Biogeochemical Inferences from the Diel Variability of Optical Properties in the NW Mediterranean (BOUSSOLE site) Morvan Barnes Post-Doc Morvan Barnes 1 David Antoine 1 I. High-resolution observations of IOPs II. Diel & Seasonal Variability of IOPs III. Vertical structure of c p IV. NCP from IOPs c) Diel NCP 0m variations a) Seasonal profiles c) ΔPOC-to-c p ratio c) Climatology c p a) Season delineation AIM: Examine variability of c p and b bp across different seasons and trophic states. AIM: Determine whether IOPs such as c p can be used to derive high resolution community or primary production data; Examine the diel and seasonal variations in community production. Introduction The interpretation and understanding of oceanographic field observations are intimately linked to their inherent spatial and temporal scales of variation. Whilst satellite observations are adapted for describing large-scale oceanographic phenomena – moored buoys are suited to study transient phenomena or to produce real-time estimates of biogeochemical properties. In particular, the link between the temporal variability of IOPs and phytoplankton production at the diurnal scale has been relatively understudied. Relationship between mean surface c p and ΔPOC, the diel increase in POC. AIM: Characterise the relationship between surface c p and its integrated content over the water column. Apply towards vertical extension of c p - based production model. Further Questions Could combining buoy data with data from profiling floats increase our understanding of optical variability? Could b bp be similarly used to estimate biogeochemical properties? What optical/biogeochemical properties can be used to characterise the anomalous optical properties of the Mediterranean? 1 Laboratoire dOcéanographie de Villefranche, CNRS and Université Pierre et Marie Curie, Villefranche sur Mer, FRANCE Contact us: Laboratoire Océanographique de Villefranche, Quai de La Darse, Villefranche, France T +33(0) E W Characteristic vertical profiles of c p by season showing mean fit to in situ profile data (r 2 ). Comparison of in situ c p with both vertical modulation (using profiles) and vertical integration from surface values. a) Seasonal NCP Funding Partners Acknowledgements This study is a contribution to the BIOCAREX and BOUSSOLE projects with funding and technical and logistic support provided by the organisations listed. The authors are grateful to the members of the BOUSSOLE staff for lab analyses and data quality control, and to the crews of the research vessels for ship measurements and sampling. b) C p vertical extension d) ΔPOC vertical extension Differences between Σ ΔPOC (total water column) calculated using both methods of vertical extension. Vertical distribution of c p can be accurately depicted by 4 characteristic seasonal profiles. A strong relationship between surface ΔPOC (daily increase in POC calculated from c p ) and mean daily c p allows for application of profiles to surface ΔPOC. These profiles improve on previously assumed vertical homogeneity which often underestimated c p from m and overestimated at depths below 50 m. However, the use of vertical c p profiles had no significant effect on Σ ΔPOC. Aims In the framework of the BIOCAREX (BIOoptics and CARbon Experiment) and BOUSSOLE (BOUée pour lacquiSition dune Série Optique à Long termE) projects, our aims are: To better understand bio- optics in Mediterranean waters and the links with biological carbon production by exploiting continuous high-frequency observations on a fixed site; To understand the daily and seasonal changes in optical properties. Methods Time series ( ) of high frequency (15 min) and hyper-spectral optical observations in surface waters at the BOUSSOLE site. Vertical profiles of c p (660 nm) at a lower frequency (monthly) from CTD profiles. Net community production (NCP) is calculated from the diel increase in c p following Claustre et al (2008, Biogeosci 5) whereby: Example of a diel cycle of c p at Boussole from Gernez et al (2011, L&O 56). The top axis represents fractions of the normalized day where 0 is sunrise and 0.5 is sunset. Also shown are the rate of variation (r) and the daily rate (µ). Map showing Boussole station (black square) in case 1 Mediterranean waters. b) NCP vs PP estimates Climatology of c p -derived NCP showing 10-day mean (blue line) and associated standard deviation (blue area). Ten-fold variations in the 10-day mean were observed. Time series of c p -derived NCP (open circles) and primary production estimates from calculated from a chlorophyll-based model (Morel 1991, P in O 26). Mean (black line) and 95% confidence intervals (white lines) of NCP calculated at 30 min intervals based on variations in cp. Percentage of total data for pooled NCP values are indicated as the colour scale. Community carbon production is greatest in March at the beginning of the increase in surface c p. Climatology of surface c p at Boussole showing 10-day mean (blue line) and associated standard deviation (blue area). Time series of the mixed layer depth (full circles with standard deviation in grey) and surface chlorophyll measurements (dotted line) from the monthly sampling at Boussole. Data were separated into 4 seasons: Mixed (MLD > 100 m), Bloom & Collapse (Chl > 0.8 mg m -3 ), and Oligotrophy (Chl < 0.4 mg m -3 ). Optically-derived NCP measurements are comparable to traditional chlorophyll-based primary production measurements both in terms of magnitude and temporal variability. However, this technique enables the calculation of diel variations in NCP. This reveals, for example, maximal carbon fixation before midday with very little variability during periods of oligotrophy. For more information - on diel cycles of Boussole optics (Poster Session 3: 111.Kheireddine), data quality control (Poster Session 2: 227.Vellucci) and other Boussole achievements (Poster Session 1: 61.Diamond). d) Seasonality of c p & b bp Climatology of surface c p at Boussole showing 10-day mean (blue line) and associated standard deviation (blue area). b) Diel cycles of c p & b bp KEY POINTS High-frequency transmissiometer data can offer more than sole beam attenuation. Rate of diel c p variation can be used to investigate carbon accumulation of particle assemblage. Characteristic seasonal vertical profiles of c p may be used to extend the IOP-based production model through the water column, although assuming vertical homogeneity of ΣΔPOC yields comparable results. Seasonal estimates of net community production reveal production maxima in March, whilst diel variations confirm a late-morning maxima. NCP values are comparable to chl-based PP estimates. Mean diel cycles of c p and b bp during bloom periods. Differences between diel cycles of c p and bbp including a notable lag in the daily maxima of c p during blooming periods in particular. Strong seasonal differences in diel variation of c p and b bp and in the mean daily values. Both c p and b bp on average twice as high during April than during periods of oligotrophy or strong mixing.