Using a litter decay experiment, we tracked monthly changes in detrital processing rate as part of a broader study investigating seasonal patterns in energy flow and nutrient cycling in a perennial Arctic spring-fed stream. Ivishak Spring (North Slope of Alaska, USA) exhibits stable discharge (~80 L sec^-1) and temperature (4-7 °C) throughout the year but experiences dramatic fluctuations in daylength between summer and winter solstices. Variation in light availability drives large seasonal shifts in gross primary production (9.6 to 0.1 g C·m^-2·d^-1), in turn altering ambient NH₄⁺ and NO₃^- concentrations in this N-limited system. Winter increases in nutrient availability, along with higher biomass of the dominant invertebrate shredder (Zapada sp.), combine to generate an order of magnitude difference in the decay rate of Salix alaxensis litter placed in coarse-mesh bags over successive ~30 d periods (from 0.5 %·d^-1 in May to 5 %·d^-1 in October).  Rapid processing of detritus during winter months, combined with the collapse of primary production, may drive the apparent mid- to late-winter energy limitation of the food web at this site, illustrated by low winter growth rates and body condition of the stream’s top predator, Dolly Varden char.