Extreme seasonality of litter breakdown in an arctic spring-fed stream is driven by shredder phenology, not temperature

Abstract

Summary 1. Using degree-days to calculate ‘temperature-corrected’ breakdown rates is a useful approach for comparing litter breakdown across sites with different thermal regimes. We used an alternative approach to investigate the importance of temperature by quantifying seasonal patterns in litter breakdown in an arctic spring-fed stream (Ivishak Spring, North Slope, Alaska) that experiences extreme seasonality in light availability and energy inputs while fluctuations in water temperature are relatively small. 2. We incubated mesh bags of senesced Salix alaxensis litter in Ivishak Spring for successive c. 30-day periods for 2 years. During our study, water temperature was very stable [5.7 ± 0.03 °C (daily mean ± 1 SE), range 3.7–7.8 °C]. Discharge was only slightly more variable (mean 112 ± 1 L s−1, range 66–206 L s−1), with lowest values occurring in late winter. Dissolved nutrient concentrations were low (52–133 μg L−1, <1–3 μg L−1, <1–6 μg L−1 soluble reactive phosphorus) and also showed evidence of seasonality (i.e. highest values in winter). 3. Litter breakdown rates were sharply seasonal, ranging from 0.05 day−1 in mid-winter. Invertebrate assemblage structure in litter bags showed pronounced seasonal cyclicity; total invertebrate biomass peaked in summer. Biomass of two dominant shredders (the nemourid stonefly Zapada haysi and the limnephilid caddisfly Ecclisomyia conspersa) showed the opposite trend, however, with mid-winter peaks in both population biomass and cohort growth rates that closely matched those we observed in litter mass loss. 4. Water temperature appeared to have negligible influence on litter breakdown rates in our study. Seasonal shifts in nutrient uptake may have increased rates of microbial activity in winter. The processing of litter inputs in Ivishak Spring, however, appeared to be most tightly coupled to shredder phenology. Our results demonstrate that extreme seasonality in the processing of allochthonous detritus can occur even in the absence of substantial temperature variation, driven by the activity of shredder taxa that have evolved to take advantage of pulsed organic matter inputs.