Nutrient cycling in glacial meltwater streams of the McMurdo Dry Valleys Antarctica is strongly dependent on stream-sediment interactions

Background/Question/Methods

Glacial meltwater streams in the McMurdo Dry Valleys flow for 6-12 weeks each austral summer.  Streambeds are occupied by extensive microbial mats where channel stability allows, and are thawed to less than 1 m deep. This thawed sediment facilitates extensive hyporheic exchange to facilitate interactions between dissolved stream nutrients and hyporheic microbial communities. Using nutrient injection techniques and monitoring stream nutrient, discharge, and electrical conductivity changes seasonally we have sought to characterize the importance of hyporheic exchange on stream nutrient cycling.

Results/Conclusions

Results of stream nutrient additions indicate that hyporheic exchange is extensive and uptake of nutrients occurs by communities both in the channel and in the hyporheic zone, but that assimilation by microbial mats, not hyporheic denitrification, was responsible for a majority of nitrate removal. Nutrient cycling in the hyporheic zones of Dry Valley Streams is affected by physical and chemical characteristics that are spatiotemporally variable. For example, while denitrification potential was not found to be carbon limited, overall levels of microbial activity and nitrate loss are dependent on DOC levels in some streams. In addition, results from monitoring nutrient, water, and solute fluxes in streams over weeks indicate that hyporheic nitrification was positively correlated with exchange, which is highest during low flow periods. Thus, exchange of water and nutrients between the channel and hyporheic zone significantly influences stream nutrient fluxes.