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Title: Links between hydrology, community ecology and ecosystem services in the hyporheic zone of streams and rivers : an interdisciplinary perspective
Author: Peralta-Maraver, Ignacio
ISNI:       0000 0004 7960 2361
Awarding Body: University of Roehampton
Current Institution: University of Roehampton
Date of Award: 2018
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Streambed sediments are an interface between the surface water and the groundwater systems characterised by the simultaneous occurrence of multiple physical, biological and chemical processes. Transformation and biodegradation of nutrients and contaminants are among the important ecological services the streambed provides. Streambeds are also the biotope of a diverse and productive biota. This system is divided into the benthic zone (BZ), which is located in the upper centimetres of the streambed and the hyporheic zone (HZ), which encompasses the volume of sediment beneath the BZ where surface water interacts with groundwater. The nature of the biota and the ecosystem processes differ depending on the streambed compartment. I first reviewed how depth gradient and hydrological flux conditions shape the structure of the hyporheic assemblage (hyporheos). Metadata analysis predicted a reduction in net biomass and productivity through depth in the streambed. However, my models were based on data derived from the literature, which focussed on macroinvertebrates only, and it is likely that model predictability will increase by including Protozoa and meiofauna. Assessing the organization of the whole hyporheos is also important to better understand the self-purifying capacity of the HZ as it removes nutrients and emerging organic contaminants (EOCs) from the medium (hyporheic bioreactor ability). Based on this premise, I conceptualize how the residence time of water in pore sediments (resulting from hyporheic exchange flow) and the rest of the hyporheos might drive the hyporheic bioreactor efficiency. Then, I carried out a local-scale survey study in which I combined sampling techniques from hydrology, biochemical engineering and community ecology to determine accurately hydrological conditions and to characterise the resident assemblages at a cm resolution in the streambed. My findings showed for the first time that decline in biomass and secondary production of the different taxa is body-size dependent. Smaller organisms (i.e. protozoa) penetrate deeper and colonise more compacted and anoxic sediment layers. My results also evidenced that down-welling (DW) sites are hot spots of productivity, and therefore carbon processing in freshwater systems. I also demonstrated that hyporheos and benthic assemblages (benthos) are two measurable ecological communities with individual integrity, whose demarcation boundary reached deeper under DW conditions. Subsequently, I analysed in a microcosms experiment how dissolved organic carbon (glucose), cell density of colloidal bacteria degraders and predator-prey interactions drive the capacity of hyporheic sediments to process a model EOCs (Ibuprofen). Glucose and the presence of the predator at medium density levels significantly promoted the degraders population growth. The increase in degraders cell density resulted in a higher consumption of Ibuprofen. Furthermore, the positive effect of predator presence interacted synergistically both with glucose availability and degrader cell density, producing an intensification of degrader population growth and Ibuprofen removal, respectively. These findings evidenced the importance of preserving the water interchange between the open channel and the HZ (and consequently the nutrient loading) and the natural predator-prey dynamics in order to promote ecosystem services upon which human well-being depends. Finally, I conducted a regional-scale study across 30 different rivers in order to further investigate leaf litter processing in the streambed. I measured different leaf litter breakdown rates by combining the cotton-strips assay and the tea bag index. Then, I modelled the obtained breakdown rates as a response of the streambed compartment (BZ and HZ) and the biological features of the streambed assemblage by involving a large range of organisms (Prokaryota, Protozoa and Eumetazoa invertebrates). Results from my models predicted a higher leaf litter breakdown in the BZ than in the HZ. Furthermore, the biomass of all the studied groups, α-diversity of Eumetazoa invertebrates and functional diversity of Prokaryota were important predictors that were positively related with the decay rates. The inferential models I presented are a suitable tool to predict the efficiency of streambed systems in the processing of leaf litter, based on the biological features of the assemblage and the difference between the BZ and the HZ. My findings covered significant knowledge-gaps on the structure of benthos and hyporheos, the mechanistic understanding of the processes and services that occur in the HZ, and their relative importance for the whole ecosystem functioning compared with those that take place in the BZ. Since the HZ was first defined the existence of a measurable transition between benthos and hyporheos had never been delimited before. Therefore, from my findings it is now possible to determine the contribution of benthos and hyporheos to the ecosystem processes and services. My research also enhanced our understanding of the processing of EOCs and alochtonous coarse organic carbon (leaf litter) in the streambed. Here I emphasise that the bioreactor ability of the streambed is sustained and maintained by diverse and active assemblages and that all size categories play an important role in its functioning.
Supervisor: Robertson, Anne ; Reiss, Julia Sponsor: EU Horizon 2020
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available