Biogeochemical Inferences from the Diel Variability of Optical

Biogeochemical Inferences from the Diel Variability of Optical Properties in the NW Mediterranean (BOUSSOLE site) Morvan Barnes 1 David Antoine 1 Morvan Barnes Post-Doc 1 Laboratoire d’Océanographie de Villefranche, CNRS and Université Pierre et Marie Curie, 06238 Villefranche sur Mer, FRANCE I. High-resolution observations of IOPs II. Diel & Seasonal Variability of IOPs c) Climatology cp AIM: Examine variability of cp and bbp across different seasons and trophic states. 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. Aims a) Season delineation Map showing Boussole station (black square) in case 1 Mediterranean waters. In the framework of the BIOCAREX (BIOoptics and CARbon Experiment) and BOUSSOLE (BOUée pour l’acqui. Sition d’une Série Optique à Long term. E) projects, our aims are: To better understand biooptics 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. 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). Climatology of surface cp at Boussole showing 10 -day mean (blue line) and associated standard deviation (blue area). d) Seasonality of cp & bbp b) Diel cycles of cp & bbp Methods Time series (2006 -2011) of high frequency (15 min) and hyper-spectral optical observations in surface waters at the BOUSSOLE site. Vertical profiles of cp (660 nm) at a lower frequency (monthly) from CTD profiles. Net community production (NCP) is calculated from the diel increase in cp following Claustre et al (2008, Example of a diel cycle of cp at Boussole from Gernez et al (2011, L&O 56). The Biogeosci 5) whereby: 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). III. Vertical structure of cp a) Seasonal profiles Climatology of surface cp at Boussole showing 10 -day mean (blue line) and associated standard deviation (blue area). KEY POINTS High-frequency transmissiometer data can offer more than sole beam attenuation. Mean diel cycles of cp and bbp during bloom periods. Rate of diel cp variation can be used to investigate carbon accumulation of particle assemblage. Characteristic seasonal vertical profiles of cp 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 AIM: Characterise the production maxima in March, whilst diel variations relationship between surface confirm a late-morning maxima. cp and its integrated content over the water column. Apply towards vertical extension of cpbased production model. NCP values are comparable to chl-based PP estimates. c) ΔPOC-to-cp ratio • Differences between diel cycles of cp and bbp including a notable lag in the daily maxima of cp during blooming periods in particular. • Strong seasonal differences in diel variation of cp and bbp and in the mean daily values. • Both cp and bbp on average twice as high during April than during periods of oligotrophy or strong mixing. IV. NCP from IOPs AIM: Determine whether IOPs such as cp can be used to derive high resolution community or primary production data; Examine the diel and seasonal variations in community production. c) Diel NCP 0 m variations a) Seasonal NCP Characteristic vertical profiles of cp by season showing mean fit to in situ profile data (r 2). b) Cp vertical extension Relationship between mean surface cp and ΔPOC, the diel increase in POC. d) ΔPOC vertical extension Climatology of cp-derived NCP showing 10 -day mean (blue line) and associated standard deviation (blue area). Ten-fold variations in the 10 -day mean were observed. b) NCP vs PP estimates Time series of cp-derived NCP (open circles) and primary production estimates from calculated from a chlorophyll-based model (Morel 1991, P in O 26). Comparison of in situ cp with both vertical modulation (using profiles) and vertical integration from surface values. Differences between ΣΔPOC (total water column) calculated using both methods of vertical extension. • Vertical distribution of cp can be accurately depicted by 4 characteristic seasonal profiles. • These profiles improve on previously assumed vertical homogeneity which often underestimated cp from 10 -50 m and overestimated at depths below 50 m. • Community carbon production is greatest in March at the beginning of the increase in surface cp. • 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. • A strong relationship between surface ΔPOC (daily increase in POC calculated from cp) and mean daily cp allows for application of profiles to surface ΔPOC. • However, the use of vertical cp profiles had no significant effect on ΣΔPOC. Further Questions Could combining buoy data with data from profiling floats increase our understanding of optical variability? Could bbp be similarly used to estimate biogeochemical properties? What optical/biogeochemical properties can be used to characterise the anomalous optical properties of the Mediterranean? 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. 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). Acknowledgements Funding Partners 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. Contact us: Laboratoire Océanographique de Villefranche, Quai de La Darse, 06238 Villefranche, France T +33(0)493763736 E barnes@obs-vlfr. fr W www. obs-vlfr. fr/LOV/OMT/