Weathering the storm: How benthic macrofauna respond to dynamic intertidal sediments

Abstract

Benthic macrofauna are a key component of intertidal ecosystems. Not only do they provide food for birds and fish, their mobility and behavior determine processes like nutrient cycling and the biogeomorphic development of intertidal flats. Local environmental conditions shape benthic macrofauna assemblages as well as the functions these perform. Because benthic macrofauna assemblages and behavior underpin many ecosystem services, it is vital to know how they respond to the highly variable environmental conditions of the intertidal. While some effects of sediment characteristics (like grain size) on benthic macrofauna are already well-studied, others (like bulk density) are less well-known. In addition, because climate change may increase the frequency and intensity of winter storms, the ability to tolerate extremes in sediment dynamics may become more important for shaping benthic macrofauna assemblages in the future. Though extreme sediment deposition has been investigated for its effects on benthic macrofauna, the effects of extreme sediment erosion have been rarely studied. Finally, the coasts are some of the most widely used and impacted areas in the world. Knowing which suite of sediment characteristics are suitable for which assemblages is very important for planning restoration initiatives which seek to improve habitat quality for benthic macrofauna. In this thesis, I examine the response of benthic macrofauna assemblages and behavior to dynamic sediment drivers. In particular, I focus on detecting behaviors and traits that confer resilience to benthic macrofauna against disturbances and extremes in sediment dynamics. In Chapter 1, I introduce benthic macrofauna-sediment interactions and how these are modulated by environmental disturbances and biotic resilience. In Chapter 2, I use a mesocosm experiment to uncover the effects of sediment bulk density, a little studied but important sediment characteristic, on benthic macrofauna burrowing and bioturbation behavior. This study shows that bulk density had a strong effect: benthic macrofauna burrowed faster and bioturbated more intensely in softer sediments, regardless of grain size. In Chapters 3 and 4, I examine the effects of storm-induced erosion on bivalves, which are vulnerable to storms due to their low mobility. These chapters show how species-specific behaviors (Chapter 3) and size-dependent traits (Chapter 4) regulate tolerances to extreme sediment erosion, which has consequences for bivalve population trajectories and long-term species success in a more storm-disturbed intertidal. In Chapter 5, I analyze the concurrent development of the hydrogeomorphology and benthic macrofauna community at three restoration projects in the Netherlands’ Western Scheldt. This chapter suggests that while the creation of a low-dynamic habitat can stimulate benthic macrofauna biomass, extremely high silt content, which is typical for low-dynamic habitats, may slow the benthic community development. In Chapter 6, I summarize the thesis results and explore how we can increase the resilience of intertidal benthic macrofauna assemblages against climate change and anthropogenic pressure.