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

We measured total surfactant activity (SA; Triton T-X-100 equivalent) and CDOM absorbance (cm-1) in the surface 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 custom designed air-sea gas exchange tank. Spatial SA variability exceeded its temporal variability. Overall, SA varied five-fold between all samples (0.08 - 0.38 mg L-1 T-X-100), being highest in the SML during summer. SML SA enrichment factors relative to SSW were ~ 1.0 - 1.9, except for two values (0.75; 0.89: February 2013). CDOM absorbance (250 - 450 nm), the CDOM spectral slope ratio (SR = S275–295 / S350–400) and the 250:365 nm CDOM absorbance 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 °C) 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) strongly correlates 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 varying surfactant in these coastal waters. Such variability should be taken account of when evaluating marine trace gas sources and sinks.