The LEVECO project bio-optics experiment in the northwestern Levantine Sea: preliminary results

Abstract

The main aim of the bio-optics experiment was to collect optical data in the northwestern Levantine Sea, south of Crete Island, to be used in climatology analysis and remote sensing products. All measurements and sampling were conducted on board the R/V Aegaeo during LEVECO cruise from 9 to 18 April 2016 within a grid of 9 stations. The experiment included the measurement of apparent and inherent optical properties of sea water by several optical sensors deployed in and above water (TriOS radiometers, ac-s absorption-scattering meter, LISST particle size distribution profiler, Eco-bb backscattering meter, fluorometers, JAZ spectroradiometer). Penetration depth estimations were on the average around 30 m while a well-developed deep chlorophyll maximum was recorded at 70-90 m depth. The horizontal distribution of chlorophyll-a, shows that the eastern part of the study area is affected by the cyclone of Rhodes resulting in slightly elevated values (typically 0.07 vs 0.04 mg m-3). The particulate matter field was very weak throughout the water column (range: 19-255 μg/l), underpinning the oligotrophic nature of the area and the overall scarcity of suspended particles. Particle size was rather high (D50<70 μm) suggesting the predominance of micro-aggregates. All reflectance band ratio derived chlorophyll-a concentrations were compared against the HPLC derived ones. Between global OC4v6 and MedOC4.2018 versions, the latter performed better predicting values very close to field observations. On the contrary, the locally tuned algorithm for Kd retrieval, gave values much lower than field observations.