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Regional and intra‐annual stability of dissolved organic matter composition and biolability in high‐latitude Alaskan rivers

Nutrient availability and molecular composition of dissolved organic matter (DOM) determine whether the carbon (C) associated with DOM is respired during decomposition or exported from stream networks. High‐latitude ecosystems are changing rapidly due to permafrost thaw, shifts in vegetation, and increasing runoff, which alter sources of DOM and nutrients to rivers. To address how changes in the composition of riverine DOM and nutrients may influence C cycling, we quantified biolability of DOM across a latitudinal gradient in Alaska and assessed temporal patterns associated with intra‐annual variation in hydrology. We measured biolability under ambient and fertilized conditions and characterized optical and chemical properties of DOM to determine the relative influence of nutrients and molecular composition of DOM on its fate in rivers. Biolability was low, comprising 4% of the DOM pool on average and 0–42% across all sites and seasons. Optical indicators of the molecular composition of the DOM pool were more constant in space and time than bulk concentrations of dissolved C and nutrients, and described a DOM pool dominated by humic‐like functional groups. Phosphorus limited biolability at freshet, but biolability was not linked to DOM composition. Strong hydrologic connection of high‐latitude rivers to a large pool of soil organic matter likely results in export of recalcitrant DOM to receiving rivers and coasts in this region, contributing to stability of the high‐latitude C cycle. However, increased inputs of phosphorus or DOM of greater biolability will likely cause increased respiration of riverine DOM and flux of CO2 to the atmosphere.