Faculty Publication: Diatoms in hyporheic sediments

Ruth Heindel, Assistant Professor of Environmental Studies, has published an article in Journal of Geophysical Research: Biogeosciences titled “Diatoms in hyporheic sediments trace organic matter retention and processing in the McMurdo Dry Valleys, Antarctica.” Her co-authors includeJosh P. Darling,  Joel G. Singley,  Anna J. Bergstrom,  Diane M. McKnight, Braeden Lukkari,  Kathleen A. Welch, and Michael N. Gooseff.

Summary

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, 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 McMurdo Dry Valleys, 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.

Scientific Abstract

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 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 McMurdo Dry Valleys, and we suggest that biogenic silica and diatom composition can be used to identify locations where this transfer is most prevalent.

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