Estimating metabolism and nutrient assimilation in high latitude (Arctic and Antarctic) streams.

Abstract:

Despite relatively short summer seasons, high-latitude streams have the potential to be highly productive due to periods of near continual solar insolation. There is also typically limited riparian shading, though topographical shading and stream azimuth may be more of a factor because of the lower zenith angle of the sun above the horizon. Estimating metabolism from dissolved oxygen time-series in these streams can prove challenging, because data is often only collected in the summer when there may be no well-defined nighttime period from which to infer rates of ecosystem respiration. This uncertainty in respiration is then carried over into estimates of gross primary production. The same uncertainties in photoperiod may also invalidate basic assumptions inherent to using diurnal variation in high-frequency nitrate time-series to estimate autotrophic nitrogen assimilation. Here, we present data from several Arctic (Alaska and Greenland) and Antarctic (McMurdo Dry Valley) streams. We investigate several approaches, both experimental and modeling, for addressing these problems. Coupling metabolism with nutrient assimilation doubles our modes of inference, as we expect the timing of these process to be roughly synchronous, and with a relative magnitude reflected in the C:N ratios of the autotrophic communities. Accurately measuring these processes in critical to understanding the role of streams and rivers within the context of larger regional biogeochemical cycles, and as a baseline for future assessment of the effects of a rapidly changing arctic.