Diatoms in Hyporheic Sediments Trace Organic Matter Retention and Processing in the McMurdo Dry Valleys, Antarctica

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 (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.

Dissolved Organic Carbon Chemostasis in Antarctic Polar Desert Streams

“…The long‐term data sets used in these analyses do not distinguish between in‐channel and hyporheic DOC sources. However, a hyporheic DOC source is plausible based on other research: we know that the hyporheic zone contains POM that is sloughed from microbial mats (Cullis et al., 2014; Heindel et al., 2021; Singley et al., 2021), and that this organic matter contributes dissolved nitrogen and likely other solutes to the stream channel (Singley et al., 2021). We also know that the hyporheic zone of MDV streams is an active site of DOC uptake and generation that occurs on sub‐diel timescales (Koch et al., 2010).…”

“…The perennial mats thrive on rocky, stable desert pavement and are sparse or nonexistent on sandy, mobile reaches, which do not accommodate much establishment and growth (McKnight et al., 1998). Thus hyporheic POM, as a source for hyporheic and ultimately stream DOM, is also expected to be spatially dispersed (Heindel et al., 2021).…”

“…(McKnight et al, 1998). Thus hyporheic POM, as a source for hyporheic and ultimately stream DOM, is also expected to be spatially dispersed (Heindel et al, 2021).…”

“…Further, DOC generation is somehow supplied across several orders of magnitude of q from autochthonous OC alone. The breakdown of POM primarily occurs in the stream channel and hyporheic zone; it is largely a biological process driven by the physical sloughing (particulate organic matter release) of in-channel mats and by decomposer microorganisms in the mats and hyporheic sediments (Cullis et al, 2014;Heindel et al, 2021). While there are differences between biogenic (DOC) and geogenic (Si) solutes, we expected that some aspect of the numerical model would help to explain DOC chemostasis, similar to the findings of Wlostowski et al (2018) when they applied the same reactive transport model to chemostatic weathering solutes in these streams.…”

“…Living phototrophs may release some of their fixed carbon to streams, and both phototrophs and heterotrophs are subject to decomposition and DOC releases (Spencer et al., 2012). At daily high flow conditions, mat material from the black, Nostoc ‐dominated mats in the stream margins is scoured, flushed downstream, and may be trapped within the substrate matrix by hyporheic downwelling (Heindel et al., 2021), eventually providing DOC to both the hyporheic zone waters and the stream water. The presence of chlorophyll pigment, biogenic silica, and preserved diatom frustules supports a microbial mat source of this hyporheic‐zone POM (Heindel et al., 2021).…”