Nutrient cycling is a key parameter that determines ecosystem function. The McMurdo Dry Valleys [MDV], Antarctica, contain streams with relatively simple ecology and hydrology. These streams flow 5-12 weeks/year during the austral summer and are fed only by glacial melt. Most remain first order streams from their glacial source to their delta. Average annual precipitation is less than 100mm/year, all falling as snow. Most of this precipitation sublimates within weeks. There are no vascular plants, no lateral inputs of water or organic matter to streams, and no deep groundwater interaction. MDV streams do not contain vertebrates or insects. They do support thriving colonies of benthic microbial mats and hyporheic microbial communities. These microbial communities drive stream ecosystem productivity and function.
Examining nutrient dynamics in this simplified system may reveal basic patterns of ecological function that are applicable across ecosystems. Other studies have established the presence of two opposite nutrient gradients for N and P in sediments, glaciers, lakes and streams along the Taylor Valley, MDV. At the coast, P is high and N is low; moving inland, N increases and P decreases across terrestrial and aquatic systems.
This study is the first to examine nutrient uptake dynamics of NO3, NH4 and PO4 across these gradients, and also the first to examine uptake along the entire Taylor Valley. We conducted TASCC-style nutrient addition experiments in seven streams, chosen for their spread along the nutrient gradients. On each stream, we performed three sequential addition experiments, assessing whole-stream uptake of NO3-N, PO4-P, and NH4-N. We also analyzed the NH4-N addition samples for NO3-N, an indicator of nitrification in the mat or hyporheic microbial communities. While it has been assumed to exist, stream nitrification has not previously been studied in MDV streams.
Results/Conclusions
Preliminary results indicate that both mat and hyporheic communities contribute to nutrient uptake for N and P. Nitrification does occur in measurable amounts, and may account for up to 14% of the N uptake/transformation from NH4-N addition experiments. We expected to see variation in uptake capacity along the nutrient gradients, with the less-available nutrient at any location experiencing more rapid uptake and higher total uptake. However, preliminary results do not show any impact of location or nutrient gradient on nutrient uptake dynamics. By examining uptake dynamics across the valley and examining nitrification, this study has expanded our understanding of nutrient uptake dynamics in the MDV.