Partitioning total spectral absorption in phytoplankton and colored detrital material contributions
Résumé
A method based on spectral information is used to derive spectral absorption coefficients of phytoplankton a(phi)(lambda) and colored detrital material, CDM, which includes non-algal particle and colored dissolved organic matter, a(CDM)(lambda), from total minus water absorption coefficients measurements. This method is first validated over a dataset of more than 300 simultaneous measurements of phytoplankton, non-algal particle, and colored dissolved organic matter absorption coefficients spectra acquired with a laboratory spectrophotometer in various oceanic and coastal European waters. The validation is presented for measurements made with a high spectral resolution (hyper-spectral case), and a limited spectral resolution (multi-spectral case)-the case of most devices routinely used for in situ profiling, such as the WETLabs ac-9. In order to examine the various sources of error in the method, we test its performance considering various levels of a priori knowledge of phytoplankton absorption properties over the study area: for each site, over each region, or over a global dataset only. When the method is applied to the multi-spectral case without introducing any ``local'' information on phytoplankton absorption properties, we obtain a good performance with a relative Root Mean Square Error equal to 17.8%, 14.6%, and 40.7% for a(CDM)(412), the CDM exponential slope, and a(phi)(440), respectively. Finally, the partitioning method is directly applied to in situ profiles of total minus water spectral absorption coefficient measured with an ac-9 in various oceanic Mediterranean waters, allowing the in situ description of CDM and phytoplankton absorption coefficients with a high spatial resolution.