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Source characterization of dissolved organic matter in the eastern Beagle Channel from a spring situation

Dissolved organic matter (DOM) was analyzed by its optical and fluorescent properties in order to study its distribution, source and transformation in relation to microbial abundances, chlorophyll and physicochemical gradients along a transect in the eastern Beagle Channel (BC) during austral spring 2019. Moreover, DOM evolution was followed during three tidal cycles in two fixed stations (F1, F2) separated by the Mackinlay Strait, a hydrographic frontier between water masses under stronger continental influence (inner sector) and that of less modified Sub-Antarctic waters (outer sector). The fluorescence signals of protein- and humic-like compounds were used as a proxy for labile and non-labile material, respectively, while the concentration of nano-, pico- and virioplankton populations, chlorophyll-a (Chl-a) and dissolved organic carbon (DOC) were used as a proxy for biological activity. Moreover, several spectroscopic indices, such as the indicator of microbially produced DOM (BIX), the Freshness index (FIX), an indicator of molecular weight, the Slope Ratio (SR) and an indicator of chromophoric DOM (CDOM), were used to trace the quality and origin of DOM. Chl-a concentration was generally low in the sampling area (< 1 μg L−1) but increased towards the end of the campaign and especially during the visit of F1 along with the concentration of picoeukaryotes and the protein-like fluorophores evidencing the onset of a bloom event. Protein-like fluorophores peaked in the euphotic zone of the inner sector of the BC while humic-like material of terrestrial origin was homogeneously distributed throughout the water column and showed a decreasing gradient towards the outer sector and Atlantic waters. Furthermore, the inner sector was characterized by higher amounts of CDOM, high molecular weight material and more recalcitrant DOM. Accumulation of autochthonous humic-like material in bottom waters of the inner sector along with enhanced values of FIX and BIX preceded by colored organic material suggest an efficient functioning of the microbial carbon pump fueled by terrestrial DOM sources in these waters.

Validation of standard ocean-color chlorophyll-a products in turbid coastal waters: A case study on statistical evaluation and quality control tests in the Persian Gulf

Selecting the appropriate in-situ and satellite matchups is a critical task for evaluating and application of ocean color products in optically complex coastal waters. This article investigates the performance of five single-sensor and two merged multi-sensors Chl-a products in the Persian Gulf. In this study, bio-optical and Chl-a measurements collected from 531 stations in the northern Persian Gulf from 2008 to 2019 were used. In-situ samples were initially controlled to filter out inappropriate datasets. Results indicated that surface measurements may not be representative of satellite-derived Chl-a, and satellite estimations were mostly consistent with Chl-a concentrations in the first optical depth (Chlopt). Statistical analysis showed that all satellite-derived Chl-a products overestimated the Chlopt by 48%–170%, in which Chl-a from Neural-Network and OC5 algorithms yielded the best agreement. Afterwards, the QC tests were designed based on the remote sensing reflectance (Rrs) at 555 nm as a proxy of SPM, Rrs(412)/Rrs(443) as a proxy of CDOM, and Rrs(560)/Rrs(490) as a proxy of Chl-a, to select the matchups with the least interference of Colored Dissolved Organic Matter (CDOM) and Suspended Particulate Matter (SPM) on Chl-a. The correlation between Chlopt and satellite-derived Chl-a improved significantly after applying the QC tests (R2 = 0.80–0.89). The valid pixels were determined using matching the results of QC tests and standard level-2 quality flags. Finally, statistical calibration was used to create the final quality controlled calibrated Chl-a maps, using regression coefficients obtained from training/validation exercises and bootstrapping-like techniques applied to the quality-controlled datasets. The findings showed that in turbid coastal waters, standard ocean color Chl-a products cannot be validated only by performing statistical methods, and quality control experiments are necessary for their applications.