a, CH4(aq) time series. Red points correspond to the HydroC pump power during operation. The continuous line depicts HydroC measurements, with the dashed section corresponding to times when the sensor experienced low pump power and thus a reduced water flow induced by the pump (19 June to 1 July). Open circles correspond to manual samples. The thin grey-shaded area around the CH4(aq) time series corresponds to the uncertainty of the HydroC measurements (about 3%). The uncertainty on manual measurements, indicated by the error bars, reflects the error on vial internal pressures and volumes (119 ± 0.76 ml, where the uncertainty represents standard deviation; internal pressures are derived from volumes—see Methods for details). b, Regression plot between the HydroC and manual sample measurements. Only manual samples taken during times when the HydroC pump power was above about 7 W were considered for the regression (black circles, black line); grey circles correspond to samples taken during times of lower pump power. Horizontal error bars reflect errors on manual measurements; vertical error bars are smaller than the size of the markers. The orange dashed line depicts a hypothetical 1:1 relationship between the sensor and manual measurements.