Surviving in Changing Seascapes: Sediment Dynamics as Bottleneck for Long-Term Seagrass Presence

Wouter Suykerbuyk,Kris Giesen,Peter Herman,Laura L Govers,Marieke M Van Katwijk,Tjeerd J Bouma,Dick J De Jong,Jan Hendriks

doi:10.1007/s10021-015-9932-3

Wouter Suykerbuyk, Kris Giesen + Show 6 more

Open Access

https://doi.org/10.1007/s10021-015-9932-3

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Abstract

Changes in the seascape often result in altered hydrodynamics that lead to coinciding changes in sediment dynamics. Little is known on how altered sediment dynamics affect long-term seagrass persistence. We studied the thresholds of sediment dynamics in relation to seagrass presence by comparing sediment characteristics and seagrass presence data of seven separate seagrass meadows. All meadows had a long-term (>20 years) presence. Within these meadows, we distinguish so-called “hotspots” (areas within a meadow where seagrass was found during all mapping campaigns) and “coldspots” (with infrequent seagrass presence). We monitored static sediment characteristics (median grain size, bulk density, silt content) and sediment dynamics (that is, bed level change and maximum sediment disturbance depth), bioturbation (that is, lugworm densities and induced fecal pit and mound relief), and seagrass cover. We statistically analyzed which sediment characteristic best explains seagrass cover. Densely vegetated hotspots were shown to have lower sediment dynamics than sparsely vegetated hotspots and coldspots, whereas static sediment characteristics were similar (grain size, bulk density). The vegetation cover was either low (2–15%) or high (>30%) and sediment dynamics showed a threshold for vegetation cover. From this correlative finding, we postulate a self-sustaining feedback of relatively dense seagrass via sediment stabilization and accordingly a runaway feedback once the seagrass cover becomes too sparse. The sensitivity for sediment dynamics shown in our study implies that future existence of seagrass meadows may be at risk as ongoing climate change might directly (increased environmental extremes) or indirectly (changing seascapes) negatively affect seagrass beds.

Highlights

There is an overwhelming body of literature describing the massive and still ongoing losses of seagrass over the last decades, predominantly due to anthropogenic impacts
Changes in the seascape often result in altered hydrodynamics that in turn lead to changes in sediment dynamics
Relatively little is known on how altered sediment dynamics may affect longterm seagrass persistence

Summary

Introduction

There is an overwhelming body of literature describing the massive and still ongoing losses of seagrass over the last decades, predominantly due to anthropogenic impacts (for example, Orth and others 2006a; Waycott and others 2009). A much less studied aspect is the more indirect effect of changes to the coastline due to infrastructural works. Such changes in the seascape may alter natural seagrass structuring processes, and thereby indirectly threaten continued seagrass existence (that is, Lotze and others 2006). Relatively little is known on how altered sediment dynamics may affect longterm seagrass persistence. This question may be relevant for seagrasses in temperate areas, where plants cannot adapt to burial or erosion during winter when they do not grow (Han and others 2012)

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