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Biogeochemistry of erosive material deposition in streams: Impact of particulate deposition upon biofilm carbon cycling

Final Report Summary - BIO-ERODS (Biogeochemistry of erosive material deposition in streams: Impact of particulate deposition upon biofilm carbon cycling.)

Bio-ERODS was designed to test the biophysical mechanisms driving biofilm-particle interactions and mechanisms that potentially alter the organo-mineral complexation and thus contributing to stream biogeochemistry. These organo-mineral particles act as wash load in streams and represent a major component of the riverine inputs of organic matter to coastal seas. Therefore, the factors governing their formation, transport and degradation represent important controls upon global macronutrient (carbon and nitrogen) cycling pathways. The initial hypothesis underlying Bio-ERODS was that microbial processes represent a major control upon the transportation and the biogeochemical fate of organo-minerals in streams.
The core findings of Bio-ERODS are:
• The presence of freshly eroded clay minerals provides active sites for organo-mineral sorption in streamwater.
• Organo-mineral sorption restricts the efficiency with which both streambed biofilms and the streamwater microbial assemblages recycle organic carbon and nitrogen.
• Organo-mineral sorption alters the relationship between microbial carbon and nitrogen cycling.
• Biofilm thickness and complexity are major controls upon the transport of organo-mineral particles, with increasing complexity increasing the deposition and resuspension potential of individual particles.
Bio-ERODS used state-of-the-art techniques including simultaneous 13C, 15N tracer experiments to directly quantify microbial degradation of organo-mineral particles, and fluorescent particles to quantitatively trace particle transport. These provided robust methods to quantify how microbial assemblages control organo-mineral transport and degradation.
Bio-ERODS was achieved through two major experiments. The first experiment focussed upon testing how streamwater and biofilm microbial communities processed mineral-sorbed and dissolved isotopically labelled (13C, 15N) amino acids. This provided a proof of principle, by experimentally demonstrating that organo-mineral sorption restricts and alters the C: N stoichiometry of organic matter degradation. In addition, pilot studies for this experiment demonstrated that clay minerals readily adsorb labelled amino acids and bulk organic matter dissolved in streamwater. The second experiment tested how the complexity of streambed biofilms controlled the transport of clay particles stained with the fluorescent dye rhodamine. This experiment was conducted within 18 recirculating flumes testing particle deposition and resuspension both at base flow and during a simulated flood event. Biofilm structure and thickness was measured using state-of-the-art optical coherence tomography (OCT), providing the first measurements of streamwater biofilm structure to be made using live specimens. The data from this experiment is still being processed but preliminary data indicates a strong positive relationship between biofilm complexity and particle retention at the streambed.
The Bio-ERODS project has fostered a number of collaborations, both within the University of Vienna and internationally. This includes work with Prof Wolfgang Wanek (SILVER lab, University of Vienna) to trace the 15N label into both dissolved ammonia, nitrates, and dissolved organic nitrogen during experiment 1. Compound specific analysis of the incorporation of 13C and 15N labelled amino acids into microbial biomass was completed in collaboration with Dr Bart Veuger (NIOZ, Netherlands). Experiment 2 represents a significant international project working with Prof Aaron Packman and his PhD student Kevin Roche (Northwestern University, USA), Dr Jennifer Drummond (Centre d’Estudis Avancats de Blanes, Spain) and Prof Fulvio Boano (Politecnico di Torino, Italy) to develop a stochastic model to interpret particle transport data and link this to live biofilm complexity.
During Bio-ERODS, William Hunter has had significant opportunities to advance his skills and knowledge of aquatic ecology, biogeochemistry, stable isotope analysis and mathematical modelling. He has gained experience organising and facilitating international collaborative projects and mentored a PhD student (Kevin Roche) who joined our lab in Vienna to work on experiment 2. During his tenure as a Marie Curie fellow William has gained experience writing grants, submitting proposals to the UK Natural Environment Research Council (shortlisted) and the Leverhulme Trust (Awarded). Consequently, William has now taken up a post Queen’s University Belfast (UK).