Pereira R

Schneider-Zapp K

Upstill-Goddard RC

We measured total surfactant activity (SA; Triton T-X-100 equivalent) and CDOM absorbance (cm-1) in thesurface microlayer (SML) and subsurface water (SSW) seasonally (2012-2013) along a 20 km coastal transect(North East UK), and evaluated corresponding values of the gas transfer velocity (kw; cm hr-1) using a customdesigned air-sea gas exchange tank. Spatial SA variability exceeded its temporal variability. Overall, SA variedfive-fold between all samples (0.08 - 0.38 mg L-1 T-X-100), being highest in the SML during summer. SML SAenrichment factors relative to SSW were ~ 1.0 - 1.9, except for two values (0.75; 0.89: February 2013). CDOMabsorbance (250 - 450 nm), the CDOM spectral slope ratio (SR = S275–295 / S350–400) and the 250:365 nm CDOMabsorbance ratio (E2 : E3) demonstrate the potential for terrestrially-derived CDOM to be biogeochemically processed in North Sea coastal waters. The range in corresponding k660 (kw for CO2; freshwater; 20 oC) was 6.8 -22.0 cm hr-1. The film factor R660 (the ratio of the scaled transfer velocity k660 to the one of clean water) stronglycorrelates with SA (r ≥0.70, p ≤0.002, each n = 16) with high SML SA correlated to k660 suppression ~14 - 51 %relative to clean laboratory water, highlighting strong spatio-temporal gradients in gas exchange due to varyingsurfactant in these coastal waters. Such variability should be taken account of when evaluating marine trace gassources and sinks

Surfactant control of gas transfer velocity along an offshore coastal transect: results from a laboratory gas exchange tank

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