Seasonal changes in light availability modify the temperature dependence of ecosystem metabolism in an arctic stream

Light and temperature are key ecosystem drivers, but their synchronous annual cycles typically confound partitioning of their relative influence. Arctic spring-streams, subject to extreme annual fluctuations in light but stable water temperatures, provide a rare contrast that allows the parsing of their independent effects. Over 30 months, we assessed the effects of light and temperature on ecosystem metabolism and nutrient uptake in Ivishak Spring, Alaska, USA. (latitude 69 degrees N, water temperature range approximate to 4 degrees-7 degrees C) using open-channel methods and short-term NH4+-N, NO3--N, and P additions, respectively. We predicted that rates of ecosystem respiration (ER) would mirror seasonal patterns of gross primary production (GPP), rather than temperature, due to relatively constant rates of metabolic demand year-round, resulting in carbon limitation during winter (October-March) when photosynthesis effectively ceases. Because patterns of nutrient uptake and GPP are often coupled due to assimilatory demand, we also predicted that extreme annual cycles of light would result in equally extreme cycles of nutrient uptake, with demand being relaxed during winter. In accordance with our prediction, we found that ER scaled linearly with GPP. Peak summer rates of GPP (>4.0 g Cm(-2)d(-1)) and ER (>5.0 g Cm(-2)d(-1)) were surprisingly high, being comparable to those of productive streams at temperate latitudes. Winter rates (GPP approximate to 0.0, ER <1.0 g Cm(-2)d(-1)) were low, however, and Arrhenius plots showed clear deviations from theoretical temperature dependence of GPP and ER during winter when other factors assumed primacy. For GPP, this factor was undoubtedly light availability, but for ER, carbon limitation is implicated due to low GPP. Significant nutrient uptake occurred only for NH4+-N, indicating N limitation, and rates of uptake were also synchronous with cycles of light availability. Consequently, light, rather than temperature, was the major driver of annual patterns of ER and nutrient cycles in this arctic ecosystem. Synchronous light and temperature cycles are pervasive among ecosystems. The winter onset and severity of energy limitation we document highlights the importance of this synchrony and how the confounding of light and temperature obscures details of mechanisms by which these fundamental drivers affect ecosystem processes.