Transient evolution of suspended and benthic algae in a riverine ecosystem: A numerical study

It is a well-established fact that there is no direct resource competition between pelagic and benthic primary producers when their habitats are located in physically separate areas. However, the benthic habitat located at the bottom of the water column is affected by the light rays that gets attenuated as it passes through the pelagic zone and is obstructed by the algal cells and suspended material present in the water column. Therefore, benthic primary production is very much influenced by the conditions and concentrations in the pelagic zone. Another level of complexity is added to the system while considering the impact of flowing water in a riverine environment. In the research presented here a numerical model is developed to examine the impact of flow and nutrients on pelagic and benthic primary producers. The aforementioned numerical model solves advection diffusion and reaction (ADR) equation through TVD (total variation diminishing) − MacCormack scheme. The diffusion term is solved through central difference scheme. The model results are first validated by comparing the results with analytical solutions for the simplified case. The validated model is then applied to a 30 kms stretch of the Bode River and simulations are conducted for multiple flow conditions. Impact of transient flow and nutrient boundary conditions on the evolution of algal trait is examined. Our simulation exercise highlights the importance of residence time in the temporal evolution of algae in pelagic and benthic zone and further identifies the most sensitive parameter influencing the evolution of the algal community modelled. Our research also reveals that largest uncertainty in the modelling stems from the wide range of entrainment rate that could be used for modelling resuspension of benthic algae in pelagic zone. The use of higher entrainment rate increased the algal concentration in the water column by 48% for identical flow conditions