Lac Bay

Vegetation analysis of White Mangrove, Bonaire Caribbean Netherlands

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. 

 

Date
2023
Data type
Research report
Theme
Research and monitoring
Report number
Project Internship
Geographic location
Bonaire

A study of the effect of salinity and soil composition on the survival rate of Conocarpus erectus planted on Bonaire.

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.

Date
2022
Data type
Research report
Theme
Research and monitoring
Report number
Forestry and Nature management Internship Report
Geographic location
Bonaire

Mangrove and Seagrass Restoration on Bonaire

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.

Date
2023
Data type
Research report
Theme
Research and monitoring
Report number
Internship Report
Geographic location
Bonaire

Poster - THE ROLE OF CREEKS FOR TIDAL EXCHANGE IN THE MANGROVE FOREST OF LAC BAY, BONAIRE

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.

Date
2022
Data type
Media
Theme
Research and monitoring
Geographic location
Bonaire
Author

Report on seagrass and Halimeda monitoring in Lac Bay 2022

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.

 

Date
2022
Data type
Research report
Theme
Research and monitoring
Geographic location
Bonaire

Report on Invertebrate Populations Lac Bay 2022

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.

Date
2022
Data type
Research report
Theme
Research and monitoring
Geographic location
Bonaire

Report on Queen Conch (Lobatus gigas) Population Monitoring in Lac Bay 2020

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) 

Date
2021
Data type
Research report
Theme
Research and monitoring
Geographic location
Bonaire

Report on Queen Conch (Lobatus gigas) Population Monitoring in Lac Bay 2022

1.Background

The queen conch, orLobatus gigas, is an iconic species found within the Caribbean, beingboth economically andsocially important.Famous for its unique and beautiful shell, alongwith its role as a popular item in local dishes, this species is heavily fishedand, in someareas,highly threatened.A project started in the 1980's, Marcultura, worked to boost localpopulations by cultivating conch to be released in the bay (Hensen,1983). Unfortunately,there were no long-term positive impacts to the conch populations after this project.Since November 1992, queen conch havebeen listed as an CITES Appendix II species,which means it is at risk of endangerment. However, due to concerns over local populations,taking conch from Bonaire has been forbidden since 1985. Only legal imports (from countrieswith CITES export permits) are allowed. Unfortunately, poaching is still an ongoing issue forthe island.The conch middens (old shells) that can be seen in large piles along the Lac Caibeach are from conch caught locally and brought in from the Aves Islands.Other species in the genusLobatusandStrombusareLobatus raninus (hawkwing conch),Lobatus costatus(milk conch) andStrombus pugilus (fighting conch) also occur on Bonaire.Lobatus costatusare taken incidentally but their shell is verythick,and they have very littlemeat.

Date
2022
Data type
Research report
Theme
Research and monitoring
Geographic location
Bonaire

The role of creeks for tidal exchange in the mangrove forest of Lac Bay, Bonaire

Nederlands below.

The mangrove forest of Lac Bay, Bonaire, is experiencing a die-off of trees in its northern area. Increasing the tidal exchange by creek restoration likely increases the living conditions for the mangrove trees. In the first months of 2022, a collaboration between the Mangrove Maniacs and the University of Twente was set up to investigate the hydrodynamic properties of the area and looked into the effects of creek restoration.

 

Creek in Lac Bay opened by the Mangrove Maniacs. Photo source: Rob van Zee

Lac Bay

Mangrove systems worldwide promote ecological diversity while also being economically valuable for humanity. If a mangrove forest experiences die-back, it loses the services it provides. One such case is the mangrove forest of Lac Bay, Bonaire. The northern side of Lac Bay, also known as Awa di Lodo, is experiencing a die-back of mangroves. Unsustainable overgrazing by livestock on Bonaire has depleted the area of ground cover vegetation resulting in the wind, vehicle traffic, and rainwater mobilizing and transporting sediment into Awa di Lodo. The excess sediment in combination with growing mangrove roots clogs lagoons and creeks, eventually closing off these creeks, reducing the hydrological connectivity between the front and back of the forest. High evaporation rates and a low influx of freshwater create hypersaline conditions in Awa di Lodo. The Mangrove Maniacs are trying to open the mangrove creeks so the tidal exchange (the tide-induced volume of water reaching Awa di Lodo) increases, lowering the salinity values in the area. However, it is yet unclear to what extent the existing creeks contribute to the tidal exchange in Lac Bay and to what extent creek restoration can improve the tidal exchange in Lac Bay.

 

Research

At the start of the year 2022, a group of researchers from the University of Twente monitored water levels and velocities and mapped topographic characteristics of Lac Bay. One of the goals was to create insight into the hydrodynamics of the mangrove system through in-depth analysis of this data. The propagation of the tidal wave, relations between the water levels and velocities and the tidal asymmetry were investigated. Next, a numerical model was developed to quantify the influence of creeks on the tidal exchange and to investigate the effect of creek restoration on the tidal exchange. To quantify the tidal exchange, the residence time of the water in Awa di Lodo was computed, which is the time it takes (in days) for the tidal exchange to completely replace the total water volume of Awa di Lodo.

 

Results

The obtained data shows that the diurnal tidal wave has a negligible delay in 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 tides, 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 in Awa di Lodo lowers again when the tidal range decreases during consecutive neap tides. Flow velocities in the creeks mainly depended 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. The observed 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.

The hydrodynamic model showed that creeks significantly influence the tidal exchange from the open water in Lac Bay to Awa di Lodo (Table 1). A new creek connection to Awa di Lodo, preferably by extension of the creek through the center of the mangrove system, is found to be the most efficient to increase the tidal exchange. It was also concluded that the widening of the creeks, deepening of the creeks or the extension of the eastern creek system would have a limited effect on the tidal exchange. Hence creek restoration is shown to be an effective measure to increase the tidal exchange in the mangrove forest of Lac Bay.

 

Table 1: Tidal exchange, residence time and the ratio of the residence time of any of the scenarios compared to the reference scenario. Red coloured rows indicate a decrease in tidal exchange and blue coloured rows indicate an increase in tidal exchange. A darker shade implies a greater decrease/increase

 

Impacts on the future

The data analysis and the developed hydrodynamic model will be important tools for the Mangrove Maniacs to make decisions on where to open new creeks and to study the impact of their work. By having more insight into the hydrodynamics of Lac Bay, the mangrove restoration will become more effective and thus increase the ecological value of the area.

For more information, you can read the full report using the DCBD link below.

More info in the Dutch Caribbean Biodiversity Database

 

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De invloed van kreken op de getijdenuitwisseling in het mangrovebos van Lac Bay, Bonaire

Het mangrovebos van Lac Bay, Bonaire, ervaart het afsterven van bomen in het noordelijke gebied. Er is een goede kans dat het verhogen van de getijdenuitwisseling door het herstellen van mangrove kreken de leefomstandigheden van de bomen verbeterd. In de eerste maanden van 2022 is een samenwerking opgezet tussen de Mangrove Maniacs en de Universiteit Twente om de hydrodynamische eigenschappen van het gebied en de effecten van kreekherstel te onderzoeken.

Kreek in Lac Bay die geopend is door de Mangrove Maniacs. Photo source: Rob van Zee

Lac Bay

Wereldwijd bevorderen mangrovesystemen ecologische diversiteit terwijl ze ook van economische waarde zijn voor de mensheid. Als een mangrovebos sterft, verliest het een voor een groot deel de waarde die het kan toevoegen aan de wereld. Een voorbeeld hiervan is het mangrovebos van Lac Bay, Bonaire. Het gebied aan de noordkant van Lac Bay, ook wel bekend als Awa di Lodo, ervaart het afsterven van mangroven. Niet-duurzame overbegrazing door vee op Bonaire heeft het gebied ontdaan van bodem bedekkende vegetatie, waardoor wind, autoverkeer en regenwater de kans hebben om sediment naar Awa di Lodo te transporteren. Overtollig sediment in combinatie met de groeiende mangrovewortels verstoppen de lagunes en kreken waardoor deze uiteindelijk dicht komen te zitten. Hierdoor wordt de hydrologische connectiviteit tussen de voor- en achterkant van het bos wordt verminderd. Hoge verdampingswaarden en een lage instroom van zoet water creëren extreem zoute omstandigheden in Awa di Lodo. De Mangrove Maniacs proberen de  mangrove kreken te openen zodat de getijdenuitwisseling (het volume water dat door het getij Awa di Lodo bereikt) toeneemt, waardoor het zoutgehalte in het gebied daalt. Het is echter nog onduidelijk in hoeverre de bestaande kreken bijdragen aan de getijdenuitwisseling in Lac Bay en in hoeverre kreekherstel de getijdenuitwisseling in Lac Bay kan verbeteren.

 

Onderzoek

Begin 2022 heeft een groep onderzoekers van de Universiteit Twente de waterstanden en snelheden gemeten en topografische kenmerken van Lac Bay in kaart gebracht. Een van de doelen een diepgaande data-analyse om inzicht te krijgen in de hydrodynamica van het mangrovesysteem. Er is gekeken naar de voortplanting van de vloedgolf, relaties tussen de waterstanden en watersnelheden en naar de getijdenasymmetrie. Vervolgens werd een numeriek model ontwikkeld om de invloed van kreken op de getijdenuitwisseling te kwantificeren en om het effect van kreekherstel op de getijdenuitwisseling te onderzoeken. Om de getijwisseling te kwantificeren is de verblijftijd van het water in Awa di Lodo berekend, dat de tijd die nodig is (in dagen) voordat de getijwisseling het totale watervolume van Awa di Lodo volledig heeft vervangen.

 

 

Resultaten

Uit de verkregen data blijkt dat de dagelijkse vloedgolf een verwaarloosbare vertraging heeft in het open water van Lac Bay. In Awa di Lodo wordt het hoogwater gemiddeld meer dan vier uur later bereikt dan in de open baai. Tijdens springtij is het getijverschil in het open water groot genoeg om een ​​stijgende trend in het waterpeil in Awa di Lodo te creëren. Het waterpeil in Awa di Lodo daalt weer wanneer het getijverschil afneemt bij opeenvolgende doodtij. Stroomsnelheden in de kreken zijn vooral afhankelijk van het waterpeilverschil tussen het open water en Awa di Lodo. In de kreken worden zowel eb- als vloed-dominante pieksnelheidsasymmetrieën waargenomen. De waargenomen vloed-dominante asymmetrie in de getijdenduur in Awa di Lodo geeft aan dat de stroming tussen de mangrovebomen door tijdens hoogwater verantwoordelijk is voor de snelle stijging van het waterpeil in Awa di Lodo. Tijdens eb zijn juist de kreken verantwoordelijk voor de uitstroom en de daling van het waterpeil in Awa di Lodo.

Het hydrodynamische model toonde aan dat kreken een significante invloed hebben op de getijdenuitwisseling van het open water in Lac Bay naar Awa di Lodo (Tabel 1). Een nieuwe kreekverbinding met Awa di Lodo, ​​bij voorkeur door verlenging van de kreek door het midden van het mangrovesysteem, blijkt het meest efficiënt te zijn om de getijdenuitwisseling te vergroten. Ook werd geconcludeerd dat de verbreding van de kreken, de verdieping van de kreken of de uitbreiding van het oostelijke krekenstelsel een beperkt effect op de getijdenuitwisseling zou hebben. Daarom is aangetoond dat kreekherstel een effectieve maatregel is om de getijdenuitwisseling in het mangrovebos van Lac Bay te vergroten.

 

Tabel 1: Getijdenuitwisseling, verblijftijd en de verhouding van de verblijftijd van elk van de scenario’s ten opzichte van het referentiescenario. Roodgekleurde rijen duiden op een afname van de getijwisseling en blauwgekleurde rijen duiden op een toename van de getijwisseling. Een donkerdere tint impliceert een grotere afname/toename

 

Impact op de toekomst

De data-analyse en het ontwikkelde hydrodynamische model zullen belangrijke instrumenten zijn voor de Mangrove Maniacs om beslissingen te nemen over waar nieuwe kreken te openen en om de impact van hun werk te bestuderen. Door meer inzicht te krijgen in de hydrodynamica van Lac Bay zal het herstel van het mangrovebos effectiever worden en zal daarmee de ecologische waarde van het gebied vergroten.

More info in the Dutch Caribbean Biodiversity Database

 

 

Published in BioNews 57.

 

Date
2022
Data type
Media
Theme
Research and monitoring
Geographic location
Bonaire
Author

The role of creeks for tidal exchange in the mangrove forest of Lac Bay, Bonaire

Abstract

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 decrease the mangrove stressors in Awa di Lodo creating improved environmental conditions for mangroves to grow. The tidal exchange consists of flow through the vegetated forest (sheet flow) and through the creeks (creek flow). Awa di Lodo has two main creeks connections to the forest fringe, the eastern and the western creek system. Due to mangrove roots growing into the creeks in combination with sedimentation, the creeks eventually close off and thereby reduce the creek flow. The Mangrove Maniacs are restoring the creeks in Lac Bay to improve environmental conditions for mangroves 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 campaign spanned three spring-neap tidal cycles. The analysis of the gathered data was combined with a literature study to investigate the hydrodynamic characteristics in the area. 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.

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, meaning that larger water level differences induce larger flow velocities. In addition, the western creek connecting the bay with Awa di Lodo shows a strong flood dominant peak velocity asymmetry, while the eastern creek varies from marginally flood dominant during spring tides, to strongly ebb dominant during neap tide. 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.

The hydrodynamic model simulations of the tidal dynamics in Lac Bay replicate the magnitude of the measured flow velocities in the creeks of Lac Bay. The model does not show the measured ebb flow in the western and centre creek. Water levels in Awa di Lodo are modelled well, except for an overestimation of low water levels and an overestimation of the water level increase. It was found that creeks have a significant influence on the tidal exchange between the open water and Lac Bay. The model showed that the creation of 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. 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.

Date
2022
Data type
Research report
Theme
Research and monitoring
Report number
University of Twente
Geographic location
Bonaire
Author