Search

In low‐nutrient streams in cold and arid ecosystems, the spiraling of autochthonous particulate organic matter (POM) may provide important nutrient subsidies downstream. Because of its lability and the spatial heterogeneity of processing in hyporheic sediments, the downstream transport and fate of autochthonous POM can be difficult to trace. In Antarctic McMurdo Dry Valley (MDV) streams, any POM retained in the hyporheic zone is expected to be derived from surface microbial mats that contain diatoms with long‐lasting silica frustules. We tested whether diatom frustules can be used to trace the retention of autochthonous POM in the hyporheic zone and whether certain geomorphic locations promote this process. The accumulation of diatom frustules in hyporheic sediments, measured as biogenic silica, was correlated with loss‐on‐ignition organic matter and sorbed ammonium, suggesting that diatoms can be used to identify locations where POM has been retained and processed over long timescales, regardless of whether the POM remains intact. In addition, by modeling the upstream sources of hyporheic diatom assemblages, we found that POM was predominantly derived from N‐fixing microbial mats of the genus Nostoc. In terms of spatial variability, we conclude that the hyporheic sediments adjacent to the stream channel that are regularly inundated by daily flood pulses are where the most POM has been retained over long timescales. Autochthonous POM is retained in hyporheic zones of low‐nutrient streams beyond the MDVs, and we suggest that biogenic silica and diatom composition can be used to identify locations where this transfer is most prevalent. Streams play an important role in the global carbon cycle by processing organic matter, some of which comes from plants and microbes living within the stream itself. A fraction of this organic matter is rapidly processed (broken down) in subsurface sediments adjacent to and below the stream channel, which makes it a difficult process to observe directly. In this study, we considered if diatoms, single‐celled algae with silica cell walls that are often found within larger algal mats, can indicate the processing of organic matter in subsurface sediments of a stream in the McMurdo Dry Valleys (MDVs), Antarctica. We show that diatoms can be used to identify locations where organic matter is processed in subsurface sediments, and to identify the source of the organic matter. In the MDVs, the processing of organic matter from within the stream itself provides important nutrients for downstream organisms. This recycling of organic matter within the stream likely occurs in other ecosystems, and our results suggest that diatoms may be a powerful tool to track this process. Diatoms from stream microbial mats were found in hyporheic zone, evidence of autochthonous organic matter transfer from channel to bedBiogenic silica correlated with loss‐on‐ignition, suggesting that diatoms indicate locations that promote organic matter retentionNostocmats were the main source of hyporheic organic matter, confirming the role of atmospherically fixed N in stream nitrogen budgets Diatoms from stream microbial mats were found in hyporheic zone, evidence of autochthonous organic matter transfer from channel to bed Biogenic silica correlated with loss‐on‐ignition, suggesting that diatoms indicate locations that promote organic matter retention Nostocmats were the main source of hyporheic organic matter, confirming the role of atmospherically fixed N in stream nitrogen budgets

ABSTRACTAeolian processes are important drivers of geomorphic change in cold regions. Because these processes often occur at slow timescales over large areas, it can be difficult to quantify rates using traditional field methods. In the Kangerlussuaq region of Greenland, strong katabatic winds have shaped distinct erosional landforms, or deflation patches, that appear to expand across the landscape. The modern erosion rate along the active margins, or scarps, of these deflation patches is unknown. We use Structure-from-Motion (SfM) photogrammetry to quantify the geomorphic change of ten deflation patches between 2014 and 2016. During the two-year study period, significant positive and negative change occurred at all sites, suggesting that deflation patches are active landforms and that geomorphic change is highly heterogeneous and localized. We observed significant change primarily along the scarps, while little to no change occurred in the center of the patches. Along the scarps, the mean negative change ranged from −0.7 to −2.5 cm, and erosion dominated in eight out of the ten deflation patches. The modern erosion rate appears to be lower than the century-scale rate of 2.5 cm yr−1estimated from prior work using lichenometry, potentially because of the episodic nature of scarp retreat. Longer-term monitoring using these methods will help quantify the geomorphic response of this landscape to a rapidly changing regional climate.