st. Eustatius

Delft University of Technology (Master Thesis)

Abstract

Over the years, InSAR has become an indispensable tool in the study of ground deformation, including volcanic deformation, and this continues to be the case in times of improved technology. Since the volcanoes on the Caribbean islands of Saba and St. Eustatius are active, the implementation of an InSAR-based monitoring system is crucial to enhance the spatial resolution of volcano monitoring beyond the capabilities of the ground-based monitoring network, for instance in the case of localized deformations such as dike intrusions. However, technical challenges arise in these tropical settings, caused by dense rainforest, atmospheric artifacts and terrain variability, posing serious challenges to the use of InSAR. Time series InSAR, including SBAS and PSI, can be used to overcome these limitations. A previous study has explored the use of PSI for monitoring, using the already available DePSI software. In this research, an SBAS approach within the Delft InSAR software framework is developed using state-of-the-art Python packages, including (sar)xarray, dask and zarr, and is used to assess whether there is capability to develop SBAS into a volcanic monitoring tool for Saba and St. Eustatius. In addition, a preliminary comparison between the SBAS and PSI methodologies is performed based on a theoretical and (semi-)quantifiable approach. This study combines data from two satellites operating at different wavelengths: Sentinel-1 (C-band) and ALOS-2 (L-band).

Assuming no ongoing deformation, based on GNSS results, the variability of the results around zero can be used as an indicator of precision. The results obtained through SBAS are promising, in particular for L-band, on account of e.g., extensive spatial coverage, efficiency and relatively low variability even with the presence of atmospheric and DEM components. Overall, the results reveal mm order deviations. In the event of volcanic activity, the expected deformation signals are in the range of cm-dm's and can therefore be detected, i.e., with an estimated minimal detectable deformation of 1.5 cm/year in the worst-case scenario. The implementation of three different coherence-based masking approaches-water, single and individual-give an indication of the level of robustness and reliability of the results. Generally, a relatively high level of consistency can be observed among the different masking results of ALOS-2 for both islands, for St. Eustatius following the correction of the unwrapping errors using two testing approaches: an interferogram removal approach and an adaptive approach based on the DIA procedure. The latter procedure allows for retaining all observations and their residuals and is therefore preferred. In contrast, the Sentinel-1 results reveal a lower level of consistency. It is suspected that this inconsistency mainly arises on account of the numerous unwrapping errors within the single masking approach. The individual masking approach appears to be less susceptible to unwrapping errors, however is more prone to outliers than the single masking approach. Further research, following the correction of the atmospheric component and DEM errors, may offer insights into the preferred masking approach. Overall, the use of L-band imagery shows potential, offering spatial coverage where C-band does not, even with limited ALOS-2 data availability and large temporal baselines. The preliminary comparative analysis with the PSI approach, based on the respective strengths and limitations from literature, spatial coverage, processing steps, precision and computational requirements, suggests that a hybrid method could prove to be advantageous to minimize (potential) signal loss, e.g. either from limited spatial coverage or spatial resolution, and enhance volcanic risk assessment. SBAS excels in the extensive spatial coverage, especially using L-band, providing nearly homogeneous coverage of St. Eustatius, even on the flanks of the Quill, and on the outer flanks Mt. Scenery on Saba. However, regardless of the mask, wavelength or method, acquiring coverage around the summit of Mt. Scenery on Saba remains challenging.

The study contributes to advancing InSAR time series analysis for the volcanic monitoring on Saba and St. Eustatius through the successful implementation of an SBAS approach within the Delft InSAR software framework based on state-of-the-art packages, the implementation and evaluation of new approaches to enhance the method in terms of the efficiency and robustness and a comparison with existing software. In addition, the software can be applied in a generic sense for various applications and can be extended for further improvements.

A B S T R A C T

Fish assemblages of different types of artificial reefs can differ greatly in abundance, biomass and composition, with some reef types harboring over five times more herbivores than others. It is assumed that higher herbivorous fish abundance results in a higher grazing intensity, affecting the benthic community by means of enhanced coral recruitment, survival and growth. Territorial fish species might affect this process by chasing away other fish, especially herbivores. In this study we compared the fish assemblage, territorial behavior and grazing intensity by fish on two artificial reef types: reef balls and layered cakes, differing greatly in their fish assemblage during early colonization. In addition, the effect of artificial reef type on benthic development and coral recruitment, survival and growth, was investigated. Although layered cakes initially harbored higher herbivorous fish biomass, this effect was lost during consecutive monitoring events. This seems to be the result of the higher territorial fish abundance around the layered cakes where almost four times more chasing behavior was recorded compared to the reef balls. This resulted in a more than five times lower fish grazing intensity compared to the reef-ball plots. Although macroalgae were effectively controlled at both reefs, the grazing intensity did not differ enough to cause large enough structural changes in benthic cover for higher coral recruitment, survival or growth. The high turf algae cover, combined with increasing crustose coralline algae and sponge cover likely explained reduced coral development. We recommend further research on how to achieve higher grazing rates for improved coral development on artificial reefs, for example by facilitating invertebrate herbivore. 

MSc internship report

The health of coral reefs is threatened by anthropogenic land-based input, which is a global problem. High nutrient conditions make corals less resilient to environmental stresses like climate change and intense weather. Poor water quality is likely for the island of St. Eustatius due to the lack of sewage treatment and its erodible coastline. However, there are no data on this island’s long-term water quality monitoring. Chlorophyll-a concentrations, used to indicate water quality, were monitored at 13 locations around St. Eustatius twice a month from May to November 2022 (n=13). Additionally, images of the ocean floor at 10m were made using a remotely operated vehicle (ROV) to monitor benthic species communities and their habitat. The main conclusion of this research shows that the reefs are primarily in an algal-dominant state. This may be explained by the frequent, chronic exceedances of the 0.2 g/l chlorophyll-a threshold. Chlorophyll -a thresholds were surpassed more frequently and with higher amounts on the sites with a larger anthropogenic influence. The lower threshold for chlorophyll-a was surpassed at 5 out of the 11 sites by more than 30% of the measurements. This would point to a more pervasive low-level eutrophic condition at all sites. On many of the sand-based substrate areas, seagrass has covered it.

For full report or more information,  please contact erik.meesters@wur.nl or gulsah.dogruer@wur.nl

Summary

Over the last 10 years, the Caribbean Netherlands fisheries on Saba and St. Eustatius have been monitored and multiple assessment reports have been made by Wageningen Marine Research (WMR) in collaboration with local Data Monitoring Officers (DMOs). However, due to challenges in collecting the necessary data, there are gaps in the data which can lead to large uncertainties in the current stock assessments and make it difficult to deliver a more detailed assessment of the fisheries and the state of the stocks.

The specific objectives of this report were to present the data challenges and provide recommendations to address the shortcomings in the current data collection. By addressing these and providing solutions, improvements of the Caribbean Netherlands fisheries monitoring program can be made.

The main gaps identified in the data are:

- Limited coverage by the logbook data, especially the case in St. Eustatius. This implies that large raising factors are applied when estimating total effort and landing estimates, which leads to more uncertain estimates.

- Landings not reported by species (at least for the main species) and port sampling for species composition not frequent enough to be able to produce landing estimates and abundance indices at the species level (instead of species groups). For instance in Saba, the number of trips sampled to estimate the length-composition of the landings was on average 60 per year (excluding 2011), with mainly lobster and redfish trips being sampled. On average, around 40 trips per year were sampled for species composition, taken representatively from the different fishing methods. This is less than one catch sampled per week. This is too low and needs to be intensified if data availability and quality are to improve.

- While some species are over-sampled for length-composition, others are not sampled enough to be able to compute reliable length-based indicators.

Our key recommendations are:

o Port sampling and biological data collection-frequency must be stepped up to meet minimum targets.

o Going along with fishers on the vessels, in order to measure catches on location. (Then fishermen won’t have to wait at the harbor for the DMOs work to be done.)

o Facilitate working in morning/midday/evening shifts. This enables data collection after regular working hours, e.g. when fishers come home late in the day (5-6pm).

o Set quantitative targets for data collection. We suggest targeting for a minimum of 70% logbook declarations, activity surveys, catch species composition and weight data (tonnes), while doubling the effort on selected species of importance

o Data collection will now need to include exact biometric data to establish length-weight and fecundity curves, sex ratios and reproductive seasons for individual species, as well as the collection of otoliths from a range of sizes for each species as a basis for age and growth studies by the WMR otolith lab.

o Have DMOs sit in a workspace with a clear view of the harbor where fishers arrive with their catches, so they can immediately act when boats arrive with their catches. This is mainly an issue for the St. Eustatius DMO.

o For bycatch measurements photographing the fish on a cm grid surface can save measuring time in port or on vessels. o Increase willingness of fishers to participate in data collection. o Incentivize fishers to participate by organizing regular (bimonthly or quarterly) gettogethers where the DMOs update fishers on some monitoring results, providing snacks and drinks.

o Provide dedicated freezer storage space for fishers at the harbor, enabling DMOs more time for the port sampling. Fishers willingness to wait for port sampling is understandably limited. By providing dedicated freezer storage facility, the DMOs can take extra time needed for sufficient biological sampling (i.e. species composition, length, sex) while the catch of the fishers stays fresh. The same can be done for lobster catches if a port-based holding area is provided. 

o Provide modern technologies to the fishers and/or DMOs, e.g. Electronic Reporting Systems (ERS) such as electronic logbooks, and GPS systems such as the Vessel Monitoring System (VMS).

o Arrange for closer involvement of WMR in work planning for the island DMO’s

Coralita is an invasive plant species, rapidly spreading across St. Eustatius.  A recently published report highlighted the ability to use satellite imagery to systematically map Coralita’s distribution over the island.  The approach could provide key insights into how habitat and vegetation are changing over time to aid in conservationists’ efforts to minimize the negative effects of Coralita and similar invasive species.

Coralita overgrowth (Source: Achsah Mitchell)

Coralita is a fast-growing, climbing vine with beautiful pink or white flowers. Originally from Mexico, Antigonon leptopus started out as a popular garden plant, but has expanded its territory and is now aggressively invading natural areas. Its fast-growing nature means it can outcompete most native species for terrain, quickly making it the dominant species, and reducing overall diversity. This is especially the case on St. Eustatius, where ground surveys indicate the plant already appears on 15-33% of the island.

Mapping Techniques

One of the biggest issues in controlling invasive species is accurately accounting for its presence, particularly if data needs to be collected over a wide area. This is where satellite imagery can help by providing an affordable, high spatial resolution option. A new collaborative study from the Utrecht University, University of Zurich, Wageningen University, and the Technical University of Braunschweig provided key insight by using such satellite imagery to identify Coralita. The method is successful, as areas dominated by Coralita emit a relatively distinct electromagnetic signal that can be detected by satellites. Once the distribution of Coralita has been mapped using this technique, it is possible to identify the environmental conditions associated with Coralita’s presence. This approach provides a relatively low-cost solution that is powerful, accurate and repeatable; key in identifying and monitoring its spread in the future.

“In creating this map,” said Elizabeth Haber, first author of this study, “it was my hope to produce something that could be useful for those who are caring for and protecting the incredibly special nature on Statia.”

Results

Using this method, researchers sampled 162 locations across St. Eustatius and estimated that Coralita was the dominant canopy cover (>50%) on over 3% of the island (64 ha). Perhaps more importantly, this map also showed that Coralita was not randomly distributed but generally found, for example, in areas of water accumulation, near roads or near drainage channels. Furthermore, Coralita was often found in grasslands and areas of development and is relatively rare in natural forests, highlighting how human disturbances can promote the spread of Coralita. It is important to note that data filtering and physical limitations of using satellite imagery means that Coralita growing under trees or shrubs or in smaller patches is likely underrepresented in this study.

Map of the distribution of Coralita on St. Eustatius (Haber et al., 2021)

Conservation Implications

Even with the physical limitations, the fact that this study is cost effective and repeatable means that consistent comparisons of Coralita’s distribution can be made over time. These comparisons are vital in understanding how populations and habitats are shifting, granting conservationists a fantastic tool in forecasting the spread of invasive species. Arguably the greatest asset of the Caribbean is its vast biodiversity. Already threats of climate change, habitat loss, invasive species and urban development are upsetting this fragile balance. St. Eustatius, although small, is home to several endemic plant species, two of which are the Statia morning glory and Statia milkweed, along with the critically endangered Lesser Antillean Iguana which could all be threatened by the habitat alterations of Coralita growth.

To read more, please find the full report on the Dutch Caribbean Biodiversity Database using the link below.

https://www.dcbd.nl/document/high-spatial-resolution-mapping-identifies-...

Article published in BioNews 48

St. Eustatius is a small island in the Caribbean that belongs to the Kingdom of the Netherlands. Its existing harbour is due for upgrades and repairs on existing structures have been discussed in addition to the possibility of constructing a new harbour. The island is rugged, and the landward side of the coast is surrounded by cliffs except for a small stretch of coast where the hinterland is quite flat.

The island is not self­sufficient and all cargo comes in via the existing harbour. These cargo flows coming from the harbour are transported through the touristic centre of the island making for some unpleasant and dangerous situations. Additionally, the harbour is mostly used by large cargo vessels and leaves no room for leisure ships. The government of the island sees the potential to attract more tourists, but is also aware that the current state of the facilities could be improved. The objective of this Master’s thesis is to explore the feasibility of constructing a new harbour on the west coast of St. Eustatius and to find a suitable layout and a conceptual design for a new harbour. This will be done following the regular design approach in which the design steps are conceptualisation, verification, evaluation and selection. The location of the new harbour has already been determined in earlier research and is shown in Figure 1. The location is chosen where the hinterland is quite flat.

The conceptualisation starts with gathering relevant information about the requirements and boundary conditions of the project. The future cargo volumes and future vessel dimensions are estimated on which the dimensions of the required facilities of the harbour are based. These cargo volumes are determined with growth of population, welfare and the tourism sector in mind. Environmental conditions and hydrodynamic conditions of the proposed project location are also gathered. Data includes information of topography, bathymetry, soil characteristics, archaeological remains, environmental and aviation regulations and also data about waves, wind, currents and water levels. Here, it is found that the off­shore wave heights during hurricane conditions are quite significant but break on the foreshore and subsequently lose energy. On the contrary, the daily wave climate is relatively calm. It is confirmed by the gathered data that the location of the new harbour has potential for construction since the hinterland is relatively flat and is located out of the zones of important nature parks. However, it is expected that significant amounts of historic remains are found in the area which need to be removed before construction. For future design steps it is advised to gather more accurate data.

The government of St. Eustatius is the client and has some clear wishes and demands. First and foremost, the harbour should be built in phases. The first phase would account for the current and future cargo volumes to the island. The second phase would be the expansion of the harbour to function as a transshipment hub and develop as a competitor to the congested harbour of St. Maarten. This way, the funding of the expansion can partly come from the operational profits of the harbour. The necessary buildings and facilities, including their dimensions, have been found in conversations with the client and through calculations. The required wet areas of the port and the elevation levels were found based on the dimensions of the design vessel. The road leading to the new harbour will be constructed on the already existing dirt road which needs upgrading.

In the development of the harbour layouts three alternatives have been developed. These are developed with the small scale of the harbour in mind which lead to the choice of discarding a northern breakwater. This choice is justified since the data showed that the waves during daily conditions almost solely come from the south. The turning circle has been moved out of the harbour basin too to save space which is justified based on the limited amounts of movement. The alternatives differ in their orientation of the berth and the usage of the breakwater as a berth. The conceptualisation step is finished by discussing the modifications on these alternatives. A shift of the harbour landwards, placement of the storage on land or water and a combination of both is considered.

The verification of these layout alternatives is done by analysing the possibility of constructing a breakwater, the impact on the morphology of the island and studying the wave penetration of the harbour basin of every alternative.

The breakwater of the different alternatives is situated at different water depths which makes for the design under different conditions. For the design of the breakwater two types, rubble mound and caisson, are considered, that have (dis)advantages under certain circumstances. It needs to be verified that a design for both types of breakwater can be designed for the different water depths. It turns out that the waves at the deeper location are too large to be able to construct a rubble mound breakwater there. In some alternatives, the use of a caisson breakwater could not be verified as a result of the small water depth. This is a result of the economic disadvantage of a caisson breakwater at such a shallow depth. These designs are subsequently dropped from the analysis.

The harbour should not have a negative influence on the morphology of the island and the sediment transport should not have a negative impact on the construction and operation of the harbour. The west coast of the island experiences very minor sediment transport in the northern direction as a result of lack of sediment availability in the surf zone. It is found that the construction of the harbour does influence the coastline in a positive way as the breakwater is able to capture the limited available sediment. The harbour would be an excellent contribution in combination with beach nourishment since the historic structures on the coastline are once again protected.

The last topic of the verification step treats the wave penetration into the harbour basin. Here, the wave direction has been divided into three main wave directions. The wave penetration has been determined on a basis of refraction and diffraction diagrams and the wave height at the berths has been calculated. The downtime of every alternative per wave direction showed to be on the high side after the comparison with allowable wave heights. The designs have been altered by means of an elongation of the breakwater to create calmer conditions and lower the downtime to be under the 1.1%. This was not possible in one alternative where the safety of navigation would be diminished and this alternative has been discarded. The harbour oscillations as a result of resonance of long ocean waves is also investigated. Here, the measured waves in the existing harbour are investigated and compared to the natural periods of the alternative layouts. Even though the natural periods of the harbour concepts are situated further from the measured period of the waves in the existing harbour, there still is a chance resonance will occur. More investigation into this subject is advised in more detailed design steps.

The final step in this analysis is the evaluation and selection of the most suitable alternative. This has been done by a multi criteria analysis in which weighted scores have been determined. The ratio between the costs and the weighted score of every alternative would result in a most suitable alternative which can be seen in Figure 3. This design is partly dug in into the land and is executed with a rubble mound breakwater. It might be possible to gather the required material from the island but it is highly likely that these need to be imported.

The price of this design, excluding the construction of the harbour buildings and the road, is estimated to be in the order of $35 million (±50%) and is very much influenced by the costs of the breakwater. The cross­section of the breakwater of the final design can be found in Figure 4.

This Master’s thesis is a first step for the exploration into the possibilities regarding the construction of a new harbour on the island of St. Eustatius. Therefore, the availability of data has not always been guaranteed and resulted in the need to estimate, or assume, necessary information. It is advised, in following design steps, to gather more accurate data by performing measurements and real­life experiments. The level of accuracy of the data should match the advance of the detail of design. Specifically, the structure of the subsoil and the wave behaviour should be investigated more thoroughly. Scale model tests and on­site measurements are part of this additional gathering of information and verification. Conclusions from further research might alter the design.

The massive die-off of the herbivorous sea urchin Diadema antillarum in 1983 and 1984 resulted in phase shifts on Caribbean coral reefs, where macroalgae replaced coral as the most dominant benthic group. Since then, D. antillarum recovery has been slow to non-existent on most reefs. Studying settlement rates can provide insight into the mechanisms constraining the recovery of D. antillarum, while efficient settlement collectors can be used to identify locations with high settlement rates and to collect settlers for restoration practices. The aim of this study was to compare pre and post die-off settlement rates and to determine possible settlement peaks in the Eastern Caribbean island of St. Eustatius. Additionally, we aimed to determine the effectiveness and reproducibility of five different settlement collectors for D. antillarum. D. antillarum settlement around St. Eustatius was highest in May, June and August and low during the rest of the study. Before the die-off, settlement recorded for Curaçao was high throughout the year and was characterized by multiple settlement peaks. Even though peak settlement rates in this study were in the same order of magnitude as in Curaçao before the die-off, overall yearly settlement rates around St. Eustatius were still lower. As no juvenile or adult D. antillarum were observed on the reefs around the settlement collectors, it is likely that other factors are hindering the recovery of the island's D. antillarum populations. Of all five materials tested, bio ball collectors were the most effective and reproducible method to monitor D. antillarum settlement. Panels yielded the least numbers of settlers, which can partly be explained by their position close to the seabed. Settler collection was higher in mid-water layers compared to close to the bottom and maximized when strings of bio balls were used instead of clumps. We recommend research into the feasibility of aiding D. antillarum recovery by providing suitable settlement substrate during the peak of the settlement season and adequate shelter to increase post-settlement survival of settlers. The bio ball collectors could serve as a suitable settlement substrate for this new approach of assisted natural recovery.

ABSTRACT

Catastrophic events, like hurricanes, bring lethal conditions that can have population-altering effects. The threatened Caribbean dry forest occurs in a region known for its high-intensity hurricane seasons and high species endemism, highlighting the necessity to better understand hurricane impacts as fragmentation and clearing of natural habitat continues. However, such studies remain rare, and for reptiles are mostly restricted to Anolis. Here we used single-season occupancy modeling to infer the impact of the intense 2017 Atlantic hurricane season on the critically endangered Lesser Antillean Iguana, Iguana delicatissima. We surveyed 30 transects across eight habitats on St. Eustatius during 2017-2019, which resulted in 344 individual surveys and 98 iguana observations. Analyses of abundance and site occupancy indicated both measures for 2018 and 2019 were strongly reduced compared to the pre-hurricane 2017 state. Iguanas at higher elevations were affected more profoundly, likely due to higher wind speeds, tree damage and extensive defoliation. Overall, our results indicate a decrease in population estimates (23.3-26.5%) and abundance (22-23.8%) for 2018 and 2019, and a 75% reduction in the number of opportunistic sightings of tagged iguanas between 2017-2018. As only small and isolated I. delicatissima populations remain, our study further demonstrates their vulnerability to stochastic events. Considering the frequency and intensity of hurricanes are projected to increase, our results stress the urgent need for population-increasing conservation actions in order to secure the long-term survival of I. delicatissima throughout its range.

Abstract

We estimated population densities of the red-bellied racer (Alsophis rufiventris) on the Caribbean island of St. Eustatius in 2011, 2018 and 2019 to determine the likely influence of hurricanes Irma and Maria (September 2017), in addition to evaluating abiotic parameters which may be correlated with its presence. Surveys were conducted at seven sites in 2011 prior to the hurricanes, and at 81 and 108 sites in 2018 and 2019 respectively posterior to the hurricanes. A total of 8.2 ha was surveyed in 2011, and 11.42 ha in 2018/2019. The pre-hurricane (2011) racer density estimate was 9.2/ha (min 7.3 - max 11.6); post-hurricane estimates were 4.6/ha (min 3.4 - max 6.0) in 2018 and 5.0/ha (min 3.8 - max 6.5) in 2019. The pre-hurricane encounter rate of individual racers was 16.0 snakes/hour compared to 0.34 snakes/hour in 2018 and 0.41 snakes/hour in 2019 (post-hurricane). The decrease in encounter rates between 2011 and 2019 implies a negative impact of the hurricanes on racer abundance. Based on calculations of detection probability (0.02 in 2018 and 0.03 in 2019), post-hurricane lambda estimates were 1.82 (95% CI 0.66 - 5.01) in 2018 and 1.60 (95% CI 0.39 - 6.65) snakes/ha in 2019. Given the current small size of the remaining population and the presence of invasive species across the snake’s range, this species could be at risk of local extirpation. We suggest conservation actions such as invasive species management and habitat restoration to enable further recovery.

In oktober 2010 kregen Sint Eustatius, Bonaire en Saba de status van bijzondere Nederlandse gemeente. Dit houdt in dat Nederland de medeverantwoordelijkheid moet nemen voor de bescherming van een aantal unieke tropische natuurgebieden. Zo ook op Statia, zoals Sint Eustatius in de volksmond wordt genoemd.