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

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.

[1]  Michael J. Behrenfeld,et al.  Beam attenuation and chlorophyll concentration as alternative optical indices of phytoplankton biomass , 2006 .

[2]  P. Falkowski,et al.  The fate of photons absorbed by phytoplankton in the global ocean , 2016, Science.

[3]  D. Mills,et al.  Primary production in the deep chlorophyll maximum of the central North Sea , 2005 .

[4]  G. Zibordi,et al.  Ocean Colour Remote Sensing of the Optically Complex European Seas , 2008 .

[5]  D. Antoine,et al.  Algal biomass and sea surface temperature in the Mediterranean basin intercomparison of data from various satellite sensors, and implications for primary production estimates , 2002 .

[6]  G. Dall’Olmo,et al.  Characterization of the bio-optical anomaly and diurnal variability of particulate matter, as seen from scattering and backscattering coefficients, in ultra-oligotrophic eddies of the Mediterranean Sea , 2011 .

[7]  Jianwei Wei,et al.  Resolving the long‐standing puzzles about the observed Secchi depth relationships , 2018, Limnology and Oceanography.

[8]  A. Karageorgis,et al.  Particle characterization and composition in the NE Aegean Sea: Combining optical methods and biogeochemical parameters , 2017 .

[9]  Weilin Hou,et al.  Secchi disk depth: A new theory and mechanistic model for underwater visibility , 2015 .

[10]  M. Krom,et al.  Satellite-derived spatial and temporal biological variability in the Cyprus Eddy , 2005 .

[11]  Robert J. W. Brewin,et al.  The open-ocean missing backscattering is in the structural complexity of particles , 2018, Nature Communications.

[12]  ZhongPing Lee,et al.  On the non-closure of particle backscattering coefficient in oligotrophic oceans. , 2014, Optics express.

[13]  T. Kanellopoulos,et al.  Spatial and seasonal variability of particulate matter optical and size properties in the Eastern Mediterranean Sea , 2012 .

[14]  Martin Gade,et al.  Remote Sensing of the European Seas , 2008 .

[15]  Michael Lee,et al.  Evaluation of SeaWiFS chlorophyll‐a in the Black and Mediterranean Seas , 2005 .

[16]  C. Mobley,et al.  Removal of surface-reflected light for the measurement of remote-sensing reflectance from an above-surface platform. , 2010, Optics express.

[17]  Rosalia Santoleri,et al.  The Mediterranean Ocean Colour Level 3 Operational Multi-Sensor Processing , 2018 .

[18]  Vassilis Zervakis,et al.  Estimating chlorophyll concentrations in the optically complex waters of the North Aegean Sea from field and satellite ocean colour measurements , 2015, International Conference on Remote Sensing and Geoinformation of Environment.