bioturbation

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.

Abstract
Seagrass meadows provide essential ecosystem services. However, seagrass cover has decreased the past decades, due to climate change and other disturbing factors. To ascertain coastal stability, biodiversity and ecological well-faring, seagrass meadows need to be restored. In this thesis, we unfold complex interactions that need to be taken into account during such restoration projects.

In particular, this study explores the spatial distribution and factors influencing seagrass cover in Lac Bay, Bonaire, with a focus on the interactions between turtle grazing, bioturbation, invasive seagrass (Halophila stipulacea), and native seagrass (Thalassia testudinum). The observational study reveals competition between the two seagrass species, as well as the negative impact of high grazing pressure and bioturbation on T. testudinum. A linear mixed model identifies significant predictors for T. testudinum cover, including macroalgae cover, turtle grazing pressure, H. stipulacea cover, and mound cover. Additionally, a negative influence between H. stipulacea and T. testudinum is observed. Bioturbation is found to negatively affect T. testudinum but does not significantly impact H. stipulacea. The experimental study investigates the effect of bioturbation on seagrass growth using different planting techniques, indicating that transplanting T. testudinum fragments can be successful for restoration. However, no significant differences are found between lattice and mesh treatments. The study concludes that the complex interactions between these factors contribute to the decline of T. testudinum and the proliferation of H. stipulacea. Future research is suggested to further investigate these interactions and evaluate the long-term effects of bioturbation and planting techniques on seagrass growth. The findings emphasize the importance of considering multiple variables when studying seagrass ecosystems and propose transplanting as a potential restoration measure in areas with high bioturbation.

4. Discussion and Recommendations
Seagrass

Overall, there has been a decrease in the native species of T. testudinum and an increase in the invasive species H. stipulacea. S. filliforme populations appear to be stable, with a slight increase in coverage. Native seagrass Thalassia testudinum (Tt) has had an overall decrease in coverage from 48.78% in 2011 to 20.61% in 2022 . Over this same time period there has been a slight
increase in native seagrass Syringodium filiforme, from 3.85% in 2011 to 6.44% in 2022.

Lastly, there has been an alarming increase in the invasive seagrass Halophila stipulacea, growing from 6.01% in 2011 to 35.24% in 2022. A table with the annual averages for the three seagrasses can be found below in Table 1.

Sargassum has been an issue within Lac Bay, with several of the survey sites being locations where decaying sargassum has created a thick mat, which in most cases was slowly removed with the tide. Physical impact of the sargassum landings can be seen by the seagrass dieback all along the mangrove border from the south until just north of Punto Kalbas. This is noticeable at G.
Additionally, at location E, a very fluffy sediment was found to be covering the substrate. A likely explanation is that this is the result of decomposed sargassum settling at this site. The overall cover by all species together seems to be stable, but in terms of biomass it would appear to be lower. The ecosystem services provided by Halophila stipulacea are significantly lower than those of
Thalassia testudinum due to its shallow root structure (Smulders et al., 2017) and the fact that it is less nutrient rich than native seagrass species (Boman et al., 2019). The shift towards this nonnative species is of concern and should be closely monitored.

Benthic Species
Since 2018 Halimeda species and in 2022 bioturbation observations were added to the methodology of these surveys. Although bioturbators were noted in 2020, they were not quantified in such a way to allow objective, quantitative comparisons moving forward. Overall cover by Halimeda seems to have decreased but a longer time series is required to draw more definitive conclusions.
Two students have looked into carbonate sand production by Halimeda during the Lac Ecological Restoration project: Laura Timmermans (2018) and Valeria Pesch (2019). Results from these studies were inconclusive, highlighting the need for additional research to fully understand the contribution of Halimeda to carbonate sands and infilling of the bay.

In addition, more information is needed on the influence of eutrophication (Slijkerman et al., 2011) on this process. Table 2 below shows the overall averages for both species of Halimeda from 2018 to 2022. Table 2: Overall Halimeda averages between 2018 and 2022.

Sand particles size in Lac was measured during the Conch Stock Restoration project. Largerfractions often show Halimeda segments next to small shells and other carbonate particles(Figure 5). For this reason, it is believed thatHalimeda sp.are a major contributor of sandwithin the bay.

Sediments have been analyzed for carbonate content in several other studies such as theEHLZK projectand duringthe baseline surveys conducted in 2012 (Debrot et al, 2012).Although the findings have not been published, the data showed that sediments towards thecenter of the bay have a higher CaCO3content, and the distribution sand, silt, clay changes(Appendix VI). In addition, it was foundthat terrigenous sediments were most prevalent alongthe borders of Lac-mainly in the northwestern sector, whereas endogenous sediments werefound in the central part of the bay and towards the reef. 

Bioturbators have also been added to recent surveys as it is believed to be important as itmay cause a loss of sequestered carbon, and new sediment may facilitate settlement ofH.stipulacea. Bioturbators are considered to be ecosystem engineers, changing the substratelandscape.Common bioturbators are callianassid (burrowing ghost) shrimp, the lugworm,mantis shrimp and the burrowing sea cucumber.

Summary
Thalassia testudinum seagrass is defined as important foundation species by serving as nursery area, giving coastal protection, maintaining high water qualities and their function as carbon sink. In the east Caribbean Sea, T.testudinum beds are under threat caused by the decomposition of excessive amounts of holo-pelagic sargassum blooms (Sargassum natans and Sargassum fluitans), creating anoxic conditions and causing the production of hydrogen sulfide( H2S) in the porewater. Hence, sulfide intrusion and toxicity can occur in T.testudinum causing lower plant performances and big scale seagrass mortalities. Positive effects of bioturbation were thought to relieve sulfide stress from T.testudinum by modifying the geo-chemistry by a process called ‘bio-irrigation’. This research assessed the effects of Bio-irrigating Upogebia shrimps on the porewater sulfide and sulfide plant intrusion within T.testudinum habitat. Firstly, a monitoring study was carried out to determined the effects of habitat and sargassum decomposition on Upogebia hole densities(indicator for Upogebia densities). Secondly, an Upogebia hole manipulation experiment was setup to investigate the effect of adding and removing bio-irrigating shrimps(Upogebia affinis) on porewater sulfide and sulfide intrusion within T.testudinum seagrass. In addition to the experimental effects, the overall effects of Upogebia hole densities on porewater sulfide and sulfide intrusion taken over the whole experimental period were investigated to account for possible delayed effects on sulfide intrusion within T.testudinum. Our results showed highest Upogebia hole densities in bare habitat with lower sargassum decomposition. Besides the possible negative effects of T.testudinum habitat reducing burrowing space for Upogebia, the bare habitat mainly consisted of Halimeda calcified sediment which likely facilitated the Upogebia burrow construction. Within the overall effects, Upogebia hole density showed negative effects on both porewater sulfide and total sulfur within T.testudinum leaf suggesting some possible bio-irrigation. Within the treatment effects, the removal intervention caused higher TS and lower δ34S in the T.testudinum leaf, but this was just visible after 16 days, indicating delayed effects of the removal intervention. Future mesocosm experiments are recommended to account for side effects and for more accurate sulfide measurements. This will fill more scientific knowledge gaps on the role of Upogebia shrimps as potential sulfide stress relievers for T.testudinum seagrass.

In tropical ecosystems, autotroph organisms are continuously competing for space, with some plant species benefiting from disturbances such as fire, grazing, or bioturbation that clear habitat (Pulsford et al. 2016). These disturbances can open up layers of vegetation, thereby promoting colonization of opportunistic species that would have been competitively inferior without disturbance (Castorani et al. 2018). Opportunistic fast-growing species also include often invasive species that are therefore also likely to increase in dominance after disturbance (Altman and Whitlatch 2007). In seagrass meadows in the southern Caribbean, we observed that the marine invasive plant Halophila stipulacea uses bioturbation mounds, created by burrowing infauna such as sea cucumbers and shrimp (see Suchanek 1983), to colonize new habitats (Figure 1a, b). On Bonaire and Curaçao, in habitats with ~100% native Thalassia testudinum cover, invasive H. stipulacea often at first only occurred on bioturbation mounds that smothered native T. testudinum seagrass, likely due to fragmentation and subsequent settlement (Smulders et al. 2017). These observations suggest that bioturbation mounds serve as starting points for further invasion (Fig. 1c).