The influence of hydrogeology and the Devensian glaciation on hyporheic communities of the UK
MetadataShow full item record
Ecologically, the hyporheic zone (HZ) performs numerous roles within stream ecosystems (e.g. habitat, refugia from floods and droughts, nutrient cycling, pollution attenuation) with the dominant control on these various roles sediment composition. Recently, the body of literature on the role of sediment in the HZ has grown, though these studies rarely extend beyond the reach scale with little regional scale research undertaken in England and Wales. In this thesis, meio- and macrofaunal hyporheic assemblages at two depths (20cm and 50cm) across four geological regions (chalk – fine sediment, sandstone - fine to medium sediment and limestone – coarse sediment) two seasons (summer (n = 396) and winter (n = 192)) are described. The influence of recent glaciations (Devensian) on the distribution of stygobite fauna (summer (n = 192) and winter (n = 98)) is also considered. A reach scale experimental manipulation of sediment composition in the HZ of a highly dynamic, species rich and diverse study site (limestone) was also undertaken. I found the influence of glaciation on stygobite fauna still apparent in the two limestone areas with macrofaunal sized stygobite species rare or absent in both limestone areas. Meiofaunal sized stygobite fauna Antrobathynella stammeri (Crustacea: Syncarida) were recorded from the glaciated limestone site. These results suggest large stygobite fauna are rare or absent in the limestone areas of Northern England with meiofaunal stygobites possibly surviving in sub-glacial refugia. Stygobite fauna were abundant in southern England with alternative migratory routes north discussed (e.g. River Severn catchment and chalk aquifers). Geological regions had characteristic fauna with species richness and abundance of meio- and macrofauna high in the HZ of both limestone areas. Conversely, species richness and abundance was low in the chalk and sandstone HZ. The chalk HZ had a high abundance of macrofaunal sized Crustacea (Gammarus pulex) and low abundance of meiofaunal sized Crustacea (Copepoda). This suggested mechanical properties (burrowing) rather than morphology (body size and shape) was important in fine sediments, whereas in the limestone HZ morphology rather than mechanical properties was important. In the experimental study results were more intuitive with meiofauna abundant across all sediment treatments (fine, mixed, coarse and natural) with macrofauna abundant in sediment treatments containing a high proportion of coarse material. These results suggest Copepoda assume a greater role in ecological processes in fine sediment patches within a dynamic HZ, whereas when fine sediment dominates a system (e.g. chalk HZ) then the role of Copepoda is reduced. One implication could be the use of Copepoda as indicators of colmation and hydrologic exchange in the assessment of ecosystem health and give an indication of the refugial capacity of the HZ from projected high and low flow events in light of UK climate change forecasts.