New insight into and characterization of DOC and DON for inland waterbodies in the Songnen Plain, Northeast China

Dissolved organic matter (DOM), usually measured as concentrations of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) in inland waterbodies, plays a vital role in the global carbon and nitrogen cycles. The spatial patterns of DOC and DON concentrations, DOC:DON weight ratios and their correlations with water physicochemical variables for 19 waterbodies in the Songnen Plain of Northeast China were first examined with data collected in May 2021. These 19 waterbodies can be further divided into two groups: fresh and brackish waterbodies according to electrical conductivity (EC) (EC threshold = 1200 mu S cm(-1)) and mesotrophic and eutrophic waterbodies according to trophic state index (TSI) (TSI threshold = 50). Results showed that significant differences in average DOC and DON concentrations were observed between fresh and brackish waterbodies as well as between different trophic states (t-test, p < 0.01), respectively. A significant positive linear relationship between DOC and DON concentrations was found (R-2 = 0.791, p < 0.01). DOC was moderately related to both EC (R-2 = 0.617, p < 0.01) and TSI (R-2 = 0.629, p < 0.01). DON showed a strong significant positive linear correlation with both EC (R-2 = 0.734, p < 0.01) and TSI (R-2 = 0.757, p < 0.01). These results suggested that DOC and DON were impacted by various parameters associated with TSI and most importantly EC could be used to estimate dissolved salts in DOC and DON through in-situ or remote sensing methods on large regional scales for these brackish waterbodies in the semiarid Songnen Plain of Northeast China.