Recruitment of the European smelt in a changing estuary: An Individual-based modelling assessment of environmental driver, climate change and management implication in the Elbe estuary

Abstract

Estuarine environments are highly productive transition zones between freshwater and marine habitats and play an important role as nursery grounds and migration corridors for diadromous fish species. At the same time, they are subject to intense anthropogenic modification and accelerating climate change, altering hydrodynamics, water quality and habitat connectivity. This thesis examines how interacting environmental drivers control early life stage development and recruitment of the European smelt (Osmerus eperlanus) in the Elbe estuary and evaluates how future climate change and estuarine management decisions may impact recruitment dynamics and ecosystem functioning. Using a multi-stage individual-based model (IBM), explicitly developed for the European smelt, the development of eggs, yolk-sac larvae and exogenous feeding larvae was determined by linked of extrinsic environmental drivers (temperature, oxygen availability, prey biomass, currents, turbidity and salinity) to intrinsic physiological processes governing development, growth and mortality. The thesis includes the model’s parameterisation, validation and extensive sensitivity analysis as well as the use case application using both a one-dimensional observation based forcing setup and a three-dimensional configuration coupled offline to a Lagrangian particle-tracking framework. The latter framework was used using high-resolution hydrodynamic and biogeochemical forcing provided by a coupled SCHISM-ECOSMO modelling system setup of the Elbe estuary. IBM results demonstrate that temperature and oxygen availability consistently emerge as the dominant drivers of successful development and survival. Yet significance varied across life stages. During endogenous development, increasing temperatures accelerate development rates thereby shortening vulnerable life stages and advancing the onset of first feeding, all of which can enhance individual survival. However, during the exogenous feeding stage, development is governed by the balance between prey intake and the individual metabolic energy demand, which both are heavily impacted by local temperatures. Once thermal optima are exceeded, metabolic rates increase disproportionately, reducing individual growth and an increased risk of starvation and ultimately mortality. Oxygen limitations impose an additional and often decisive constraint, with sharply increasing mortality if concentrations fall below the individual’s critical threshold and are often connected to a temperature-driven component further decreasing survival chances. Simulations using a RCP8.5-type climate scenario reveal substantial changes in habitat suitability within the estuary. Increased warming, decreased oxygen concentrations and changes in the salinity gradients drive a downstream shift of favourable nursery habitats and a contraction of suitable habitats in the central estuary. This shift results in an increasing exposure of early life stages to combined thermal and hypoxic stress. Although moderate warming may temporarily result in enhanced development conditions during spring, warmer summer temperatures result in increased mortality and vulnerability to frequent hypoxic events. The models result further indicate that future recruitment is even more shaped by a combination of interacting stressors, making the prediction of recruitment variability even more challenging. Beyond climate driven changes, this thesis highlights the importance of estuarine management decisions for the smelt’s recruitment success. Using scenario analysis of potential flood-adaptation measures reveal significant contrasts between hard-engineered structures and nature-based solutions. Barriers such as flood gates severely reduce longitudinal connectivity by restricting access to spawning habitats and promote aggregation affects that could further increase the individual’s mortality, especially for migratory species. In contrast, nature-based solutions such as controlled flood polders and channel reconstruction can enhance spawning habitat availability, partially restore connectivity and provide local growth benefits. These results demonstrate that recruitment dynamics of a key species such as the European smelt can be used as a sensitive indicator for the ecological impact of large-scale engineering projects. Overall, this thesis demonstrates that mechanistic, spatially explicit individual-based model provide a powerful framework for integrating multiple stressors across life stages and habitats. By linking physiological processes to environmental drivers, the developed smelt IBM offers insights that extend beyond correlative analysis and enables robust assessment of climate change impacts and management trade-offs. The findings emphasise that sustainable estuarine management, especially under the increasing influence of climate change should be using key-species based assessments to improve the measures ecological benefit.