Morvan Barnes Post-Doc

1. Biogeochemical Inferences from the Diel Variability of Optical Properties in the NW Mediterranean (BOUSSOLE site) Morvan Barnes1 David Antoine1 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. a) Season delineation Map showing Boussole station (black square) in case 1 Mediterranean waters. 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). Aims In the framework of the BIOCAREX (BIOoptics and CARbon Experiment) and BOUSSOLE (BOUée pour l’acquiSition d’une 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. 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, Biogeosci 5) whereby: Climatology of surface cp at Boussole showing 10-day mean (blue line) and associated standard deviation (blue area). KEY POINTS • Differences between diel cycles of cp and bbp including a notable lag in the daily maxima of cp during blooming periods in particular. High-frequency transmissiometer data can offer more than sole beam attenuation. 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 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 cp and bbp during bloom periods. Example of a diel cycle of cp 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). • 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. III. Vertical structure of cp 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. a) Seasonal profiles AIM: Characterise the relationship between surface cp and its integrated content over the water column. Apply towards vertical extension of cp-based production model. c) Diel NCP0m variations a) Seasonal NCP c) ΔPOC-to-cp ratio Characteristic vertical profiles of cp by season showing mean fit to in situ profile data (r2). b) Cp 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. Relationship between mean surface cp and ΔPOC, the diel increase in POC. b) NCP vs PP estimates d) ΔPOC vertical extension 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). 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. 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. • 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. • 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. • 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. 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). • 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? 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. Funding Partners 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/