Highly variable physical and biological characteristics of hyporheic zones among young streams undergoing primary succession

Abstract

Opportunities are rare to study ecological primary succession and physical channel development in brand-new streams. An exception exists in streams recently formed atop extensive pyroclastic materials deposited during the 1980 eruption of Mount St Helens (USA), an area known as the Pumice Plain. Recent work demonstrated unexpectedly high diversity of benthic invertebrates in Pumice Plain streams at <4 decades post eruption, both within and among streams. Given the ecological importance of the hyporheic zone (HZ), we asked if differential development of HZs, both physically and biologically, could help explain the among-stream variability in surface assemblages. In 2019 (39 y post eruption), we measured abiotic variables and collected hyporheic invertebrates with a modified Bou–Rouch method from 4 Pumice Plain streams already understood to vary substantially in terms of successional trajectory. The HZ was accessible to just ~30 cm below the stream beds because of a nearly impermeable layer of compacted ash overlain by alluvium. Hyporheic invertebrate assemblages were low-diversity subsets of the benthos, dominated by Chironomidae, and lacking true groundwater taxa. Hence, the benthic zone appears to influence the underdeveloped HZ assemblages in these young streams, more so than the reverse. Substantial among-stream variation in HZ assemblage structure also reflected previously documented spatial patterns of the benthos. Mean densities of hyporheic invertebrates varied by >2 orders of magnitude, and standing biomass varied by >4 orders of magnitude among streams. The stream with greatest density and richness had large median particle size, a stable bed, and dense riparian woody vegetation and was dominated hydraulically by downwelling. Streams occupying eastern vs western sides of the Pumice Plain were strongly differentiated abiotically and biotically, and snapshot water stable isotope data suggest hydrological sources drove these differences, with eastern streams likely fed by glacier melt and western streams fed by warmer groundwater sources. This survey provides novel insight into HZ development in newly formed streams and points to further avenues of research, including exploring the development of groundwater assemblages in Pumice Plain aquifers and evaluating the role of woody riparian plants in establishing surface-water/groundwater connections to develop the ecotone characteristic of mature hyporheic zones.

Department(s)

Biology

Document Type

Article

DOI

10.1086/734314

Publication Date

3-1-2025

Journal Title

Freshwater Science

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