Seagrass

After the 2021 Workshop on Mangrove Restoration held on Bonaire, Jessica Johnson (Coastal Dynamics; Bonaire), Jimena Samper Villareal (Centro de Investigación en Ciencias del Mar y Limnología (CIMAR) of the University of Costa Rica (UCR)) and Sabine Engel (Internos/Mangrove Maniacs; Bonaire) joined forces to establish the Tropical Restoration Network. The aim of this network is to provide a platform for the collaboration and methodological harmonization of the many efforts for restoration of blue carbon habitats, focused on seagrasses and mangroves, in the Caribbean, Gulf of Mexico and Eastern Tropical Pacific region.

Building off the momentum of the 2021 Workshop, a follow on Tropical Restoration Workshop was planned for 6-8 June 2023 hosted by the University of Costa Rica in San Jose. Participants from across the region were invited to attend, either virtually or in person

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.

Abstract
Seagrass meadows provide essential ecosystem services. However, seagrass cover has decreased the past decades, due to climate change and other disturbing factors. To ascertain coastal stability, biodiversity and ecological well-faring, seagrass meadows need to be restored. In this thesis, we unfold complex interactions that need to be taken into account during such restoration projects.

In particular, this study explores the spatial distribution and factors influencing seagrass cover in Lac Bay, Bonaire, with a focus on the interactions between turtle grazing, bioturbation, invasive seagrass (Halophila stipulacea), and native seagrass (Thalassia testudinum). The observational study reveals competition between the two seagrass species, as well as the negative impact of high grazing pressure and bioturbation on T. testudinum. A linear mixed model identifies significant predictors for T. testudinum cover, including macroalgae cover, turtle grazing pressure, H. stipulacea cover, and mound cover. Additionally, a negative influence between H. stipulacea and T. testudinum is observed. Bioturbation is found to negatively affect T. testudinum but does not significantly impact H. stipulacea. The experimental study investigates the effect of bioturbation on seagrass growth using different planting techniques, indicating that transplanting T. testudinum fragments can be successful for restoration. However, no significant differences are found between lattice and mesh treatments. The study concludes that the complex interactions between these factors contribute to the decline of T. testudinum and the proliferation of H. stipulacea. Future research is suggested to further investigate these interactions and evaluate the long-term effects of bioturbation and planting techniques on seagrass growth. The findings emphasize the importance of considering multiple variables when studying seagrass ecosystems and propose transplanting as a potential restoration measure in areas with high bioturbation.

This dataset contains ecosystem metabolism and seagrass meadow data from five locations in the Greater Caribbean and Gulf of Mexico regions at which green turtle populations had established foraging areas. Ecosystem metabolic rates were compared between grazed and adjacent ungrazed areas of seagrass (Thalassia testudinum) to investigate the effects of green turtle grazing on metabolic carbon capture rates in seagrass meadows across a wide geographic area. Seagrass data are provided for site descriptions and drivers of variation in metabolic rates. Ecosystem metabolic rates are also included for meadows of the invasive seagrass Halophila stipulacea from two locations for comparison to rates in the native seagrass meadows where this invasive seagrass is encroaching upon green turtle foraging areas. Data were collected from one location (Little Cayman) in 2016, and from the remaining four locations (Bonaire; St. Croix; Eleuthera, Bahamas; west coast of Florida) in 2018.

Dutch, Papiamento, and Papiamentu below

On Tuesday, May 9th, 2023, the Minister for Climate and Energy, Drs. Rob Jetten, and his delegation visited Bonaire’s mangrove forest at Lac Bay. He was received by the Dutch Caribbean Nature Alliance (DCNA) and the Mangrove Maniacs Foundation that gave him a tour. He learned about the forest and the mangrove restoration taking place as well as the seagrass and coral restoration efforts by STINAPA. The Minister also got into the mud helping the volunteers with opening a channel.  

Minister Jetten and DCNA director Arno Verhoeven. Photo credit: Casper Douma / DCNA

Mangrove ecosystems are very important for biodiversity and strengthen coastal areas against extreme weather conditions. They are important for tourism and fishery, and they are true carbon storage champions. Together with sea grass and wetlands, mangroves can play an important role in nature-based solutions to climate change, both in adaptation and mitigation strategies.  

The mangrove forest on Bonaire, the largest of the Kingdom, is struggling. Erosion and siltation cause bad water quality – including high salinity. This has resulted in continuing die offs of mangrove trees. Based on fisherman’s knowledge, satellite imagery, and local research, the Mangrove Maniacs open up channels to restore water circulation. A tough job, hence, their name ‘Maniacs’. In the last 10 years, the volunteers have already maintained more than 3km of channels by hand.  

The Minister for Climate and Energy, Drs. Rob Jetten, is co-hosting the Caribbean Climate & Energy Conference on Aruba from the 10th to the 12th of May. Before traveling there, he visited Bonaire. On the 9th of May 2023, the Mangrove Maniacs and the Dutch Caribbean Nature Alliance (DCNA) escorted the Minister and his delegation on a tour through Bonaire’s mangrove forest in Lac Bay. During this tour, the Minister and the two organizations discussed the importance of mangrove forests, the stress the trees experience, and the need for mangrove and seagrass restoration. Not just for Bonaire, but also for the other five Dutch Caribbean islands. 

Globally, research is being conducted into how mangroves, sea grass, coral and wetlands can play a role in tackling the climate crisis. Through the local park management organizations and many other local NGOs in conservation, the Dutch Caribbean islands already possess a lot of knowledge and expertise on these ecosystems, how they can be restored, and how they can be used in climate mitigation and adaptation. The DCNA emphasized, however, that capacity building and structural financing is necessary and that cooperation between and with the six Dutch Caribbean islands is key.   

Minister Jetten working in the mangroves. Photo credit: Casper Douma / DCNA

Besides the fruitful conversations between the Minister and his delegation, the Mangrove Maniacs, and the DCNA, the Minister also got his hands ‘dirty’ while helping the volunteers of the Mangrove Maniacs with opening a channel.  

Op dinsdag 9 mei 2023 bezocht de Minister van Klimaat en Energie, drs. Rob Jetten, samen met zijn delegatie het mangrovebos van Bonaire in Lac Bay. Hij werd begeleid door de Mangrove Maniacs en de Dutch Caribbean Nature Alliance (DCNA), die hem rondleidden en uitleg gaven over het belangrijke werk van mangroveherstel door de Mangrove Maniacs. Ook het zeegras- en koraalherstel in het gebied door STINAPA werd onder de aandacht gebracht. De minister stak ook zijn handen uit de mouwen toen hij de vrijwilligers van de Mangrove Maniacs hielp bij het openen van een kanaal. 

Minister Jetten en DCNA director Arno Verhoeven. Foto: DCNA

Mangrove-ecosystemen zijn erg belangrijk voor de biodiversiteit en spelen een rol in het versterken van kustgebieden tegen extreme weersomstandigheden. Ze zijn belangrijk voor toerisme en visserij en ze zijn echte kampioenen in koolstofopslag. Samen met zeegras en wetlands kunnen mangroven een belangrijke rol spelen in op natuur gebaseerde oplossingen voor klimaatverandering, zowel in aanpassings- als mitigatiestrategieën. 

Maar het mangrovebos op Bonaire, het grootste van het Koninkrijk, heeft het moeilijk. Erosie en verzanding hebben een slechte invloed op de waterkwaliteit – zo veroorzaakt het een hoog zoutgehalte. Dit heeft geresulteerd in een voortdurende afsterving van mangrovebomen. Op basis van de kennis van vissers, satellietbeelden en lokaal onderzoek openen de Mangrove Maniacs kanalen om de watercirculatie te herstellen. Een zware klus, vandaar hun naam ‘Maniacs’. In de afgelopen 10 jaar hebben de vrijwilligers al meer dan drie kilometeraan kanaal met de hand onderhouden. 

De Minister voor Klimaat en Energie, drs. Rob Jetten is van 10 tot en met 12 mei mede-gastheer van de Caribische Klimaat & Energie Conferentie op Aruba. Voor zijn reis naar Aruba, bezocht hij Bonaire. Op 9 mei 2023 begeleidden de Mangrove Maniacs en de Dutch Caribbean Nature Alliance (DCNA) de minister en zijn delegatie op een tour door het mangrovebos van Bonaire in Lac Bay. Tijdens deze rondleiding bespraken de minister en de twee organisaties het belang van mangrovebossen, de druk die de bossen ervaren en de noodzaak van mangroveherstel. Niet alleen voor Bonaire, maar ook voor de andere Caribische eilanden van het Koninkrijk waar mangroven voorkomen. 

Wereldwijd wordt onderzocht hoe mangroven, zeegras, koraal en wetlands een rol kunnen spelen bij het aanpakken van de klimaatcrisis. Natuurbeheerorganisaties en de vele lokale ngo’s  zorgen er voor dat er op de eilanden al over veel kennis en expertise over deze ecosystemen kan worden beschikt. Wat is er mogelijk en wat moet er gebeuren? Vooral zogenaamde nature based solutions kunnen een belangrijke rol spelen bij klimaatadaptatie. Zeker om dat de natuur al een cruciale rol speelt in vrijwel alle facetten van het leven op de eilanden. Tijdens de ontmoeting, benadrukte de DCNA dat capaciteitsopbouw en structurele financiering echter noodzakelijk zijn en dat samenwerking tussen en met de zes Nederlands Caribische eilanden centraal moet staan. 

Minister Jetten working in the mangroves. Foto: DCNA

Naast de vruchtbare gesprekken tussen de minister en zijn delegatie, de Mangrove Maniacs, en de DCNA, heeft de minister ook de handen uit de mouwen gestoken en hielp hij de vrijwilligers van de Mangrove Maniacs bij het openen van een kanaal. 

Diamars dia 9 di mei 2023 Minister di Clima y Energia, Drs. Rob Jetten hunto cu su delegacion a bishita e mondi di palo di mangel di Boneiro na Lac Baai. El a wordo compaña pa Mangrove Maniacs y Dutch Caribbean Nature Alliance (DCNA), kendenan a mustr’e e alrededornan y a splica tocante e trabou importante di restauracion di e palo di mangelnan door di Mangrove Maniacs y restauracion di lima di lama door di STINAPA y e importancia di conservacion di naturalesa en general. E minister tambe a saca man yuda e boluntarionan di Mangrove Maniacs habri un canal.  

Minister Jetten and DCNA director Arno Verhoeven. Foto: DCNA

Ecosistema di palo di mangel ta masha importante pa biodiversidad y ta fortalece e zonanan di costa contra condicion climatico extremo. Nan ta importante pa turismo y pesca, y ta berdadero campeon den almacenamento di carbon. Hunto cu lima di lama y e humedalnan, e palo di mangelnan por desempeña un papel importante den solucion di cambio climatico basa riba naturalesa, tanto den strategia di adaptacion como di mitigacion.  

Pero e mondi di palo di mangel na Boneiro, esun mas grandi di Reino, tin e dificil. Erosion y sedimentacion ta provoca un mal calidad di awa, incluyendo un nivel halto di salo. Esaki a resulta den murimento continuo di palo di mangel. Basa riba conocemento di e piscadonan, imagen di satelite y investigacion local, Mangrove Maniacs ta habri canal pa restaura circulacion di awa. Un trabou duro, door di esey e nomber “Maniacs”. Den e ultimo 10 añanan e boluntarionan ya a mantene mas di 3 km di canal cu man.  

Minister di Clima y Energia, Drs. Rob Jetten, ta coanfitrion di e Conferencia Caribense di Clima y Energia na Aruba di dia 10 pa 12 di mei. Prome di biaha bay eynan, el a bishita Boneiro. Dia 9 di mei di 2023 Mangrove Maniacs y Dutch Caribbean Nature Alliance (DCNA) a compaña Minister y su delegacion den un recorido door di e mondinan di palo di mangel di Boneiro na Lac Baai. Durante e gira aki Minister y e dos organisacionnan a discuti importancia di e mondinan di palo di mangel, e presion cu e matanan ta experimenta y e necesidad pa restaura e palonan di mangel. No solamente pa Boneiro, sino tambe pa e otro islanan di Caribe Hulandes den cuanan bo ta haya mata di mangel.  

Na nivel mundial, nan ta realisando investigacion tocante con e palo di mangelnan, e limanan di lama y e humedales por desempeña un papel den e lucha contra e crisis climatico. Entre e organisacionnan di maneho di parke y otro organisacion di conservacion, e islanan di Caribe Hulandes ya ta posee hopi conocemento y experencia tocante e ecosistemanan aki, con por restaura nan y con por uza nan den mitigacion y adaptacion climatico. Sinembargo, DCNA a enfatisa cu desaroyo di e capacidadnan y financiamiento structural ta necesario y cu e cooperacion entre y cu e seis islanan di Caribe Hulandes ta central.  

Minister Jetten working in the mangroves. Foto: DCNA

Ademas di e combersacionnan fructifero entre e Minister y su delegacion, Mangrove Maniacs y DCNA, e Minister tambe a saca man, yuda e boluntarionan di Mangrove Maniacs habri un canal.  

Riba djamars 9 di mei 2023, e Minister di Klima i Energia, Drs. Rob Jetten, huntu ku su delegashon a bishitá e mondinan di palu di mangel di Boneiru na Bahia di Lac. E tabata kompañá pa The Mangrove Maniacs i Dutch Caribbean Nature Alliance (DCNA), ku a duna un guia na e sitio i tambe un splikashon di e trabou importante di restorashon di mondi di palu di mangel ku The Mangrove Maniacs ta hasi i e restorashon di yerba di laman ku ta na enkargo di STINAPA i e importansia di konservashon di naturalesa en general. E Minister a pone man na obra tambe dunando asistensia na e boluntarionan di The Mangrove Maniacs den nan trabou sushi di habri un kanal den lodo. 

Minister Jetten and DCNA director Arno Verhoeven. Foto: DCNA

E ekosistemanan di Palu di mangel ta hopi importante pa biodiversidat i pa fortalesé áreanan na kosta kontra kondishonnan ekstremo di wer. Nan ta hopi importante pa turismo i peska, i nan ta un mangasina kampion pa almasená karbon. Huntu ku yerba di laman i áreanan akuátiko, palu di mangelnan por tin un papel importante den solushonnan basá riba naturalesa pa kambio di klima, tantu den strategianan di adaptashon komo den strategia di suavisá. 

E mondi di palu di mangel na Boneiru, ku ta esun di mas grandi den Reino hulandes, ta luchando pa sobrebibí. Eroshon i oumentu di sedimento ta kousa mal kalidat di awa – inkluso grado haltu di salu. Esaki a duna resultado ku palu di mangelnan ta muri bai kontinuamente. Basá riba konosementu di piskadó, mapanan di satélite, i investigashon lokal; The Mangrove Maniacs ta koba lodo pa habri kanal pa por restorá sirkulashon di awa. Un trabou duru, no ta pa nada e nòmber ‘Maniacs’. Den e último 10 añanan, e boluntarionan a logra kaba di habri mas ku 3 km di kanal ku trabou na man. 

E Minister di Klima I Energia, Drs. Rob Jetten, ta kompartí e funshon di anfitrion di e Caribbean Climate & Energy Conference na Aruba di 10 pa 12 di mei. Promé di biaha pa Aruba, el a bishitá Boneiru. Riba 9 di mei 2023 The Mangrove Maniacs ku Dutch Caribbean Nature Alliance (DCNA) a duna e Minister ku su delegashon un eskurshon rònt di e mondinan di palu di mangel di Boneiru den Bahia di Lac. Durante e eskurshon, e Minister huntu ku e dos organisashonnan a diskutí e importansia di mondi di palu di mangel, e strès ku e palunan ta eksperensiá, i e nesesidat pa restorashon di palu di mangel. No solamente pa Boneiru, pero tambe pa e otro islanan di Hulanda Karibense ku tin palu di mangel. 

Na nivel global, ta hasiendo investigashon pa wak kon palu di mangel, yerba di laman i área akuátiko por hunga un papel den atendé ku e krísis di klima. Denter di e organisashonnan di maneho di parke natural i otro organisashon di konservashon di naturalesa, e islanan di Hulanda Karibense tin kaba hopi konosementu i ekspertisio di e ekosistemanan akí den nan man, tokante kon pa restorá nan i kon por usa nan den suavisá i adaptá kambio di klima. DCNA sinembargo a enfatisá, ku krea kapasidat i finansiamentu struktural ta nesesario i ku koperashon entre e seis islanan di Hulanda Karibense ta importante.  

Minister Jetten working in the mangroves. Foto: DCNA

Fuera di e kombersashonnan fruktífero entre e Minister i su delegashon, ku The Mangrove Maniacs, i DCNA, e Minister a pone tambe man na obra sushi di lodo pa asistí e boluntarionan di The Mangrove Maniacs den kobamentu pa habri un kanal. 

Published in BioNews 65

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.

Historical Data and Background

The first known qualitative seagrass survey in Lac Bay was conducted in 1969. Sincethen, additional quantitative surveys were conducted in 1999 and 2007 and on a regularbasis since 2011 (2011, 2013, 2015, 2017, 2018,2019, 2020).After each survey, raw dataforseagrass observationswereuploaded to the DutchCaribbeanBiodiversityDatabase (DCBD)and have beenreferenced innumerouspublications, most notably the seagrassandsea turtle research byChristianen andSmulders from Wageningen University and Research (WUR).In 2010, the invasive speciesHalophila stipulaceawas first documented in Lac Bay, andhas continued to spread since. This is a concern as this species is fast growing andhas been known to outcompete local species of seagrasses. If left unchecked,Halophila stipulaceais a threat to native seagrasses and the species of which dependon it (such as green turtle and queen conch).

Discussion

In recent monitoring (2019, 2020) pictures were taken of all quadrants surveyed as wellas of the surroundingarea.SeagrassOverall, there has been a decrease in the native species ofT. testudinumand anincrease in the invasive speciesH. stipulacea. S. filliformepopulations appeartobe stable, with a slight increase in coverage.Sargassum has been an issue within Lac Bay, with several of the survey sites beinglocations where decaying sargassum has created a thick mat, which in most cases wasslowly removed with the tide. Two ofthese sites, Locations A and G were completelycovered by sargassum. Physical repercussions of the sargassum landings can be seenby the seagrass dieback all along the mangrove border from the south until just north ofPunto Kalba.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 thissite.The overall cover by all species together seems to be stable, but in terms of biomass itwould appear to be lower. The ecosystem services provided byHalophila stipulaceaaresignificantly lower than those ofThalassia testudinumdue to its shallow root structure(Smulders et al., 2017) and the fact that it is less nutrient rich than native seagrassspecies (Boman et al., 2019).The shift towards this nonnative species is of concern andshould be closely monitored.

Halimeda

Since 2018 Halimeda species and bioturbation observations were added to themethodology of these surveys. 2020Data onHalimeda sp.occurrence can be found inAppendix III.Overall cover by Halimeda seems to have decreasedbut a longer time series isrequired to draw more definitive conclusions. Two students have looked into carbonatesand production by Halimeda during theLac Ecological Restoration project: LauraTimmermans (2018)and Valeria Pesch (2019). Results from these studies wereinconclusive, highlighting the need for additional research to fully understand thecontribution of Halimeda to carbonate sands and infilling of the bay. In addition, moreinformation is needed on the influence of eutrophication (Slijkerman et al., 2011) on thisprocess.

Sand particles size in Lac was measured during the Conch Stock Restoration project.Larger fractions often show Halimeda segments next to small shells and othercarbonate particles (Figure 5). For this reason, it is believed thatHalimeda sp.are amajorcontributor of sand within the bay.Sediments have been analyzed for carbonate content in several other studies such asthe EHLZK project and during the baseline surveys conducted in 2012 (Debrot et al,2012). Although the findings have not been published, the data showed that sedimentstowards the center of the bay have a higher CaCO3content, and the distribution sand,silt, clay changes (Appendix V). In addition, it was found that terrigenous sedimentswere most prevalent along the borders of Lac-mainly in the northwestern sector,whereas endogenous sediments were found in the central part of the bay and towardsthe reef.

Bioturbators

Bioturbators have also been added to recent surveys as it is believed to be important asit may cause a loss of sequestered carbon, and new sediment may facilitate settlementofH. stipulacea. Bioturbators are considered to be ecosystem engineers, changingthesubstrate landscape. Common bioturbators are callianassid (burrowing ghost) shrimp,the lugworm, mantis shrimp and the burrowing sea cucumber. To date, surveys haveonly included rough estimates as to the number of mounds per square meter.After discussing this issue among local researchers, it has been decided to amend theprotocol for next year to count all mounds in the same quadrant as the seagrass cover.If more than half of the mound falls out of the quadrant the mound will not be counted

Abstract

The seagrass Halophila stipulacea is native to the Red Sea. It invaded the Mediterranean over the past century and most of the Caribbean over the last two decades. Understanding the main drivers behind the successful invasiveness of H. stipulacea has become crucial. We performed a comprehensive study including field measurements, a mesocosm experiment, and a literature review to identify ‘superior growth traits’ that can potentially explain the success story of H. stipulacea. We assessed meadow characteristics and plant traits of three invasive H. stipulacea populations growing off the Island of Sint Eustatius (eastern Caribbean). We compared similar parameters between native (Eilat, northern Red Sea) and invasive (Caribbean) H. stipulacea plants in a common-garden mesocosm. Lastly, we compared our field measurements with published data. The newly arrived H. stipulacea plants from St. Eustatius were characterized by higher percent cover, higher below- and above-ground biomasses, more apical shoots, and faster leaf turnover rates than those measured in both native and older invaded habitats. These results were further confirmed by the mesocosm experiment where the invasive H. stipulacea plants grew faster and developed more apical shoots than the native plants. Results suggest that increased growth vigour is one of the main invasive traits that characterize successful invasive H. stipulacea populations in the Caribbean and potentially in other invaded areas. We encourage long-term monitoring of H. stipulacea in both native and invaded habitats to better understand the future spread of this species and its impacts on communities and their ecosystem functions and services.

Summary
Thalassia testudinum seagrass is defined as important foundation species by serving as nursery area, giving coastal protection, maintaining high water qualities and their function as carbon sink. In the east Caribbean Sea, T.testudinum beds are under threat caused by the decomposition of excessive amounts of holo-pelagic sargassum blooms (Sargassum natans and Sargassum fluitans), creating anoxic conditions and causing the production of hydrogen sulfide( H2S) in the porewater. Hence, sulfide intrusion and toxicity can occur in T.testudinum causing lower plant performances and big scale seagrass mortalities. Positive effects of bioturbation were thought to relieve sulfide stress from T.testudinum by modifying the geo-chemistry by a process called ‘bio-irrigation’. This research assessed the effects of Bio-irrigating Upogebia shrimps on the porewater sulfide and sulfide plant intrusion within T.testudinum habitat. Firstly, a monitoring study was carried out to determined the effects of habitat and sargassum decomposition on Upogebia hole densities(indicator for Upogebia densities). Secondly, an Upogebia hole manipulation experiment was setup to investigate the effect of adding and removing bio-irrigating shrimps(Upogebia affinis) on porewater sulfide and sulfide intrusion within T.testudinum seagrass. In addition to the experimental effects, the overall effects of Upogebia hole densities on porewater sulfide and sulfide intrusion taken over the whole experimental period were investigated to account for possible delayed effects on sulfide intrusion within T.testudinum. Our results showed highest Upogebia hole densities in bare habitat with lower sargassum decomposition. Besides the possible negative effects of T.testudinum habitat reducing burrowing space for Upogebia, the bare habitat mainly consisted of Halimeda calcified sediment which likely facilitated the Upogebia burrow construction. Within the overall effects, Upogebia hole density showed negative effects on both porewater sulfide and total sulfur within T.testudinum leaf suggesting some possible bio-irrigation. Within the treatment effects, the removal intervention caused higher TS and lower δ34S in the T.testudinum leaf, but this was just visible after 16 days, indicating delayed effects of the removal intervention. Future mesocosm experiments are recommended to account for side effects and for more accurate sulfide measurements. This will fill more scientific knowledge gaps on the role of Upogebia shrimps as potential sulfide stress relievers for T.testudinum seagrass.

Dutch below

Sea turtles have been overharvested in the past, resulting in their endangered population status. But since two decades, their populations have recovered in certain areas around the world, and their ecological role is unfolding. The seagrass meadows they feed on are returning to their naturally grazed state as a result. Some are even becoming overgrazed. How does the return of the sea turtle affect the functioning of coastal ecosystems?

A research team analysed the seagrass ecosystem value (such as coastal protection and carbon storage) along a turtle grazing area from low to high. In experiments, they determined the impact of recovering sea turtle populations on ecosystem functioning. The team, led by Wageningen University, with colleagues from the Netherlands and the Dutch Caribbean, recently published their findings in Global Change Biology.

Seagrass meadows provide us with a range of goods and services. For example, they provide a habitat and food for many commercially important fish species. They also help protect the coast by stabilising sediment, and reducing wave energy. With the return of the large herbivore – the sea turtle – it is important to quantify their impact on the ecosystem services a seagrass meadow can provide. Often these services are measured separately, but in the new study, the most essential services have been assessed together to assess the ecosystem’s multifunctionality.

Combining ecosystem services in one index

“Our approach to measuring different ecosystem functions simultaneously and combining these in one ecosystem-multifunctionality-index is new for marine habitats and led to striking results,” says Marjolijn Christianen, lead investigator of the study. “We found that medium turtle grazing pressure increased carbon storage and nutrient cycling in seagrass meadows. On the other hand, fish biomass and other services were higher in meadows without turtle grazing. More importantly, we found a simultaneous collapse of all these services under severe turtle grazing pressure.”

Cages were used to keep turtles from grazing the seagrass to mimic the – no-grazing scenario

Integrative approach for balanced ecosystems

Based on these results, the authors argue that the successful return of the sea turtle should be accompanied by the protection of their habitat, seagrass meadows, as well as their predators, who can influence turtle grazing behaviour through fear effects. “Based on recent insights from studies in the Bahamas, the resulting spatial variation in grazing pressure can probably prevent habitat collapse,” says Fee Smulders, co-author of the study. Christianen: “By taking an integrative ecosystem approach to management, we can maintain high ecosystem multifunctionality, as well as balanced ecosystems that can sustain natural densities of charismatic sea turtles”.

Pristine seagrass meadows

Green sea turtles are considered the megaherbivores of the ocean. Since their populations have been decimated, scientists have become accustomed  to studying and experiencing ungrazed seagrass meadows. These lush seagrass meadows dominated by sturdy species are considered healthy, in ‘pristine’ condition, and very valuable. But with the return of the green sea turtle in many coastal areas in the past decades – due to successful conservation – many seagrass meadows have been transformed into grazed seascapes.

An earlier paper published in Nature, Ecology and Evolution by Wageningen researchers discussed how we must reconsider our view of a pristine seagrass meadow, which likely consists of a mosaic of (heavily) grazed patches and ungrazed patches with high plant species diversity and higher biodiversity in general. This mosaic seascape may be more natural, even though the ecosystem value may be slightly lower than ungrazed meadows. Now that the different grazing scenarios (high turtle abundance, low abundance, sometimes hyper-abundant) coincide in all three ocean basins where turtles are found, the outcomes of these new studies are even more globally relevant and urgent. The findings can also be used to predict ecosystem impacts of future shifts.

Three scenarios of megaherbivore grazing intensity can be observed in tropical seagrass ecosystems with green turtles as megaherbivores worldwide (Fig a). The turtle’s ecological role is rapidly unfolding in numerous foraging areas where populations are recovering through conservation after centuries of decline, with an increase in recorded overgrazing episodes. In field experiments, researchers assessed the effects of simulated grazing intensity scenarios on ecosystem functions and multifunctionality (Fig b) in a tropical Caribbean seagrass ecosystem over an 18-month period.

More info in the Dutch Caribbean Biodiversity Database

De zeeschildpad komt terug, wat betekent dat voor het functioneren van ecosystemen?

In het verleden is er veel op zeeschildpadden gejaagd, waardoor ze met uitsterven worden bedreigd. Maar sinds twee decennia hebben ze zich in een aantal gebieden over de hele wereld hersteld. Hun rol in het ecosysteem komt daarmee terug. Zo wordt zeegras – hun voedingsbron – weer op een natuurlijke manier begraasd. Op sommige plekken wordt zeegras zelfs overbegraasd. Wat betekent de terugkeer van de zeeschildpad voor het functioneren van ecosystemen langs de kust?

Een onderzoeksteam heeft in een gebied met grazende zeeschildpadden de waarde van zeegras-ecosystemen geanalyseerd (waarbij bijvoorbeeld naar kustbescherming en koolstofopslag werd gekeken). Via experimenten bepaalden ze het effect van de zich herstellende populaties schildpadden op het functioneren van het ecosysteem. Het team, onder leiding van de Wageningen University, met collega’s uit Nederland en het Caribisch gebied, publiceerde hun bevindingen onlangs in Global Change Biology.

Velden met zeegras voorzien ons van verschillende diensten. Ze vormen bijvoorbeeld een habitat voor veel vissoorten die commercieel belangrijk zijn, en bieden hen voedsel. Ook helpt zeegras de kust te beschermen, doordat het sediment stabiliseert en golfslag vermindert. Nu de grote grazer – de zeeschildpad – is teruggekeerd, is het belangrijk om te bepalen wat voor impact dat heeft op de diensten die een zeegras-ecosysteem kan leveren. Vaak worden deze diensten afzonderlijk gemeten, maar in de nieuwe studie zijn de belangrijkste diensten samen onderzocht om de multifunctionaliteit van het ecosysteem te beoordelen.

Combinatie van ecosysteemdiensten in één index

“Onze aanpak om verschillende functies tegelijkertijd te meten, en die te combineren in één multifunctionaliteitsindex voor ecosystemen, is nieuw voor mariene habitats,” zegt Marjolijn Christianen, hoofdonderzoeker van de studie. “Het leidde tot opvallende resultaten. We zagen dat koolstofopslag en nutriëntencyclus hoger lagen in velden zeegras met een gemiddelde graasdruk. Anderzijds bleken de hoeveelheid vis, en andere diensten, hoger te zijn in onbegraasde velden. Maar nog belangrijker: we zagen dat al deze diensten bezweken bij een zware graasdruk.”

Om het scenario ‘geen-begrazing’ na te bootsen, werden er kooien gebruikt om te voorkomen dat schildpadden bij het zeegras konden.

Integrale aanpak voor een evenwichtig ecosysteem

Op basis van de resultaten stellen de auteurs dat de succesvolle terugkeer van de zeeschildpad moet samengaan met de bescherming van hun habitat (zeegras) en hun natuurlijke vijanden. Deze predatoren kunnen het graasgedrag van schildpadden beïnvloeden door het effect van angst. “Recent zijn er daar op de Bahama’s studies naar gedaan. Op basis van die inzichten, denken we dat ruimtelijke variatie in graasdruk die daaruit ontstaat, waarschijnlijk kan voorkomen dat habitats bezwijken,” aldus Fee Smulders, co-auteur van de studie. Christianen: “Met een integrale benadering van ecoysteembeheer kunnen we de hoge multifunctionaliteit van ecosystemen behouden. En het leidt tot evenwichtige ecosystemen, die natuurlijke aantallen van de zeeschildpad kunnen dragen.”

Ongerepte velden met zeegras

Groene zeeschildpadden worden beschouwd als de grote herbivoren van de oceaan. Sinds hun populaties zijn uitgedund, zijn wetenschappers gewend geraakt aan het bestuderen van onbegraasd zeegras. Deze weelderige velden – gedomineerd door sterke soorten – worden beschouwd als gezond, in ‘ongerepte’ staat, en als zeer waardevol. Maar dankzij succesvolle natuurbescherming is de groene zeeschildpad de afgelopen decennia in veel kustgebieden teruggekeerd. De velden met zeegras zijn daardoor weer veranderd in begraasde gebieden.

Een eerder artikel, gepubliceerd in Nature, Ecology and Evolution door Wageningse onderzoekers, beschreef hoe we ons idee van ‘ongerept’ zeegras moeten bijstellen. Waarschijnlijk bestaat dat uit een mozaïek van (zwaar) begraasde plekken, afgewisseld door onbegraasde plekken met een hogere dichtheid aan plantensoorten, en een hogere biodiversiteit in het algemeen. Dit mozaïek is wellicht natuurlijker, ook al ligt de ecosysteemwaarde iets lager dan bij onbegraasd zeegras. De uitkomsten van deze studies worden wereldwijd nog relevanter en urgenter, nu de verschillende graasniveaus (veel schildpadden, weinig, of soms uitermate veel) samenvallen in de drie oceaanbekkens waar schildpadden voorkomen. De bevindingen kunnen ook worden gebruikt om de gevolgen voor ecosystemen te voorspellen, die toekomtige verschuivingen kunnen veroorzaken.

In tropische ecosystemen met zeegras, waar groene schildpadden als mega-herbivoor voorkomen, worden wereldwijd drie niveau’s aan graasintensiteit waargenomen (a). Na eeuwenlange achteruitgang ontvouwt de ecologische rol van de schildpad zich snel in talrijke foerageergebieden waar populaties zich door natuurbescherming herstellen. Hier wordt een toename van het aantal overbegrazingen geregistreerd. Onderzoekers hebben met veldexperimenten de effecten beoordeeld van gesimuleerde graasintensiteit (op verschillende niveaus) op de functies van ecosysteemfuncties, en op multifunctionaliteit (b). Deze experimenten vonden gedurende 18 maanden plaats in een tropisch zeegras-ecosysteem in het Caribisch gebied.

More info in the Dutch Caribbean Biodiversity Database

Published in BioNews 59