The hydrochemistry of the hyporheic zone:
Assessing ecotone properties for juvenile freshwater pearl mussel (Margaritifera margaritifera L.) survival in the River Esk, NE England

Abstract

The hyporheic zone is intrinsic to river ecosystems. Nutrient breakdown rates in shallow riverine sediment occur at rates faster than in marine sediments. This means that where water exchanges between stream and substrate, key alterations occur to the chemistry of sediment pore waters and lower river water column. A river unit’s hydrogeomorphological setting and spatial configuration is linked to exchange flow generation between terrestrial and aquatic ecotones – a hyporheic flow pathway. Hydrological process connectivity occurs at scales from valley-scale enclosures through to meanders and riverbed forms, such as pool-riffles. Together, the hyporheic pathways coupled with the surface turbulent oxygenation processes in riffle flow, are significant in altering river water chemistry over short distances. This is considered vital, as the central focus of the European Union Water Framework Directive (2000/60/EC (EU WFD) is to achieve good ecological status, hyporheic processes significantly contribute to this. In this thesis bio-indicator species abundance, in the form of Margaritifera margaritifera Linnaeus (1758), an oligotrophic bivalve, is investigated as having an affinity with hyporheic processes. The impact of point-source pollution, and species absence, is also investigated.

Hyporheic zone, in-stream and groundwater hydrochemistry and hydraulic measures are assessed at three sites in this study – a braided woodland reach, an agricultural reach and a sewage treatment works, using a dense monitoring approach comprising piezometers and automatic pump water samplers augmented by manual sampling. The findings show that through a 23-metre pool-riffle sequence, where water injects at riffle-head and subsequently ejects at the riffle-tail, there is a 5 percent reduction in mean in-stream nitrate-N, and in the hyporheic zone, 73% reduction in pore water concentration. A similar pattern is presented for phosphate-P. Nitrate-N, dissolved oxygen, ammonium and phosphate-P are all subject to acute nocturnal sag, often doubling daytime concentrations. These water quality stresses and the impact of sewage outfall are assessed in respect of the last remaining Yorkshire freshwater pearl mussels, Margaritifera margaritifera L., an endangered species in decline throughout its geographical range and a bio-indicator under the EU WFD. The Freshwater Biology Association now has a cohort of hatchery-reared molluscs awaiting reintroduction. This study concludes that riffle-tails are the prime site for reintroduction of these juveniles, due to cool alluvial oligotrophic discharges at the riffle-tail creating metabolic hotspots and refugia. Sewage outfall is presented as a major risk imperilling long-term Margaritifera margaritifera viability. Statistical tests confirm river gravel compaction and pore water hypoxia at a degraded reach, compared with high pore water oxygenation at a much less impacted river reach.