Lac Bay

This presentation discusses the impacts of Avrainvillea impacts on Lac Bay

Powerpoint presentation

BSc Thesis

Abstract

Mangrove forests are among the most productive ecosystems on the planet. However, the global mangrove area is decreasing annually by 0.7% - 3%. For mangrove areas in semi-arid to arid climates, salinity is one of the causes for a decrease in mangrove canopy and tree die-off. This process occurs in mangrove forests and the backwaters near the main land. Because of their location and the presence of sediments, water circulation from the seaside becomes limited.

The backwaters near the mainland become shallow, warmer and isolated compared to the rest of the seawater, which results in increasing evapotranspiration and salinity rates. Lac Bay on Bonaire is a place where the salinity of the backwaters increase and where sediments limit the water circulation. In this case study, the electrical conductivity (EC) and sediment depths (SD) are measured to assess the current situation concerning the EC and SD variety in the backwaters of the mangrove forest at Lac Bay. This is done in two different areas in the backwaters: Area 1 and Area 2. In addition, the EC is measured twice to see if the EC changes over time and a third area is used as a reference site. The results show that the measured range of the EC in Area 1 and Area 2 is between 85 mS/cm - 128 mS/cm. The measured range of the sediment depths in Area 1 and Area 2 is between 1 cm – 379 cm. Furthermore, the EC values change over time and, with some exceptions, the greatest values are found the furthest from the feeder channels, which provide water from Lac Bay towards the backwaters. The tides are also a possible factor for the water to flow over a broader mangrove area towards the backwaters. This causes exceptions on the general pattern where EC increases with distance to feeder channels. Overall, the SD gradually increases with distance to the mainland. Some local exceptions from this pattern were measured in Area 2 and could be possible due to irregularities in the underlaying bedrock. In addition, the sediment inflow in the northern part of Area 1 causes some greater values than the surround areas.

These results and conclusions provide a baseline for follow-up research. This follow-up research should focus on factors which will prevent sediment inflow and help to reduce the EC values to make it possible to restore the previous state of the mangroves.

Dutch below

A collaborative effort between Maynooth University, University of Portsmouth and Wageningen University & Research explored the use of satellite technology to offer a cost-effective solution for accurate mangrove mapping within Bonaire’s Lac Bay Forest. This innovative approach empowers small island states to make informed decisions for the management and protection of these vital ecosystems. 

Mangrove ecosystems, crucial for ecological balance and human well-being, are facing severe degradation globally. This issue is particularly true within the Caribbean, where mangroves have declined by 7.9% between 1996 and 2020.  In this context, monitoring and managing these ecosystems has become imperative, yet challenges persist due to accessibility and limited resources availability. A recent study conducted in Lac Bay, Bonaire, presents a groundbreaking solution leveraging Sentinel-2 satellites. 

(Red mangroves. (Rhizophora mangle). Photo Credit: Henkjan Kievit)

Mapping Mangroves 

This recent study evaluated the use of Sentinel-2 satellite imagery to map the extent and species composition of mangrove forests in Lac Bay. Results showed that Sentinel-2 data are a valuable tool, providing accurate maps with a mean overall accuracy of over 95%. Using five Sentinel-2 images, the extent of mangrove forests in Lac Bay was estimated to be approximately 222.3 hectares, comprising mainly of red mangroves (Rhizophora mangle) and black mangroves (Avicennia germinans). 

Remote Sensing Indicators  

This research also attempted to assess the ecological condition of the mangrove forests through biophysical variables, namely Effective Leaf Area Index (used to assess the density of vegetation) and Net Primary Productivity (NPP) (how much energy plants are storing as biomass.   Despite the success in mapping, there were challenges in validating estimates, stressing the need for further refinement.  Estimating NPP based on remote sensing showed promise but needs to be further developed to fully replace traditional monitoring methods.  

Implications for Conservation and Management: 

Using satellites imagery has proven to be a game-changer for monitoring mangrove ecosystems. The study in Lac Bay showcases the potential of this technology to overcome challenges in mapping, assess ecological conditions, and support conservation efforts. As mangroves have recently gained attention for their value as a powerful Nature Based Solution, it will become increasingly important to monitor and preserve these vital coastal ecosystems for the future. 

(Thematic map of the distribution of the black mangrove A. germinans (in blue) and the red mangrove R. mangle (in red) in Lac Bay derived from the Sentinel-2 image registered on 23/03/2022.)

DCNA  

The Dutch Caribbean Nature Alliance (DCNA) supports (science) communication and outreach in the Dutch Caribbean region by making nature related scientific information more widely available through amongst others the Dutch Caribbean Biodiversity Database, DCNA’s news platform BioNews and through the press. This article contains the results from several (scientific) projects but the projects themselves are not DCNA projects. No rights can be derived from the content. DCNA is not liable for the content and the in(direct) impacts resulting from publishing this article. 

Een samenwerking tussen Maynooth University, University of Portsmouth en Wageningen University & Research onderzocht het gebruik van satelliettechnologie om een kosteneffectieve oplossing te bieden voor het nauwkeurig in kaart brengen van mangroven in het Lac Bay-bos op Bonaire. Deze innovatieve aanpak stelt kleine eilandstaten in staat om weloverwogen beslissingen te nemen voor het beheer en de bescherming van deze vitale ecosystemen.

(Rode mangrove. Foto: Henkjan Kievit)

Mangrove-ecosystemen, cruciaal voor het ecologisch evenwicht en het menselijk welzijn, worden wereldwijd geconfronteerd met ernstige degradatie. Dit probleem geldt met name in het Caribisch gebied, waar mangroven tussen 1996 en 2020 met 7,9% zijn afgenomen.  In deze context is het monitoren en beheren van deze ecosystemen absoluut noodzakelijk geworden, maar er blijven uitdagingen bestaan als gevolg van de toegankelijkheid en de beperkte beschikbaarheid van middelen. Een recente studie uitgevoerd in Lac Bay, Bonaire, presenteert een baanbrekende oplossing die gebruik maakt van Sentinel-2-satellieten.

Mangroven in kaart brengen

Deze recente studie evalueerde het gebruik van Sentinel-2-satellietbeelden om de omvang en soortensamenstelling van mangrovebossen in Lac Bay in kaart te brengen. De resultaten toonden aan dat Sentinel-2-gegevens een waardevol hulpmiddel zijn, omdat ze nauwkeurige kaarten opleveren met een gemiddelde algehele nauwkeurigheid van meer dan 95%. Aan de hand van vijf Sentinel-2-beelden werd de omvang van mangrovebossen in Lac Bay geschat op ongeveer 222,3 hectare, voornamelijk bestaande uit rode mangroven (Rhizophora mangle) en zwarte mangroven (Avicennia germinans).

Remote Sensing-indicatoren 

Dit onderzoek probeerde ook de ecologische toestand van de mangrovebossen te beoordelen aan de hand van biofysische variabelen, namelijk Effective Leaf Area Index (gebruikt om de dichtheid van vegetatie te beoordelen) en Net Primary Productivity (NPP) (hoeveel energie planten opslaan als biomassa). Ondanks het succes bij het in kaart brengen, waren er uitdagingen bij het valideren van schattingen, wat de noodzaak van verdere verfijning benadrukte. Het schatten van NPP door middel van remote sensing was veelbelovend, maar moet verder worden ontwikkeld om traditionele monitoringmethoden volledig te vervangen.

Implicaties voor behoud en beheer

Het gebruik van satellietbeelden is een gamechanger gebleken voor het monitoren van mangrove-ecosystemen. De studie in Lac Bay toont het potentieel van deze technologie om uitdagingen bij het in kaart brengen te overwinnen, ecologische omstandigheden te beoordelen en natuurbehoud te ondersteunen. Aangezien mangroven de laatste tijd aandacht hebben gekregen voor hun waarde als een krachtige Nature Based Solution (op de natuur gebaseerde oplossing), zal het steeds belangrijker worden om deze vitale kustecosystemen voor de toekomst te monitoren en behouden.

(Thematische kaart van de verspreiding van de zwarte mangrove A. germinans (in blauw) en de rode mangrove R. mangle (in rood) in Lac Bay, afgeleid van de Sentinel-2-afbeelding geregistreerd op 23/03/2022.)

DCNA

De Dutch Caribbean Nature Alliance (DCNA) ondersteunt (wetenschaps) communicatie en outreach in de Nederlandse Caribische regio door natuurgerelateerde wetenschappelijke informatie breder beschikbaar te maken via onder meer de Dutch Caribbean Biodiversity Database, DCNA’s nieuwsplatform BioNews en de pers. Dit artikel bevat de resultaten van verschillende (wetenschappelijke) projecten, maar de projecten zelf zijn geen DCNA-projecten. Aan de inhoud kunnen geen rechten worden ontleend. DCNA is niet aansprakelijk voor de inhoud en de indirecte gevolgen die voortvloeien uit het publiceren van dit artikel.

Published in BioNews 72

Internship report (DRAFT)

Abstract 

A study was conducted on Bonaire, Caribbean Netherlands, focusing on the White white mangrove (Laguncularia racemosa) to gather information regarding its abundance and distribution, sediment characteristics, state and herbivory. The distribution range of the wWhite mangrove species is quite extensive on Bonaire, Caribbean Netherlands. The species showed different types of adaptations to their environment, such as root system composition and tree height. The overall sediment composition of the White white mangrove species on Bonaire is sand with a limestone foundation underneath. Furthermore, the study on the herbivory brought forth several species that are potentially responsible for the herbivory on the White white mangrove. Overall, this study provides a basebase line data for filling up the knowledge gap on the White white mangrove species on the island of Bonaire and offers a foundation basis for further research and conservation efforts on the species. 

This research was conducted as part of my bachelor internship for the Tropical Forestry specialization of the forest and nature management studies at Van Hall Larenstein, University of Applied Sciences. The internship was conducted at Mangrove Maniacs, an Bonaire based NGO that mainly works to restore the mangrove forests on the island. Field work was conducted between May and July 2022.

Summary
Vegetated coastal ecosystems provide important ecosystem services on which humans depend. Mangrove and seagrass ecosystems function as a nursery for fish, sequester large amounts of carbon and protect our coasts. Mangroves and seagrasses worldwide are threatened by human disturbances like coastal development, tourism, pollution, and climate change. Therefore, the protection of these valuable ecosystems is crucial and understanding underlying dynamics becomes increasingly important. Monitoring restoration efforts of mangroves and seagrasses provides more knowledge on effective restoration measures. On the Dutch Caribbean island of Bonaire, both large areas of mangrove forest and seagrass beds are present. Nature organisations like Mangrove Maniacs and STINAPA work together on mangrove and seagrass restoration. However, there are still knowledge gaps on the most suitable restoration measures for certain areas and there is a lack of monitoring. Therefore, this four month professional internship with Mangrove Maniacs focussed on monitoring mangrove and seagrass restoration efforts. In consultation with the internship host, activities included monitoring a new mangrove restoration pilot in the mangrove forest of Lac Bay, a reforestation area near Lac Bay and a new seagrass restoration experiment at Klein Bonaire. Besides, helping to set up a regional blue carbon network, analysing data and conducting a literature review were also part of this internship, next to joining the weekly Tuesday morning of channel maintenance with the Mangrove Maniacs. With this internship research, I was able to provide new insights on mangrove and seagrass restoration on Bonaire which could help steering future research and restoration plans of the host organisation.

The mangrove forest in Lac Bay, Bonaire, experiences a die-off of trees in its northern area (Awa di Lodo). This die-off is caused by a combination of hypersaline conditions, long inundation periods and excess sedimentation. It is expected that an increase in the tidal exchange between Lac Bay and Awa di Lodo will improve environmental conditions for mangroves to grow. Due to mangrove roots growing into the creeks in combination with sedimentation, the creeks eventually close off, reducing the creek flow. The Mangrove Maniacs are restoring the creeks in Lac Bay (figure 2) to improve creek flow and they want a better understanding of the impacts of their work. This study aims to create more insight into the tidal-induced hydrodynamic processes in Lac Bay and the contribution of creeks in the mangrove forest to the tidal exchange.

During a field campaign from January to March 2022 field data were collected on flow velocities, water levels and topographic characteristics of Lac Bay. The field measurements show that the tidal wave is diurnal and has a negligible delay propagating through the open water of Lac Bay. In Awa di Lodo, high water is reached on average more than four hours later than in the open bay. During spring tide, the tidal range in the open water is sufficiently large to create an increasing trend in the water level in Awa di Lodo. The water level lowers again when the tidal range decreases during neap tide. Flow velocities in the creeks mainly depend on the water level difference between the open water and Awa di Lodo. Both ebb and flood dominant peak velocity asymmetries are observed in the creeks. A flood dominant tidal duration asymmetry in Awa di Lodo indicates that sheet flow during high tides is responsible for the fast increase of the water level in Awa di Lodo while during low tides the creeks are responsible for the outflow.

Based on the data from the field campaign, a hydrodynamic model (Delft3D) was built to analyse the effects of tidal creeks restoration on flow velocities, tidal exchange and water levels of Awa di Lodo (figure 1). The model shows that creeks significantly influence the tidal exchange between the open water and Lac Bay. A new creek connection to Awa di Lodo, either by extending the centre creek or by creating a new creek, is found to be the most efficient to increase the tidal exchange (table 1). It was concluded that the widening of the creeks, deepening of the creeks or extension of the eastern creek system would have a limited effect on the tidal exchange. Creek restoration is shown to be an effective measure to increase the tidal exchange in the mangrove forest of Lac Bay.

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.

1. BackgroundInvertebrates play a critica

l role in maintaining a resilient and healthy environment. These speciesare one of the most globally abundant and diverse animal groups, comprising nearly 80% of alldocumented species to date (Brusca &Brusca, 2002). In fact, these species occupy a wide range oftropic levels, interacting with species throughout the food web.One study (Prather et al, 2020) broke down the importance of invertebrates into the following fourecosystem services:

1) Supporting services. This includes primary production, decomposition, nutrient cycling,hydrologic flux and habitat formation and modification. Within the sediment, invertebratescan dramatically influence water movement, increasing soil porosity (Derouard etal., 1997)and decreasing litter quantity (Wardle, 2002).

2) Providing services. These contributions include serving as a food source, or generatinghousehold goods, inclusion in biochemical or pharmaceutical products as well as a boundlesssupply ofscientific study. For Lac, thequeen conch is an iconic species, whose meat washistoricallyfeatured in local cuisine and shellis still usedas decoration.

3) Regulating services. This includes ability to improve water quality, food web stability,disease regulation within populations as well as pest and invader control. In shallow marineecosystems, bivalves (such as mussels and oysters) can provide substantial water filtrationthroughout the water column.

4) Cultural services. This includes benefits obtained from recreational services and theircultural significance. Many iconic invertebrate species, such as octopus, corals, sponges andconch create a vibrant landscape for scuba divers and snorkelers alike to explore.

Lac Bay has great economic,environmental and cultural value, none of which would be possiblewithout a healthy and robust invertebrate population. These invertebrates are help build resiliencewithin the sandy plateaus, seagrass beds and mangroves, serve as a point of interest forvisitors, andareeither themselves iconic, or vital to the success of other iconic species (such as the flamingo andsea turtle)within the bay.

Historical Data

A study conducted in 1969 by Hummelinck and Roosgave the first qualitative data forqueen conch general distribution throughout Lac Bay. In 2000, a study by Lottpresented the first quantitative data within the same study area. This was followed by asecond survey in 2007. From 2010 onwards (Conch Stock Restoration Project)assessment of queen conch population has been done at irregular intervals (2010,2013, 2015, 2016, 2020).

Results
A total of 43,200 m2 was surveyed and 85 live conchs were found and measured. This resulted in a population density of 19.27 conchs / Ha. To use the Allee effect, only sexually mature conchs should be considered. Over the course of this study, no sexually mature conchs were found , the oldest conch had a lip thickness which measured 6 mm.

Below, Table 1 has been included to summarize the results of the last 5 surveys (2010, 2013, 2015, 2016, 2020). It is interesting to note the dramatic increase between conch populations in 2010 and 2013, and then the rapid decrease in follow on surveys. The results of the 2020 survey highlight a significant issue with only 85 conchs found, none of which having reached sexual maturity. Figure 4 shows a map with the total number of live conchs found per location. There was also a significant number of poached conchs (Table 2/Figure 5) found within the bay. In fact, there were more poached conch shells (100) found than live conch (85), demonstrating that poaching is still an issue which needs to be addressed before the conch population can rebound. 

Each quadrant (quantity 49) equates to 0.09 ha. Using this value, the following densities per year were calculated: 2010 (6.35 conchs/ha), 2013 (51.70 conchs/ha), 2015 (46.49 conchs/ha), 2016 (21.54 conchs/ha), 2020 (19.27 conchs /ha)