Laurin Steidle , Johannes Pein, Adrian Burd, Ross Vennell

Within the Elbe estuary, a sudden change in depth occurs when the river enters the shipping
channel in the Port of Hamburg. This change in depth correlates with a sharp decline in phytoplankton
concentrations. This decline affects the estuarine food web and shifts the ecosystem from autotrophic
to heterotrophic during the summer months. Previous studies have hypothesized that this collapse
is primarily driven by zooplankton grazing. We question this narrative and investigate the effect
of phytoplankton aggregation with inorganic suspended matter and its impact on light limitation.
In this study, we present a novel individual-based Lagrangian model to investigate the influence of
aggregation on phytoplankton mortality. By incorporating data from the hydrodynamic model SCHISM
and the sediment transport model SediMorph, we perform a Langragian simulation of the movement
and aggregation of phytoplankton in the estuary. Our results show that aggregation with inorganic
particles significantly increases sinking rates, leading to increased light limitation-induced mortality of
phythoplankton, suggesting that aggregation processes may play an important role in explaining the
collapse of phytoplankton concentration.