Abstract
Abstract. The diurnal fluctuations in solar irradiance impose a fundamental frequency on ocean biogeochemistry. Observations of the ocean carbon cycle at these frequencies are rare, but could be considerably expanded by measuring and interpreting the inherent optical properties. A method is presented to analyze diel cycles in particulate beam-attenuation coefficient (cp) measured at multiple wavelengths. The method is based on fitting observations with a size-structured population model coupled to an optical model to infer the particle size distribution and physiologically relevant parameters of the cells responsible for the measured diel cycle in cp. Results show that the information related to size and contained in the spectral data can be exploited to independently estimate growth and loss rates during the day and night. In addition, the model can characterize the population of particles affecting the diel variability in cp. Application of this method to spectral cp measured at a station in the oligotrophic Mediterranean Sea suggests that most of the observed variations in cp can be ascribed to a synchronized population of cells with an equivalent spherical diameter around 4.6±1.5 μm. The inferred carbon biomass of these cells was about 5.2–6.0 mg m−3 and accounted for approximately 10% of the total particulate organic carbon. If successfully validated, this method may improve our in situ estimates of primary productivity.
Highlights
The Earth’s climate is profoundly influenced by the ocean biological pump, which absorbs CO2 from the atmosphere and transfers it to the deep ocean in particulate and dissolved form
The suitability of the pumping system for conducting these measurements was verified by comparing C-star based cp(660) values measured on the flow-through system with surface cp values derived from another C-star transmissometer that was mounted on the CTD
The novelty of the present work is that the dynamics of both the magnitude and the spectral shape of cp are exploited to characterize the phytoplankton population that most likely is responsible for the diel variations in cp
Summary
The Earth’s climate is profoundly influenced by the ocean biological pump, which absorbs CO2 from the atmosphere and transfers it to the deep ocean in particulate and dissolved form. Be investigated by means of the inherent optical properties, which have strong potential to improve understanding of the biological pump. The ocean carbon cycle is dependent on the Earth’s rotation. Imposes a fundamental cycle on photosynthesis and the resulting conversion of atmospheric CO2 into particulate carbon. Diel cycles of biogeochemical (Burney et al, 1982; Gasol et al, 1998) and optical (Siegel et al, 1989; Cullen et al, 1992; Marra, 1997) properties have been observed in the sunlit ocean
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