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Optical properties of chromophoric dissolved organic matter during a phytoplankton bloom. Implication for DOC estimates from CDOM absorption

Coastal waters of the English Channel and the North Sea are affected by a massive spring phytoplankton bloom event of the prymnesiophyte Phaeocystis globosa every year. The impact of this bloom on DOM dynamics in coastal waters of the western English Channel was investigated through a 45 d long mesocosm experiment and field survey (2012–2014). Our results highlight that, over short time scales, marine production of DOM mostly affects CDOM absorption properties in the UV domain. Specifically, it has been evidenced that the acdom spectral slope between 320 nm and 412 nm (S320‐412) can be used as a relevant indicator of the DOM marine origin. The presence of a strong linear relationship between S320‐412 and the DOC specific absorption coefficient in phytoplankton bloom conditions reveals that this optical parameter provides complementary information to those obtained from the spectral slope between 275 nm and 295 nm, S275‐295 which has been recently described as a relevant tracer of the terrestrial DOM and a good descriptor of the specific absorption coefficient of dissolved organic carbon variability in the confining river‐influenced ocean margins. This study has further demonstrated that the joint use of S275‐295 and S320‐412 significantly improves the estimation DOC‐specific absorption coefficient in the presence of varying contribution of marine and terrestrial DOM. While such bands are still not available on ocean color sensors, this methodology represents a valuable tool for estimating DOC concentration from CDOM absorption measurements from in situ data obtained from classical discrete measurements or from autonomous platforms equipped with adapted spectrophotometer.