Modelling Phytoplankton Light Availability under Aggregation-Induced Buoyancy Changes in the Elbe Estuary from a Lagrangian Perspective

We present a Lagrangian modelling study focusing on the loss of phytoplankton observed in the Elbe estuary, where an estimated 95% of the upstream population is lost in the dredged harbour channel before reaching the North Sea. The collapse has often been attributed to factors such as light limitation and zooplankton grazing; however these explanations lack evidence. Current Eulerian models, while producing reasonable results in predicting phytoplankton population dynamics, fall short in explaining the causes of the loss by tuning the mortality parameters to fit the observed data. A recent study by Walter et al. (2017) suggests that light limitation alone does not fully explain the phytoplankton collapse. Our study explores an alternative hypothesis. We propose that high turbidity in the estuary may increase the coagulation rate of phytoplankton with inorganic sediments. Our approach involves modelling these coagulation interactions from a Lagrangian perspective. We combine schism hydrodynamics from Pein et al (2021), suspended particle concentrations based on the SediMorph model with the Lagrangian model OceanTracker to calculate collision and coagulation frequencies based on Burd (2013). This allows us to track changes in buoyancy and investigate their effects on light availability, providing a new approach to estimate phytoplankton losses due to sinking. The poster presents our methodology and preliminary results on the potential influence of aggregation-induced sinking on phytoplankton light availability and its subsequent impact on population dynamics. This research aims to contribute to a more nuanced understanding of the factors driving phytoplankton loss in turbid estuarine systems and to refine the modelling approaches used in such ecological studies.