Engel, S.

Healthy mangroves act as carbon sinks, storing a variety of greenhouse gases.  As mangroves degrade this ability is lost, but to what extend is still unknown. A 2019 study of Bonaire’s mangroves worked to analyze the differences between intact and degraded mangroves’ ability to store carbon. This work proved that preserving healthy wetlands is crucial in the fight against climate change.

Greenhouse gases, such as carbon dioxide (CO2) and methane (CH4), play an important role in accelerating global warming, worsening the conditions of climate change. When in balance, there are environmental systems in place which can trap greenhouse gases and allow an equilibrium to be reached.  Wetlands serve as a prime example of areas where productive plant communities are capable of storing and using large amounts of carbon through decomposition and photosynthesis.

Blue Carbon

Carbon which is stored within coastal environments has become known as “blue carbon” and could become key in building resilience against climate change moving forward. Of these blue carbon areas, mangroves are some of the most carbon-rich ecosystems on earth.  Through their dense leaf canopies and complex root systems, it is estimated that they are able to store carbon at a rate of 50 times higher thantropical rainforests.

Unfortunately, these areas are under threat.  Recent estimates have found that nearly one-third of these forests have been removed due to coastal development and land conversion.  In the Caribbean alone, nearly 24% of mangrove area was lost between 1980 and 2005.  When these areas are destroyed, not only do we lose the benefits of future carbon storage, but we begin adding carbon to the atmosphere that had been previously trapped in the sediment.

Scientists are just beginning to understand the importance of these ecosystems.  New policies are being drafted to advocate for these areas as important carbon sinks and policymakers are working to imbed these concepts into future climate change mitigation strategies.  Although the differences between healthy and clear-cut mangrove forests have already been studied, there is still a lack of information concerning forests which slowly degrade.  This slow degradation is generally the result of deteriorating environmental conditions, which causes trees to gradually die off.  As climate conditions continue to harshen, it can be expected that the remaining mangrove forests could see an increase in gradual die off, so understanding how this impacts their ability to function as a carbon sink will become critical.

Lac Bay

This is the case on Bonaire, where the mangrove forests around Lac Bay have been in gradual decline for decades.  A recent study conducted by the University of Bremen and the Leibniz Center for Marine Tropical Research and STINAPA Bonaire worked to understand these differences by quantifying the carbon sink capabilities of healthy and gradually degrading mangrove areas.

The area which was studied experience high sediment run off, as overgrazing and urban development have removed ground vegetation which would normally minimized erosion.  This high sediment run off has caused infilling within the mangrove forest, minimizing water circulation and creating areas of stagnant and hypersaline waters.  These conditions have led to the gradual die back of mangroves.  The presence of healthy and degraded mangroves within the same forest made Bonaire the perfect location to study the differences in these environments to better understand the carbon dynamics of these areas.

The Study

Measurements were taken between January and March of 2019.  17 plots of intact mangroves and 15 plots of degraded mangroves were selected.  In the end, a striking difference was found between these two areas.  Healthy, intact mangroves were seen to have larger amounts of both above ground (leaves, branches, trunks) and below ground (roots, sediment) biomass than those in degraded areas.  Degraded areas had very little aboveground biomass, resulting in less photosynthesis, less sedimentation and more erosion, chemical weathering and higher rates of decomposition within the sediment.  This complete loss of aboveground carbon capture and erosion of sediment meant that these areas could no longer be considered a carbon sink, but in fact act more as a carbon source, allowing previously trapped carbon to reenter the atmosphere or neighboring waters.

The Future of Mangroves

Interestingly, this study found that carbon left the slowly degrading areas slower than in forests where mangroves were intentionally cleared.  This could be important for future climate change mitigation plans as scientists believe that climate change will increase aridity in parts of the Caribbean, Central and South America and South Asia altering hydrology and causing seasonal hypersalinity which will lead to the gradual die off of large amounts of remaining forests. Understanding these differences will be key in forecasting the ability for natural areas to serve as carbon sinks in the future. This study proved that slowly degrading mangroves are no longer functioning as carbon sinks and efforts must be made to keep the remaining forests intact and healthy if we hope to find more natural solutions to minimizing our carbon footprint.

https://www.dcbd.nl/document/impacts-wetland-dieback-carbon-dynamics-com...

Article publish in Bionews 41

Abstract
In recent decades, concern about mangrove die-back has intensified. One of the last remaining mangrove areas in the Dutch Antilles is the mangrove forest in Lac bay on the island of Bonaire.
It has a relatively large biodiversity and is essential for many bird species. Not only the inhabitants of the forest itself – also many other species outside the forest – depend on it. This is because the filtering effect of mangrove roots prevents excessive sediment deposition on sea grass beds and coral reef. Unfortunately, extensive mangrove die-back in Lac bay has been observed in recent years. The hypothesis is that this die-back is caused by hyper-salinity. This study focuses on subsurface freshwater flow from southern Bonaire into Lac bay as a source of refreshment to the mangrove area.
In the rainy season 2012-13 (October to January), the hydrological system in the area surrounding Lac bay was intensively investigated. Measurements included rainfall, hydraulic conductivity, groundwater levels and open water salinity in the backwaters of Lac bay.

The geo-hydrological properties of limestone were found to largely determine groundwater flow on both regional and local scale. On a regional scale hydraulic conductivity and degree of karstification determines gradients, hydraulic contact to Lac bay and responses to rainfall. 4 and 5km north of Lac bay, hydraulic conductivities were found to be 1.3 and 2.7m d-1, respectively. The relatively low values are accountable for: 1) steep gradients of 300-380cm km-1 of the southward groundwater flow; 2) poor hydraulic contact to Lac bay; and 3) strong groundwater responses to heavy rainfall. For instance, a rise in groundwater level of 4m was observed in only 9 days.
Further south, extensive karstification exists. Here, a relatively high hydraulic conductivity of 12.5m d-1 was found, resulting in: 1) low gradients of 2-10cm km-1 of the eastward groundwater flow west of Lac bay; 2) high tidal impact on hydraulic heads; and 3) weak responses to rainfall.

On a local scale, karstified limestone makes groundwater flow complex, because it facilitates underground streams with unpredictable flow paths. 700m northwest of Lac bay, a northeastern flow direction away from Lac bay was found. This may explain the relatively high salinity greater than 50p.p.t. observed in the northwest backwaters of Lac bay.

Mangroves are effective blue carbon sinks and are the most carbon rich ecosystems on earth. However, their areal extent has declined by over one-third in recent decades. Degraded mangrove forests result in reduced carbon captured and lead to release of stored carbon into the atmosphere by CO2emission. The aim of this study was to assess changes in carbon dynamics in a gradually degrading mangrove forest on Bonaire, Dutch Caribbean. Remote sensing techniques were applied to estimate the distribution of intact and degraded mangroves. Forest structure, sediment carbon storage, sediment CO2 effluxes and dissolved organic and inorganic carbon in pore and surface waters across intact and degraded parts were assessed. On average intact mangroves showed 31% sediment organic carbon in the upper 30 cm compared to 20% in degraded mangrove areas. A loss of 1.51 MgCO2 ha−1 yr−1for degraded sites was calculated. Water samples showed a hypersaline environment in the degraded mangrove area averaging 93 which may have caused mangrove dieback. Sediment CO2 efflux within degraded sites was lower than values from other studies where degradation was caused by clearing or cutting, giving new insights into carbon dynamics in slowly degrading mangrove systems. Results of water samples agreed with previous studies where inorganic carbon outwelled from mangroves might enhance ecosystem connectivity by potentially buffering ocean acidification locally. Wetlands will be impacted by a variety of stressors resulting from a changing climate. Results from this study could inform scientists and stakeholders on how combined stresses, such as climate change with salinity intrusion may impact mangrove's blue carbon sink potential and highlight the need of future comparative studies of intact versus degraded mangrove stands.

Raw Invertebrate monitoring data. During monitoring the species and the number of individuals of that species is counted at 66 different locations.

Monitoring took place in: 1999, 2007, 2013, 2016 and 2017.

Distinguished species: 

Please contact the DCBD administrator or STINAPA for access to the raw data.

Raw data of seagras monitoring on Lac, Bonaire. 

At 49 locations 6 square meter quadrants are investigated. The coverage per seagrass species is measured per quadrant using a 10 by 10 cm grid. 

Observed species:

• Thalassia testudinum (Turtlegrass, Species code: Tt) - IUCN Red List
• Syringodium filiformi (Manatee Grass, Species code Sf) - IUCN Red List
• Halodule beaudettei (Shoal grass, Species code Hy - IUCN Red List
• Halophila stipulacea (Species code: Hs) - IUCN Red List
   
• 

Please contact STINAPA for more information

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Depth of Lac Bay in meters. Data acquired with lowrance HDS7 Gen 3 Totalscan transducer. The borders are not realiable. No map to snap to the coast/borders was available.

• Microsoft Excel file (longitude, latitude, depth in feet) [original file]
• Line shape file (*.shp) with depth isolines (m)
• Geotiff at a resolution of 2x2 m. The original file was interpolated using Inverse Distance Weighting (IDW) with a maximum number of 12 points for a single cell value. Interpolation boundaries at 30 meter from any given point.

Legend:

• -0.5 m    light yellow
• -5 m       green 
• -13.5 m  dark blue

Seagrasses are essential components of coastal zones ecosystems due to their extremely high productivity and the high biodiversity they support. Inside Lac, Bonaire, seagrasses cover the sea floor and provide a key-habitat to a growing population of endangered green sea turtles (Chelonia mydas). 

Invasive seagrass Halophila stipulacea (originating from the Red Sea) appears to be outcompeting native seagrasses such as Thalassia testudinum. Using 49 fixed locations, we observed that between 2011 and 2015 the occurrence of H. stipulacea in the bay increased significantly from 6% to 20% while native T. testudinum occurrence decreased significantly from 53% to 33% (Smulders et al., 2017).  The consequences for the seagrass ecosystem services are still not known. In February 2017, several fieldwork projects were conducted on seagrass ecosystem services and foraging behaviour of sea turtles in Lac by a team of local experts (Sabine Engel, STINAPA and STCB), together with researchers from Groningen University, NIOZ and NIOO led by Marjolijn Christianen.

Also a pilot experiment was installed to test the use of “BESE-elements”, biodegradable potato starch polymer structures (https://www.bese-elements.com) for seagrass restoration. 

This news-item was published in BioNews 6-2017.

Although Bonaire’s coral reefs remain among the healthiest and most resilient in the Caribbean, this IUCN report based on the IUCN Resilience Assessment of Coral Reefs highlights some of the threats that exist to Bonaire’s coral reefs, and which could have serious implications for resilience to future climate change and other threats. The report identi ed recommendations for addressing the current threats, as well as high and low resilience sites.

The threats and recommendations identified include:

Coastal development and artificial beaches.Recommendation: All coastal construction on Bonaire should be strictly regulated and follow the construction guidelines. The guidelines should become law in order to be enforced appropriately.

Leaching from septic tanks. Recommendation: It is strongly recommended that Bonaire invest in appropriate sewage treatment facilities to improve water quality and increase the resilience of its valuable coral reefs. It is also recommended that a water quality monitoring program be set up and sustained.

Increasing damsel sh populations. Recommendation: It is recommended that the shing of preda- tory sh species on Bonaire’s coral reefs be controlled and managed to a sustainable level to prevent population explosions of prey sh capable of modifying the reef habitat.

Trididemnum and Lobophora. Recommendation: It is recommended that the populations of Trididemnum and Lobophora are closely monitored and the factors contributing to the unnatural abundance of these coral-overgrowing organisms should be studied and then eliminated. 

The distribution and abundance of juvenile corals were examined at depths from 3 to 37 m on the reefs of Curaçao and Bonaire (Netherlands Antilles). Juveniles of Agaricia agaricites were most abundant (60.6%), followed by Helioseris cucullata (8.3%). The large massive corals such as Montastrea annularis, M. cavernosa and branched species such as Madracis mirabilis and Acropora palmata had few juveniles. This, combined with species characteristics, shows that these species employ very different life history strategies. In some species the abundance of juveniles over the reef paralleled that of larger colonies, but not for example in Agaricia agaricites. The composition of the coral community was apparently no direct function of juvenile abundance. A change in angle of settlement of A. agaricites juveniles with increasing depth, from vertical to horizontal surfaces, seems to reflect the preferred light intensity. We studied the survival of juvenile corals during a half-year period. One-third remained unharmed, one-third died or disappeared, and one-third was limited in growth by factors such as spatial competition. This was the same for all depths, but factors influencing survival varied with depth.

Researchers from the Wageningen Marine Research, under the leadership of water quality specialist and ecologist Dr. Diana Slijkerman, have been working on the question whether sun care products are an emerging risk for marine life in the Caribbean. More specifically, UV-filters in sunscreen, such as Oxybenzone, were subject of study since these were recently reported to be of serious concern for corals.

This news article was published in BioNews 2-2017.

BioNews is produced by the Dutch Caribbean Nature Alliance and funded by the Ministry of Economic Affairs.