Nutrient Uptake Dynamics in Antarctic Polar Desert Streams

Stream nutrient uptake rates are indicators of biotic activity and nutrient bioavailability in both the stream channel and hyporheic zone. The intermittent streams of the Taylor Valley [TV], Antarctica have relatively simple ecology and hydrology: they are bounded by permafrost and fed only by glacial melt. There are no vascular plants or higher animals. Thriving colonies of benthic microbial mats and hyporheic microbial communities drive stream ecosystem productivity and function. Examining nutrient uptake dynamics in this simplified system may reveal basic patterns of ecological function that are applicable across ecosystems. Background N and P levels vary along opposing spatial gradients across the TV. At the coast, P is high and N is low; moving inland, N increases and P decreases across terrestrial and aquatic systems. While we know that these streams are capable of rapid nutrient uptake and transformation, no previous studies have examined nutrient uptake dynamics at the valley scale or across these background gradients. We conducted NO3-N, PO4-P, and NH4-N pulse addition experiments in six streams across TV and analyzed the results with both breakthrough curve and TASCC-style analyses. We expected uptake of each nutrient to vary inversely with the gradient, i.e. for uptake rates to be higher where nutrient concentrations were lower. Results indicate there are strong uptake patterns that do not follow these gradients. Instead, we see three general uptake typologies: increasing uptake in the tail of the injection curve, which may indicate stronger uptake in the hyporheic zone than in the channel; steadily high uptake, which may indicate rapid nutrient processing in both the channel and hyporheic zone; and decreasing uptake across the injection curve, which may indicate either stronger channel uptake or saturation. These typologies do not follow a consistent pattern based on geographic location or nutrient type. In addition, two streams show no apparent uptake for NO3-N but do show uptake for PO4-P and NH4-N, which may indicate either co-limitation or a preference for the more readily bioavailable NH4-N. These findings shed new light on the spatiotemporal dynamics of nutrient demand across these polar desert streams.