Spatial and Temporal Patterns of Dissolved Carbon in a North Pacific Coastal Temperate Rainforest Stream

The coastal temperate rainforests of Southeast Alaska are terrestrial-aquatic interfaces that mediate the transfer of vast quantities of water and materials from land to sea, largely through thousands of streams draining small, steeply sloping watersheds. Stream export processes are an important part of the coastal temperate rainforest carbon cycle, including supporting carbon sequestration through enhanced nearshore productivity and sedimentation, and may play a role in regulating future carbon-climate feedbacks. Yet, the ecohydrological mechanisms that regulate stream export of carbon, and would mediate any future climate feedback, are less well understood at the sub-watershed spatial scale and at sub-seasonal temporal scales. In this study, we quantified continuous, high-frequency dissolved organic carbon flux and methane concentrations in Peterson Creek, near Juneau, AK, during the peak growing season (June-August 2023). We explored how three antecedent meteorological states (dry, drought, rainy) affected three regionally representative hydropedological units (upland forest, forested wetland, and poor fen) to contribute to the Peterson Creek carbon export dynamics, using continuous water table depth and discharge, and discrete dissolved organic carbon and methane measurements in the soils and connecting tributaries of each unit. We found that methane was typically a very small component (<1%) of total dissolved organic carbon, but that exceptionally high concentrations could occur under a drought-to-rain sequence and that the source appears to be the poor fen. Our study therefore builds on previous work demonstrating the importance of lateral flushing of carbon out of coastal temperate rainforests during rain events by showing that this same general mechanism is applicable to methane during rarer drought-to-rain sequences.