Transmissometer

Abstract: An analysis is presented based on a large dataset (N = 2,787) made up of recent measurements of the beam attenuation coefficient at 660 nm and of the chlorophyll concentration by using the SeaTech transmissometer and the high-pressure liquid chromatography technique, respectively. This analysis, restricted to case 1 waters, aims at reassessing a previous nonlinear relationship established between the particle scattering coefficient, b,, (very close to the particle attenuation coefficient, c,,), and the chlorophyll concentration, [Chl]. As a first result, nonlinearity is fully confirmed over the whole range of oceanic chlorophyll concentration (about 3 orders of magnitude). Despite more accurate measurements, the scatter in this relationship remains large and is actually comparable to that observed within the old dataset. Rather than establishing a single relationship between c,, (or b,,) and [Chl] for the entire upper water column, the deep layer and the near-surface layer (important for remote-sensing application) have been studied separately. This separation has led to two distinct expressions. A more appropriate parameterization is thus proposed when dealing specifically with, and modeling, the near-surface layer. As a consequence, a modified criterion is also suggested with a view to identifying turbid case 2 waters. Understanding or predicting the propagation of radiant energy within a water body requires that the boundary conditions (at the interface and bottom) and the inherent optical properties (IOP) within the medium are both known or prescribed. Strictly speaking, these properties (IOP) within the medium are both known or prescribed. Strictly speaking, these properties comprise the absorption coefficient, a, and the volume scattering function p(0); the scattering coefficient, b, derives from p(0) by integrating over the whole space, and the attenuation coefficient, c, represents the sum of a and b. These last three coefficients are expressed as m--l. The IOP result from the presence in a water body of colored dissolved organic substances and of scattering as well as absorbing particulate matter. In the open ocean, far from notable terrigenous influence, these optically active materials are locally and permanently Acknowledgments OMEX data were made available with the originators’ permission from the EU-MAST OMEX 1 Programme by the British Oceanographic Data center (BODC). The HPLC chlorophyll measurements for this project were undertaken by R. Barlow and R. E C. Mantoura at the Plymouth Laboratory. They are gratefully acknowledged for having made their data available to us. The transmissometer data for the same cruise were calibrated and quality controlled by the BODC. We thank R. Lowry for his help in the control of, and access to, these data. The BOFS data, published on a CD-ROM, were also produced under the BODC, and are duly appreciated here. The HPLC data for the EUMELI and OLIPAC (French JGOFS) cruises and those (unpubl.) for the MlNOS campaign were performed by H. Claustre with the collaboration of J. C. Marty, C. Cailliau, and E Vidussi. We thank them for these data. We also thank H. Claustre for helpful suggestions and discussions on a first draft of this paper: Substantial help with fieldwork and maintenance-calibration of the transmissometers used during the French cruises were provided by D. Tailliez, who is gratefully thanked. We acknowledge and thank our colleagues who worked at the two U.S. JGOFS-WOCE timeseries and during the EqPac cruises or were involved in the related databanks-these colleagues rendered this study feasible through their continuous effort. This work is a contribution to the French JGOFS Programme “Prosope.”