The facilitating effects of the seagrasses Halophila stipulacea, and Halodule wrightii on Thalassia testudinum through lowering pore-water sulfide levels

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.