Modeling the Adaptation of Estuarine Morphology and Macrobenthic Bioturbators under Sea Level Rise

Estuaries are one of the most valuable marine environments, but their functioning is threatened by predicted sea level rise. Macrobenthic organisms occupy estuarine sediments, where they forage for food and can destabilize local mud. This bioturbation activity can cause erosion and modify local and system-scale morphology. Simultaneously, sea level rise may increase bed shear stresses affecting estuarine morphology and the habitat and activity of bioturbators. We still lack understanding of how sea level rise will alter the feedbacks between morphology and bioturbator abundance and activity, and how these interactions control estuarine morphology.

We use a two-dimensional hydro-morphodynamic model of an idealized estuary that is coupled with rules for abundance and biomass-dependent bioturbation of two contrasting bioturbators, a sand-prone bioturbator (lugworm) and a mud-prone bioturbator (mud shrimp). Local mud erodibility is based on species pattern predicted through hydrodynamics, soil mud content and competition. The model allows us to test 1) the response of estuarine morphology to the combined effects of sea level rise and bioturbation, and 2) the response of the two bioturbators to changes in their habitat induced by environmental adaptation. In addition to two rates of sea level rise, we investigate if increasing hinterland erosion through higher fluvial sediment supply can offset potential estuarine erosion and habitat loss.

The model results show that species-specific bioturbation locally increases accommodation space and reduces mud content in the estuary, which amplifies the impacts of sea level rise. Intertidal profiles are smoothened, affecting intertidal habitat extent. Species-dependent adaptation leads to the lugworm adapting to an increasingly dynamic habitat, whereas the mud shrimp declines as it prefers calm, muddy habitat. Colonization and mortality are a result of the combined rates of drowning of existing and generation of new habitat. Increasing sediment input from the hinterland can reduce accommodation space, but that effect might be neutralised under high rates of bioturbation. Our findings show that bioturbation alters estuarine adaptation and resilience under sea level rise, illustrating the need to consider macrofauna when predicting sea level rise impacts.