Experiments by Invited Researchers


The Reciprocal Role of Microbial Colonization and Bed Sediment Patchiness in Biostabilization: the Critical Interface in Ecohydraulics

Project acronym: HyIV-Hull-09
Name of Group Leader: Professor Tom Battin
User-Project Title: The Reciprocal Role of Microbial Colonization and Bed Sediment Patchiness in Biostabilization: the Critical Interface in Ecohydraulics
Facility: TES
Data Management Report: Report

User-Project Objectives

Biofilms are prevalent in all substrates and hydraulic regimes, however we still know very little regarding the spatial patterns of colonisation, growth or failure. Field studies show microbial preference for depositional sites and finer substrates where the high grain surface:volume ratios, higher organic content, surface charge and reduced turbulence intensity appear important controls on local species and succession. That said, we also find microbial colonization on very coarse beds with high shear velocities. Whilst no specific evidence exits, it appears intuitive that relationships between grain and turbulence length scales would control the distribution of first-colonization. Subsequent positive feedbacks in local ecohydraulics would then likely account for spatial heterogeneity of growth, possibly explaining differences in the macro scale architecture (bio-‘form’) of mature biofilm over fine and rough beds. The overall aim of the research programme is to determine the biostabilization of spatially distributed patches and to assess the interactions of fluid turbulence, grain parameters and biofilm development. Using a groyne field to induce spatial patchiness and segregation of sediments the specific research objectives of this proposal are to: 1. Quantify bio-physical changes at the boundary during biofilm growth, including interactions and feedbacks. 2. Determine the biophysical relationships underpinning differences in the mode of failure and entrainment (i.e. discrete particle, floc or carpet-like). 3. Resolve the hydrodynamic and sediment controls on spatio-temporal heterogeneity of microbial colonization. 4. Inform how best to parameterize numerical models on sediment transport and spatial ecology.

Short description of the work carried out

For this project a set of five symmetrical embayments were created along both sides of the flume tank separated by a central section unimpeded flow. Initially, poorly mixed bed sediments were water-worked at a higher flow rate to generate spatial patchiness in sediment sorting and bed morphology. Each bay had similar pattern of morphology and sediment sorting enabling different studies to be carried out within each embayment. Nutrient rich saline water was used to promote the rapid growth of biofilms so that the experiments could be conducted within the access period Detailed ADV measurements were taken in one embayment prior to biofilm colonization and during the biofilm growth to establish the properties of the turbulent flow field. Samples of the biofilm were taken twice each week for detailed laboratory analysis. A novel measurement technique ‘Mag-Pi’ was deployed to investigate the cohesion of the biofilm in situ as it grew. Towards the end of the experiments, the flow rate was increased to investigate the effect of the biofilm on the entrainment threshold of the sediment substrate. Unfortunately, attempts to collect Particle image Velocimetry were unsuccessful due to the opacity of the flow.

Highlights of important research results

Analysis of the data from the experiment is currently being undertaken. Laboratory analysis is currently being undertaken to quantify the differences in biofilm colonization and growth in areas with different sediment substrates and hydraulic conditions. These results will be used to establish whether increased growth occurred in the low velocity zones of the channel embayments. However, biofilms did also successfully colonize the faster moving central region of flow with the significantly coarser substrate. The results will enable detailed comparison between the biofilm behavior and the hydraulic and sedimentological conditions within each of the patches that were characterized. Towards the end of the experiment floc samples and velocity measurements were taken as the biofilm was eroded to enable detailed analysis of the failure mechanisms when the biofilm is exposed to higher shear stresses.