Fig. 3: Significant methane formation rates from 13C-methylphosphonate incubation experiments.
a Schematic drawing of a typical water column profile at the study site, showing the chlorophyll a (Chl a), as well as nitrate and phosphate (Pi) regimes. b Rate measurements of methane (13C-CH4) formation from the four incubation depths: 10-metre surface depth, intermediate depth, deep chlorophyll maximum (DCM) and below the DCM and the three incubation experiments: methylphosphonate-addition (MPn; white), methylphosphonate + phosphate-addition (MPn + Pi; blue) and methylphosphonate + nitrate-addition (MPn + nitrate; orange) from both duplicates and all twelve stations. Boxplots illustrate the 25–75% quantile range, encompassing the median rates with whiskers extending to data points within the 1.5 times the interquartile range. Each data point represents a statistically significant (One-sided t-test, df = 2, p < 0.05, R2 > 0.81) rate calculated from the linear regression of the first four time points (24 hours), with both insignificant and exponential rates reported as zero. Filled points represent the northern stations in the low surface Chl a waters and open points the southern stations in the high surface Chl a waters. Statistical difference (Pairwise Wilcoxon test, p < 0.05) between the MPn-addition and the others is highlighted (***p < 0.01, N.S. = not significant). From the intermediate depths, the two highest rates of methane formation are not shown in the plot, namely 4.8 and 9.6 nmol CH4 L−1 d−1 from S3 and S4 of the MPn-addition experiment respectively, as well as the 9.4 nmol CH4 L−1 d−1 from the MPn + nitrate-addition experiment from S12. Source data are provided as a source data file.
