Seasonal Wetlands Make a Relatively Limited Contribution to the Dissolved Carbon Pool of a Lowland Headwater Tropical Stream

Wetlands process large amounts of carbon (C) that can be exported laterally to streams and rivers. However, our understanding of wetland inputs to streams remains unclear, particularly in tropical systems. Here we estimated the contribution of seasonal wetlands to the C pool of a lowland headwater stream in the Australian tropics. We measured dissolved organic and inorganic C (DOC and DIC) and dissolved gases (carbon dioxide-CO2, methane-CH4) during the wet season along the mainstem and in wetland drains connected to the stream. We also recorded hourly measurements of dissolved CO2 along a 'stream-wetland drain-stream' continuum, and used a hydrological model combined with a simple mass balance approach to assess the water, DIC and DOC sources to the stream. Seasonal wetlands contributed similar to 15% and similar to 16% of the DOC and DIC loads during our synoptic sampling, slightly higher than the percent area (similar to 9%) they occupy in the catchment. The riparian forest (75% of the DOC load) and groundwater inflows (58% of the DIC load) were identified as the main sources of stream DOC and DIC. Seasonal wetlands also contributed marginally to stream CO2 and CH4. Importantly, the rates of stream CO2 emission (1.86 g C s-1) and DOC mineralization (0.33 g C s-1) were much lower than the downstream export of DIC (6.39 g C s-1) and DOC (2.66 g g C s-1). This work highlights the need for further research on the role of riparian corridors as producers and conduits of terrestrial C to tropical streams. Streams and rivers play a vital role in carrying carbon to oceans. This carbon can originate from biological processes in the water or from external sources like rocks, forest and wetland soils. The proportion of carbon from each source depends on factors such as the local geology, climate, and landscape. In this study, we measured how much of the carbon transported by an Australian tropical stream was sourced from the wetlands in the catchment. We found that seasonal wetlands contributed similar to 15% of the carbon measured in the stream. We conclude that the main sources of carbon to the stream were the riparian forest, and rock-derived carbon carried by groundwater. Synoptic sampling of DIC, DOC and dissolved C gases along a tropical stream and connected wetlands Seasonal wetlands contributed similar to 15% and 16% of the stream DOC and DIC loads The riparian forest and groundwater inflows were likely the main sources of DOC and DIC to the stream, respectively