Seagrasses and coastal protection: The role of Halophila stipulacea in sediment stability at the coast of St. Eustatius, Dutch Caribbean
Hee, Francine van
MetadataShow full item record
Seagrasses are increasingly being recognized for their potential in protecting the coastlines from flooding and erosion. As ecosystem engineers, seagrass can attenuate waves and trap and stabilise sediment, causing the seabed to become more stable, which can contribute to coastal protection. Where many tropical countries lack the means to undertake traditional coastal protection measures, natural ecosystems (e.g., seagrass meadows) can provide cost-effective alternatives to protect coastal communities from natural hazards. The Caribbean region holds a large cover in seagrass relative to the coastlines, yet few studies have focused specifically on seagrass and sediment stability in this region. Over the last decades, native seagrasses have been degrading and the non-native Halophila stipulacea, originating from the Red Sea, has been spreading rapidly through the region relatively recently. A knowledge gap exists in whether this opportunistic species could provide essential ecosystem services, such as coastal protection, where native species have been lost. The extent to which seagrass species contribute to coastal protection services depends on the growth properties of these species as well as environmental conditions. Therefore, a need remains to study sediment dynamics in seagrass meadows in different scenarios. This study aimed to increase our understanding of the role of H. stipulacea in sediment dynamics, specifying on the coast St. Eustatius, Dutch Caribbean. Next to a literature review on sediment stabilising properties of seagrass species in the wider Caribbean region, an experiment was set up at a study site (18 m deep), in a monospecific H. stipulacea meadow, and in a sand patch within the meadow. Growth properties of H. stipulacea were measured, and sediment stability was investigated by measuring changes in seabed level over time, as well as using sediment traps to calculate the sediment deposition rate. Data on meteorological conditions was retrieved to investigate correlations between the weather and sediment change. Results showed that H. stipulacea at a depth of 18 m at the coast of St. Eustatius, grew a new shoot within 3.3 days, over twice as fast as measured for its native range. Average leaf length was 4 cm while 8 shoots were observed per strand. Similar values were observed in its native range. Although the leaf length was smaller than most native seagrasses in the Caribbean, H. stipulacea showed a higher density, which likely contributed to the findings of this study. Sediment deposition rate was much lower in the H. stipulacea meadow than in the sand patch, which is probably the result of limited resuspension within the seagrass meadow. After 14 weeks, sediment in the H. stipulacea meadow had eroded. This is believed to be the result of erosion of the surface sediment layer, a less stabilised layer of sediment that was potentially resuspended and carried away when the weather became more turbulent after months of calm conditions. The subsurface layer, however, is believed to be more stabilised by the root and rhizome system of H. stipulacea, which explains why smaller and relatively constant changes in sediment level were found after weather conditions had increased. The sediment level in the sand patch showed a larger variance over time and spatially between erosion and accumulation. This study showed that H. stipulacea seems to stabilise the subsurface sediment layer via its root and rhizome system, which is assumed to contribute to a more stable seabed. However, this result only became apparent after erosion of the surface layer and might even disappear with more extreme weather conditions. This study urges for future research to investigate the sediment stabilising effect of H. stipulacea at shallower depths as well as over longer terms, as different results might be found for different seasons.