophy than b bp diel cycle by a factor up to ~1.5 and by a factor 2 to 5 during the bloom.

cpcp b bp Comparison of c p and b bp cycles Results Ocean optics 2012 October 10 th

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. Results The backscattering ratio diel cycles Ocean optics 2012 October 10 th This assumes that the particle scattering coefficient is spectrally flat [b p (λ) = c p (660 nm)]. = Twardowski et al., 2001; Boss et al., 2004

Discussion Ocean optics 2012 October 10 th Origin of the diel variability ? Origin of the differences observed between c p and b bp ? Each of these phenomena implies a change of abundance, size (PSD) and refractive index (n) and thus IOPs. Diurnal increase Growth of cells (↑PSD) Fixation of carbon (↑n) Diurnal increase Growth of cells (↑PSD) Fixation of carbon (↑n) Nocturnal decrease Respiration and loss of cellular material (↓n, ↓PSD) Cellular division (↓PSD, ↑Number) Grazing (↓ Number) Nocturnal decrease Respiration and loss of cellular material (↓n, ↓PSD) Cellular division (↓PSD, ↑Number) Grazing (↓ Number) The lower magnitude observed for diel variations of b bp might be related to the high sensitivity of b bp to changes in the small particle abundance (bacteria and detritus). b bp is mostly influenced by submicrometer particles, whereas c p is mainly driven by particles with diameters between 0.5 and 20 μm (Stramski and Kieffer, 1991; Pak et al., 1988). 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; …

The Mie theory (Mie, 1908) is only used as a tool for interpretation to parameterize the dependence of c p and b bp on the daily changes in refractive index (n) and size distribution (PSD). Mie computations Ocean optics 2012 October 10 th Discussion 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 c p and b bp diel cycles. Strategy 1.Bibliography about several studies performed on the diel variability of the refractive index, size and abundance of phytoplankton cells. (Stramski and Reynolds, 1993; Stramski et 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)

microorganismsConcentration (m -3 )Size (µm)Refractive index (n)Wavelength (nm) Viruses [ ] Bacteria [ ] Detritus [0.02-4] Picophytoplankton [0.2-2] Ultra-nanoplankton [2-8] Nanophytoplankton [8-20] Microphytoplankton [20-100] Mie computations Ocean optics 2012 October 10 th Discussion Concentration, particle size distribution, refractive index and wavelength of each group of microorganisms used in Mie computations. c p and b bp dependent only on the dynamics of phytoplankton cells (variation of PSD, n & abundance). Base simulations background component time-varying component (24h)

Mie computations Ocean optics 2012 October 10 th PSDn PSD, n 1.Daily changes in c p and b bp can be related to daily changes in size and refractive index. 2.The main driving factor for c p is PSD and for b bp, n. 3.It’s necessary to stimulate daily changes in PSD & n to be in agreement with in situ observations. Discussion

Conclusion & perspectives Ocean optics 2012 October 10 th Investigate the impact of diurnal variations of IOPs on the diurnal variability of AOPs (Kd, R, …). 1.The c p and b bp time series show clear daily oscillations whatever the season. 2.The characteristics and the shape of the c p diel cycle vary seasonally. 3.Seasonal differences in c p diel cycles seem to be related to the trophic state of phytoplankton (nutrient availability, population composition, physiological state,…). 4.Differences between c p and b bp diel cycles can be related to the high sensitivity of b bp to changes in the small particle abundance (bacteria, detritus, etc…). 5.Use of b bp cycle to infer biogeochemical properties and carbon fluxes at diurnal scale will be questionable. Conclusion Perspective

Ocean optics 2012 October 10 th 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)!

Data selection Data quality control Characterize the zone where the variability is due to unstable physical conditions Data eliminated Ocean optics 2012 October 10 th