seagrass restoration

MSc Research Report 

Abstract Seagrass beds are among the world’s most productive systems with many ecosystem services such as coastal protection. The global rate of seagrass loss is high and despite legislation, the rate of loss has not slowed down substantially. A significant stressor on the Caribbean coast is the Sargassum mats that wash ashore. The coastal waters become enriched in nutrients and oxygen-depleted, releasing toxic hydrogen sulfide gas. The sediment becomes hereby uninhabitable for the native seagrass Thalassia testudinum.  

This study aimed to investigate whether there is a facilitating effect of Holophila stipulacea and Halodule wrightii on T. testudinum through sediment oxidizing properties. The following sub-questions were raised: (1) “Is there a difference in the growth rate of the seagrasses H. stipulacea, H. wrightii, and T. testudinum when they are transplanted to a post-SBT sulfide-rich environment?”, (2) “Is there a difference in sulfide levels in the sediment next to transplanted H. stipulacea, H. wrightii and T. testudinum cores?”, and lastly (3) “Is there a difference in the growth of individual T. testudinum shoots next to these transplanted cores?”.

To test this, cores of all three species were taken from healthy seagrass beds and transplanted to a sulfide-rich area. (1) The relative growth rate of the cores was measured, and, (2) sulfide levels in the sediment next to the transplanted cores compared. Lastly, (3) the growth of individual T. testudinum shoots was measured when placed next to the transplanted cores.

This study showed (1) no difference in growth rates of the transplanted cores, and (2) temporarily elevated pore-water sulfide levels in the immediate surroundings of the transplanted cores. Lowest sulfide levels near the H. wrightii transplantation and (3) no difference in the growth of the individual T. testudinum shoots between treatments. Concludingly, it is unlikely that there is a plant-plant facilitating effect of H. stipulacea, T. testudinum and H. wrightii on T. testudinum. Transplantation of T. testudinum cores with H. stipulacea or H. wrightii is not a restoration measure that may facilitate T. testudinum in bare areas with high pore-water sulfide levels. Investigating the behavior of the sulfide levels over a longer period is advised since the duration of the experiment was too short to observe the equilibrium that arose. 

Abstract
Seagrass is of great importance worldwide for coastal protection, carbon sequestration and as a nursery and feeding habitat for various species. However, due to climate change, eutrophication, turtle grazing and anthropogenic activities seagrass meadows are declining globally. Seagrass restoration might be a tool to restore the seagrass ecosystem and bring back the linked ecosystem services. In the case the area is ought to be suitable for restoration, different restoration methods can be used. This study will focus on the importance of sediment stabilisation for seagrass restoration of the native seagrass Thalassia testudinum and the invasive Halophila stipulacea, using biodegradable sheets that mimic the sediment stabilizing function of seagrass meadows. This study is executed in Lac Bay, Bonaire. It is expected that by using these stabilizing sheets, the balance between native and invasive seagrass can be shifted towards native seagrass occurrence. During this research we found that using sediment stabilizing root mats can improve restoration of the native seagrass T. testudinum, especially in environments with high wave action and currents. Sediment stability is provided and fragments are held in place by the use of these biodegradable sheets, which prevents fragments from washing away. However, for the long-term these biodegradable sheets are possibly negatively affecting seagrass growth, likely due to interference with rhizome growth. This should, however, be researched into further detail. The invasive seagrass species H. stipulacea does not experience advantages in terms of growth when using these root mats. Fragments of H. stipulacea are fragile and possibly suffer from different kinds of stress when implementing in between the sheets. It could be stated that by using the sediment stabilizing sheets, the balance between native and invasive species can be shifted towards native seagrass in this research. This will benefit the seagrass ecosystem and its ecosystem services. In general it can be stated that the effect of using these biodegradable sheets differs depending on the seagrass species and various environmental factors such as hydrodynamics. There is also an indication of a difference in efficiency of the use of these sheets between the short-term and long-term growth. Furthermore bioturbation is likely to influence seagrass expansion and the functionality of these biodegradable sheets, therefore further research is advised.