Modelling epilithic biofilms combining hydrodynamics, invertebrate grazing and algal traits

This model of stream epilithic biofilm biomass dynamics is based on the system of equations from Uehlinger etal. () and the term for autogenic detachment of biofilm from Bouletreau etal. (). Its new features are (i) a mathematical term based on estimated feeding activity of biofilm-dwelling invertebrates, (ii) local hydrodynamics considered as the principal factor governing algal traits and biofilm structure and (iii) a variable degree of parameterisation that was adjusted to biofilm biomass conditions. Biofilm biomass was monitored over a one-year period in the Garonne river in France (September 2008-2009). An allometric approach was used to estimate the feeding activity of biofilm-dwelling invertebrates based on their energetic requirements. Diatom functional diversity was also monitored to find how it varied with overall biofilm growth patterns. The one-year monitoring period was divided into six biofilm biomass cycles, with each cycle consisting of a phase of biofilm growth as the main process, followed by detachment. This model reproduced the observed data as a complex of biofilm growth/detachment cycles using different sets of empirical parameters which allowed (i) the dominant processes involved in each biofilm cycle to be evaluated and (ii) the six cycles of biofilm growth/detachment to be reproduced. This accounted for the observed patterns more effectively than a parameterisation using a single set of empirical parameters. High flow had a severe effect on biofilm dynamics through chronic and catastrophic detachment. Presumably as a result, assemblages of diatoms shifted towards species that were firmly attached and protected by mucilage. During low flow (and when temperature was high), biofilm dynamics was mainly affected by autogenic detachment and grazer activity. The grazing pressure of the dominant biofilm-dwelling invertebrates (Nematoda and larvae of Chironomidae and Trichoptera) was fairly low (a maximum of 6% of biofilm biomass ingested daily); nevertheless, their presence in the biofilm seemed to favour biofilm autogenic detachment.