Thalassia testudinum

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.

MSc thesis

The economy of Bonaire is highly dependent on tourism. Tourists are drawn to Bonaire because of the beautiful nature and biodiversity in the coastal ecosystems, e.g, mangrove forests, seagrass meadows, coral reefs. Therefore, it is important to protect these coastal ecosystems. Currently, eutrophication and pollution are serious threats to Bonaire’s mangrove forests and seagrass meadows through terrestrial run-off and influx of Sargassum. Seagrasses are known to be sensitive to local environmental changes by bioaccumulation of chemicals and nutrients through absorption in their tissues. In this study, we used turtle grass (Thalassia testudinum) as a bioindicator of spatial and temporal variation in eutrophication and pollution in five different bays on Bonaire. We found that T. testudinum is a good bioindicator of eutrophication and pollution. Analysis of stable isotope signatures (δ13C, δ15N), nutrient (%N, %P, %S) and trace metal content (%As, %Cd, %Co, %Fe, %Mn, and %Ni) in T. testudinum leaf samples
revealed that Lagun was the most eutrophic and polluted bay on Bonaire. The high eutrophic and polluted state in Lagun is mainly due to a nearby landfill, large catchment area, and influxes of pelagic Sargassum spp.. In Lac Bay, higher sulfide stress was observed in T. testudinum leaf tissues in 2019 compared to 2015, which may have hindered the uptake of N and P by T. testudinum. The difference in sulfide stress between 2015 and 2019 is due to the massive influx of Sargassum that occurred in 2018. However, we suggest a possible recovery given better uptake of N and P and lower sulfur content in T. testudinum leaf tissues in 2022. This may indicate less sulfide stress in 2022 compared to 2019. In Lac Bay in 2022, biochemical content of T. testudinum leaf tissues collected at fixed sampling sites where the direct cumulative effect of Sargassum influxes was assumed to be highest (i.e., west of Lac Bay), were similar to tissues collected in areas with no or intermediate direct impact of Sargassum. This may also suggest recovery of T. testudinum that has survived the most severe influx in 2018. We showed that Bonaire’s coastal ecosystems are threatened by eutrophication and pollution through land-based run-off and the influx of pelagic Sargassum. Hence, we want to encourage the local government with this study that nature restoration measures need to be taken immediately to protect their coastal ecosystems.
 

The success of invasive macrophytes can depend on local nutrient availability and consumer pressure, which may interact. We therefore experimentally investigated the interacting effects of nutrient (nitrogen and phosphorus) addition, the exclusion of large herbivorous fishes and mimicked grazing on the expansion rates of the invasive seagrass Halophila stipulacea. The experiments were established on Bonaire and Aruba, two islands in the southern Caribbean, which differ in fish community structure. We observed that multiple Caribbean fish species feed on H. stipulacea. At both study sites, nutrient enrichment decreased invasive leaf carbon:nitrogen ratios. However only on Bonaire, where herbivore fish abundance was 7 times higher and diversity was 4.5 times higher, did nutrient enrichment result in a significant reduction of H. stipulacea expansion into native Thalassia testudinum meadows. This effect was likely due to increased herbivory on nutrient enriched seagrass leaves, as we found that excluding large herbivorous fish (e.g. parrotfish) doubled invasive expansion rates in bare patches on Bonaire. On Aruba, H. stipulacea expansion rates were higher overall, which coincided with lower abundances and diversity of native fishes, and were limited by mimicked fish grazing. We suggest that top-down control by the native fish community may counteract eutrophication effects by increased grazing pressure on nutrient-rich invasive seagrass leaves. We conclude that diverse and abundant herbivore communities likely play an important role in limiting invasion success and their conservation and restoration may serve as a tool to slow down seagrass invasions.

Seagrasses comprise 78 species and are rarely invasive. But the seagrass Halophila stipulacea, firstly recorded in the Caribbean in the year 2002, has spread quickly throughout the region. Previous works have described this species as invasive in the Caribbean, forming dense mats that exclude native seagrass species. During a reconnaissance field survey of Caribbean seagrass meadows at the islands of Bonaire and Sint Maarten in 2013, we observed that this species was only extremely dense at 5 out of 10 studied meadows. Compared to areas with sparse growth of H. stipulacea, these dense meadows showed consistently higher nutrient concentrations, as indicated by higher leaf tissue N contents of the seagrass Thalassia testudinum (dense when C:N < 22.5) and sediments (dense when %N > 11.3). Thus, the potential invasiveness of this non-native seagrass most likely depends on the environmental conditions, especially the nutrient concentrations.

Although reef corals are dependent of the di- noflagellate Symbiodinium, the large majority of corals spawn gametes that do not contain their vital symbiont. This suggests the existence of a pool of Symbiodinium in the environment, of which surprisingly little is known. Reefs around Curac ̧ao (Caribbean) were sampled for free- living Symbiodinium at three time periods (summer 2009, summer 2010, and winter 2010) to characterize different habitats (water column, coral rubble, sediment, the macroalgae Halimeda spp., Dictyota spp., and Lobophora variegata, and the seagrass Thalassia testudinum) that could serve as environmental sources of symbionts for corals. We detected the common clades of Symbiodinium that engage in symbiosis with Caribbean coral hosts A, B, and C using Symbiodinium-specific primers of the hyper- variable region of the chloroplast 23S ribosomal DNA gene. We also discovered clade G and, for the first time in the Caribbean, the presence of free-living Symbiodinium clades F and H. Additionally, this study expands the habitat range of free-living Symbiodinium as environmental Symbiodinium was detected in T. testudinum seagrass beds. The patterns of association between free-living Symbio- dinium types and habitats were shown to be complex. An interesting, strong association was seen between some clade A sequence types and sediment, suggesting that sediment could be a niche where clade A radiated from a free-living ancestor. Other interesting relationships were seen between sequence types of Symbiodinium clade C with Halimeda spp. and clades B and F with T. testudinium. These relationships highlight the importance of some macroalgae and seagrasses in hosting free-living Symbio- dinium. Finally, studies spanning beyond a 1-yr cycle are needed to further expand on our results in order to better understand the variation of Symbiodinium in the environ- ment through time. All together, results presented here showed that the great diversity of free-living Symbiodinium has a dynamic distribution across habitats and time. 

Seagrass beds are globally declining due to human activities in coastal areas. We here aimed to identify threats from eutrophication to the valuable seagrass beds of Curaçao and Bonaire in the Caribbean, which function as nursery habitats for commercial fish species. We documented surface- and porewater nutrient concentrations, and seagrass nutrient concentrations in 6 bays varying in nutrient loads. Water measurements only provided a momentary snapshot, due to timing, tidal stage, etc., but Thalassia testudinum nutrient concentrations indicated long-term nutrient loads. Nutrient levels in most bays did not raise any concern, but high leaf % P values of Thalassia in Piscadera Bay (0.31%) and Spanish Water Bay (0.21%) showed that seagrasses may be threatened by eutrophication, due to emergency overflow of waste water and coastal housing. We thus showed that seagrasses may be threatened and measures should be taken to prevent loss of these important nursery areas due to eutrophication.