Halophila stipulacea

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.

Dutch, Papiamentu, and Papiamento below

Researchers from Wageningen University & Research and the University of Amsterdam report on a fascinating case of competition between an animal and an invasive plant. In tropical ecosystems, photosynthesizing organisms are continuously competing for space and light. The invasive seagrass Halophila stipulacea has been very successful in new habitats both in the Mediterranean and Caribbean seas. 

Photo: Erik Wurz

The seagrass can quickly colonize new habitats because small fragments break off, remain viable and spread via currents. In the new paper Battle for the mounds: Niche competition between upside-down jellyfish and invasive seagrass published in the scientific journal Ecology, the researchers report on their discovery that the invasive seagrass uses little mounds – created by burrowing animals as shrimp or seacucumbers – as a new habitat to settle and expand from. The mounds provide new space with sufficient light, opening up the dense meadows of native seagrass where the invasive seagrass otherwise cannot settle. From there, they observed that the invasive seagrass can spread. 

High and open locations are in demand 

Researching the habitat of the upside-down jellyfish (Source: Erik Wurz)  Photo: Erik Wurz

But the researchers also found that this can cause problems for native species. “The upside-down jellyfish (Cassiopea spp) lies upside down because it has photosynthetic algae in its tentacles. Therefore, these organisms also need light and prefer open spaces such as these mounds created by burrowing animals,” according Fee Smulders of Wageningen University & Research, and lead author of the study. “Msc student Naomi Slikboer recorded the presence of both invasive seagrass and upside-down jellyfish on many of these mounds on the island of Curaçao. She found that often the invasive seagrass pushes the upside-down jellyfish out of these habitats over time.” 

Jellyfish move away more often 

Researching the habitat of the upside-down jellyfish (Source: Erik Wurz) Photo: Erik Wurz

This probably increases the energetic costs for the jellyfish as it has to move more often due to rapid overgrowth of H. stipulacea. Additionally, the authors hypothesize that the interplay between invasive seagrass and burrowing mounds will lead unstable, dynamic seagrass meadows, unfavorable for valuable native seagrass species. Smulders: “We need to keep a close watch on this invasive seagrass and investigate the impact on both native species as well as the seascape patch dynamics in Caribbean seagrass meadows.” 

For more information fin the article Battle for the mounds: Niche competition between upside-down jellyfish and invasive seagrass  published in the scientific journal Ecology in the DCBD database. 

The 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 of one of those scientific studies but the study itself is not a DCNA study. 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. 

Text: Wageningen Environmental Research

Onderzoekers van Wageningen University & Research en de Universiteit van Amsterdam hebben ontdekt dat een invasieve plant de competitie aangaat met een inheemse kwal. De zeegrassoort Halophila stipulacea is erg succesvol in het koloniseren van nieuwe gebieden in de Middellandse Zee en de Cariben. 

Onderzoek naar leefgebied inheemse ‘ondersteboven’ kwal in het Caribisch gebied (Bron: Erik Wurz)

Het zeegras kan zich verspreiden doordat fragmentjes afbreken, lang in leven blijven en met de stroming mee nieuwe gebieden bereiken. In het nieuwe artikel Battle for the mounds: Niche competition between upside-down jellyfish and invasive seagrass in het wetenschappelijke tijdschrift Ecology beschrijven de onderzoekers hun ontdekking dat het invasieve zeegras kleine heuveltjes – gemaakt door gravende diertjes als garnalen of zeekomkommers – gebruikt als startpunt om nieuwe gebieden te begroeien. Deze heuveltjes bieden ruimte en licht, een ideale omgeving voor het zeegras. Vanaf daar kan de invasieve plant zich verspreiden door het inheemse grasveld. 

Lichte, hoge plekken zijn gewild 

Onderzoek naar leefgebied inheemse ‘ondersteboven’ kwal in het Caribisch gebied (Bron: Erik Wurz)

Maar de onderzoekers ontdekten ook dat dit problemen op kan leveren voor inheemse soorten. “De ‘ondersteboven’ kwal (Cassiopea spp) ligt op zijn rug omdat er fotosynthetiserende algen in zijn tentakels zitten. Daarom hebben deze kwallen ook licht nodig, en verblijven ze graag op open plekken zoals de kale heuveltjes,” zegt Fee Smulders, promovendus bij Wageningen University & Research en hoofdauteur van de studie. “MSc student Naomi Slikboer volgde een aantal van deze heuvels door de tijd, en noteerde de aanwezigheid van zowel de kwallen als het invasieve zeegras op Curaçao. Zij ontdekte dat er in het begin veel kwalletjes op de heuvels lagen, maar dat de meeste heuvels op het eind begroeid waren met invasief zeegras.” 

Kwallen verhuizen vaker 

Onderzoek naar leefgebied inheemse ‘ondersteboven’ kwal in het Caribisch gebied (Bron: Erik Wurz)

Door de komst van het zeegras moeten de kwallen vaker verhuizen en verliezen ze mogelijk hun habitat. Verder verwachten de onderzoekers dat het samenspel tussen het invasieve zeegras en de heuveltjes zal zorgen voor een onstabiel dynamisch zeegrasveld, wat nadelig is voor het waardevolle inheemse zeegras. Smulders: “Het is belangrijk om deze exoot goed in de gaten te houden en zijn invloed op zowel inheemse soorten als het gehele onderwaterlandschap te onderzoeken.” 

Meer informatie 

Het artikel Battle for the mounds: Niche competition between upside-down jellyfish and invasive seagrass in het wetenschappelijke tijdschrift Ecology. 

Tekst: Wageningen Environmental Research
 

DCNA 

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

Investigadónan di Wageningen Unisversity & Research i Universidat di Amsterdam a deskubrí un mata invasivo ku ta kompetí ku un kimakima indígeno. E espesie di yerba di laman Halophila stipulacea ta hopi eksitoso den kolonisashon di áreanan nobo den Laman Mediteráneo i den Karibe.

Investigashon pa loke ta trata e área di biba di e kimakima indígena ‘ariba abou’ den área karibense (Fuente: Erik Wurz)

E yerba di laman por plama pa motibu ku hopi fragmento chikitu ta kibra kita ‘fo, keda na bida hopi tempu largu i alkansá áreanan nobo pa medio di drif bai ku koriente di laman. Den e artíkulo nobo ‘ Battle for the mounds: Niche competition between upside-down jellyfish and invasive seagrass’ den e revista sientífiko Ecology, e investigadónan ta deskribí kon nan a deskubrí ku e yerba di laman invasivo ta usa seritu chikitu – ku bestia ku ta koba manera kabaron òf lol’i awa ta traha – komo punto di salida pa krese den áreanan nobo. E seritunan akí ta ofresé espasio i klaridat, un ambiente ideal pa e yerba di laman. For di einan e mata invasivo por plama atraves di e kama di yerba di laman. 

Sitionan haltu ku klaridat ta hopi gustá 

Investigashon pa loke ta trata e área di biba di e kimakima indígena ‘ariba abou’ den área karibense (Fuente: Erik Wurz)

Pero e investigadónan a deskubrí ku esaki por okashoná problemá tambe pa espesienan indígeno. “E kimakima ‘ariba abou’ (Cassiopea spp) ta drumi riba su lomba, pasobra tin alga fotosintétiko den su tenglanan. P’esei e kimakimanan akí tin mester di klaridat tambe, i nan gusta permanesé na sitionan habrí, manera e seritunan sin nada riba nan”, segun Fee Smulders, ku ta serka di optené su título komo dòktor na Wageningen University & Research i kende ta e outor prinsipal di e investigashon akí. “Durante algun tempu e studiante pa Master of Science Naomi Slikboer a sigui e seritunan akí, i el a nota presensia di tantu e kimakimanan komo e yerba di laman invasivo na Kòrsou. El a deskubrí ku na kuminsamentu tabatin hopi kimakima riba e seritunan, pero ku na final mayoria di e seritunan tabatin yerba di laman invasivo ta krese riba nan.” 

Kimakima ta muda mas biaha 

Investigashon pa loke ta trata e área di biba di e kimakima indígena ‘ariba abou’ den área karibense (Fuente: Erik Wurz)

Pa motibu di binida di e yerba di laman, e kimakimanan mester muda mas biaha i posibelmente nan ta pèrdè nan habitat. Mas aleu e investigadónan ta verwagt ku e interakshon entre e yerba di laman invasivo i e seritunan lo sòru pa un kama di yerba di laman dinámiko i instabil, loke ta desbentahoso pa e yerba di laman indígeno balioso. Smulders: “Ta importante pa vigilá e espesie eksótiko akí bon i investigá su influensia riba tantu e espesienan indígeno komo riba henter e paisahe bou di laman.” 

Mas informashon 

E artíkulo ‘ Battle for the mounds: Niche competition between upside-down jellyfish and invasive seagrass’ den e revista sientífiko Ecology. 

Teksto: Wageningen Environmental Research 

 DCNA 

Duthc Caribbean Nature Allianc (DCNA) ta sostené komunikashon sientífiko i alkanse den región hulandes karibense pa medio di hasi informashon sientífiko relatá na naturalesa disponibel den un forma mas amplio, via entre otro Dutch Caribbean Biodiversity Database, e plataforma di notisia BioNews di DCNA i via prensa. E artíkulo akí ta kontené resultado di un di e investigashonnan sientífiko ei, pero e investigashon mes no ta un investigashon di DCNA. No por derivá ningun derecho for di e kontenido. DCNA no ta responsabel pa e kontenido i e konsekuensianan (in)direkto ku ta surgi  pa motibu di publikashon di e artíkulo akí.  

Investigadonan di Wageningen University & Research y Universidad di Amsterdam a descubri un mata invasivo cu ta competi cu un kimakima indigeno. E especie di yerba di lama Halophila stipulacea ta hopi exitoso den colonisacion di area nobo den Lama Mediteraneo y den Caribe.

Investigacion pa loke ta trata e area di biba di e kimakima indigena “ariba abou” den area Caribense (Fuente: Erik Wurz)

E yerba di lama por plama pa motibo cu hopi fragmento chikito ta kibra kit’afo, ta keda na bida hopi tempo largo y ta alcansa area nobo door di drif bay cu coriente di lama. Den e articulo nobo Battle for the mounds: Niche competition between upside-down jellyfish and invasive seagrass den e revista cientifico Ecology, e investigadonan ta describi con nan a descubri cu e yerba di lama invasivo ta uza serito chikito – traha door di bestia cu ta coba manera cabaron of lol’i awa – como punto di salida pa crece den area nobo. E seritonan aki ta ofrece espacio y claridad, un ambiente ideal pa yerba di lama. For di eynan e mata invasivo por plama atraves di e cama di yerba di lama.

Sitio halto cu claridad ta hopi gusta

Researching the habitat of the upside-down jellyfish (Source: Erik Wurz)  Photo: Erik Wurz

Pero e investigadonan a descubri cu esaki por ocasiona problema tambe pa especienan indigeno. “E kimakima ‘ariba abou’ (Cassiopea spp) ta drumi riba su lomba, pasobra tin alga fotosintetico den su tenglanan. P’esey e kimakimanan aki mester di claridad tambe, y nan gusta permanece na sitio habri, manera e seritonan sin nada riba nan”, segun Fee Smulders, kende ta cerca di obtene su titulo como doktor na Wageningen University & Research y tambe ta autor principal di e investigacion aki. “Durante algun tempo e studiante di Master of Science Naomi Slikboer a sigui e seritonan aki, y el a nota presencia di tanto kimakima como yerba di lama invasivo na Corsou. El a descubri cu na cuminsamento tabatin hopi kimakima riba e seritonan, pero cu na final mayoria di e seritonan tabatin yerba di lama invasivo ta crece riba nan.”

Kimakima ta muda mas biaha

Researching the habitat of the upside-down jellyfish (Source: Erik Wurz) Photo: Erik Wurz

Pa motibo di binida di e yerba di lama, e kimakimanan mester muda mas biaha y posiblemente nan ta perde nan habitat. Mas aleu e investigadonan ta spera cu interaccion entre e yerba di lama invasivo y e seritonan lo percura pa un cama di yerba di lama dinamico y instabil, loke ta desbentahoso pa e yerba di lama indigeno balioso. Smulders: “Ta importante pa vigila e especie exotico aki bon y investiga su influencia riba tanto e especienan indigeno como riba henter e paisahe bou di lama.”

Mas informacion

E articulo Battle for the mounds: Niche competition between upside-down jellyfish and invasive seagrass den e revista cientifico Ecology.

DCNA

Dutch Caribbean Nature Alliance (DCNA) ta sostene comunicacion cientifico y alcanse den region Caribe Hulandes pa medio di haci informacion cientifico relata na naturalesa disponibel den un forma mas amplio, via entre otro Dutch Caribbean Biodiversity Database, e plataforma di noticia BioNews di DCNA y via prensa. E articulo aki ta contene resultado di un di e investigacionnan cientifico ey, pero e investigacion mes no ta un investigacion di DCNA. No por deriva ningun derecho for di e contenido. DCNA no ta responsabel pa e contenido y e consecuencianan (in)directo cu ta surgi  pa motibo di publicacion di e articulo aki.

Published in BioNews 62

In tropical ecosystems, autotroph organisms are continuously competing for space, with some plant species benefiting from disturbances such as fire, grazing, or bioturbation that clear habitat (Pulsford et al. 2016). These disturbances can open up layers of vegetation, thereby promoting colonization of opportunistic species that would have been competitively inferior without disturbance (Castorani et al. 2018). Opportunistic fast-growing species also include often invasive species that are therefore also likely to increase in dominance after disturbance (Altman and Whitlatch 2007). In seagrass meadows in the southern Caribbean, we observed that the marine invasive plant Halophila stipulacea uses bioturbation mounds, created by burrowing infauna such as sea cucumbers and shrimp (see Suchanek 1983), to colonize new habitats (Figure 1a, b). On Bonaire and Curaçao, in habitats with ~100% native Thalassia testudinum cover, invasive H. stipulacea often at first only occurred on bioturbation mounds that smothered native T. testudinum seagrass, likely due to fragmentation and subsequent settlement (Smulders et al. 2017). These observations suggest that bioturbation mounds serve as starting points for further invasion (Fig. 1c).  

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 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, Red Sea) and invasive H. stipulacea plants in a common-garden mesocosm. Lastly, we also 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 mesocosm experiments 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.

Seagrasses are marine flowering plants that can reproduce both sexually (through flowering and subsequent seed formation) and asexually (through clonal growth). Sexual reproduction increases genetic diversity, resilience and dispersal success of seagrasses. A recent study discovered that the first report of sexual reproduction of a successful invasive seagrass was incorrect, and therefore released a new field guide to improve future determination.

Photo credit: Henkjan Kievit

The invasive seagrass Halophila stipulacea originating from the Red Sea and Western Indo-Pacific, has been successfully invading the Mediterranean Sea since 1894 and the Caribbean Sea since 2002. It was shown to outcompete native seagrass species and affect local ecosystem functioning.

In this new paper, WUR PhD candidate Fee Smulders found that so far, only male flowers have been described of the successful invasive seagrass species H. stipulacea in the Caribbean Sea. Female flowers and fruits have not been reported. This means that fragmentation and fast clonal growth may be the only factors explaining its current success, without genetic adaptation capacity. This needs to be taken into account in further studies studying H. stipulacea expansion.

In-depth monitoring of reproductive structures in invaded seagrass meadows, both in the Mediterranean and the Caribbean Sea is important to assess further invasion potential.

Because the fruits and flowers of  H. stipulacea have been misidentified in the past, we have developed a field guide with a dichotomous key, to take into the field and easily identify the various structures by eye. We call upon (citizen) scientists to keep an eye out underwater when they are in the Caribbean, to be able to predict future invasion success of this species.

– Fee Smulders

The field guide can be found in the supplementary material of the paper, and reports can be made in the online global database www.seagrassspotter.org.

Or check the field guide directly in the DCBD:

https://www.dcbd.nl/sites/default/files/documents/FIeldguide_Hstipulacea...

Article published in BioNews 44

Abstract

Seagrasses are increasingly being recognized for their potential in protecting the coastlines from flooding and erosion. As ecosystem engineers, seagrasses can attenuate waves as well as trap and stabilise sediment, causing the seabed to become more stable, which can contribute to coastal protection. Where many tropical countries lack the means to undertake traditional coastal protection measures, natural ecosystems, such as seagrass meadows, can provide cost-effective alternatives to protect coastal communities from natural hazards and improve their well-being, as ecosystem services are continuously provided. The Caribbean regions holds a large cover in seagrass relative to the coastlines, yet few studies have focused specifically on seagrass and sediment stability in this region. Over the last decades, native seagrass meadows have been degrading and the non-native Halophila stipulacea, originating from the Red Sea, has been spreading rapidly through the region relatively recently. A knowledge gap exists in whether this opportunistic species could provide essential ecosystem services, such as coastal protection, where native species have been lost. The extent to which seagrass species contribute to coastal protection services depends on the growth properties of these species as well as the environmental conditions. Therefore, a need remains to study sediment dynamics in seagrass meadows in different scenarios. The present study aims to increase the understanding of the role of H. stipulacea in sediment dynamics, specifying on the coast St. Eustatius, Dutch Caribbean.

First, current available knowledge on sediment stabilising properties of seagrass species in the wider Caribbean region was reviewed. Second, an experiment was set up at a study site with a depth of 18 m, in a monospecific H. stipulacea meadow, and in a sand patch within the meadow. In addition to measuring the growth properties of H. stipulacea, sediment stability was investigated by measuring changes in seabed level over time, as well as using sediment traps to calculate the sediment deposition rate. Data on meteorological conditions was retrieved to investigate correlations between the weather and sediment change.

Results showed that H. stipulacea at a depth of 18 m at the coast of St. Eustatius, grew a new shoot within 3.3 days, over twice as fast as measured for its native range. Average leaf length was 4 cm while eight shoots were observed per strand. Similar values were observed in its native range. Although the leaf length was smaller than most native seagrasses in the Caribbean, H. stipulacea showed a higher density, which likely contributed to the findings of this study. The sediment deposition rate was much lower in the H. stipulacea meadow than in the sand patch, which is probably the result of limited resuspension within the seagrass meadow. After 14 weeks, the sediment in the H. stipulacea meadow had eroded. This is believed to be the result of erosion of the surface sediment layer, a less stabilised layer of sediment that was potentially resuspended and carried away when the weather became more turbulent after months of calm conditions. The subsurface layer, however, is believed to be more stabilised by the root and rhizome system of H. stipulacea, which explains why smaller and relatively constant changes in sediment level were found after weather conditions had increased. The sediment level in the sand patch showed a larger variance over time, also varying much more spatially between erosion and accumulation.

This study showed that H. stipulacea seems to stabilise the subsurface sediment layer via its root and rhizome system, which is assumed to contribute to a more stable seabed. However, this result only became apparent after the erosion of the surface layer and might even disappear with more extreme weather conditions. This study urges for future research to investigate the sediment stabilising effect of H. stipulacea at shallower depths as well as over longer terms, as different results might be found for different seasons.

The success of invasive macrophytes can depend on local nutrient availability and consumer pressure, which may interact. We therefore experimentally investigated the interacting effects of nutrient (nitrogen and phosphorus) addition, the exclusion of large herbivorous fishes and mimicked grazing on the expansion rates of the invasive seagrass Halophila stipulacea. The experiments were established on Bonaire and Aruba, two islands in the southern Caribbean, which differ in fish community structure. We observed that multiple Caribbean fish species feed on H. stipulacea. At both study sites, nutrient enrichment decreased invasive leaf carbon:nitrogen ratios. However only on Bonaire, where herbivore fish abundance was 7 times higher and diversity was 4.5 times higher, did nutrient enrichment result in a significant reduction of H. stipulacea expansion into native Thalassia testudinum meadows. This effect was likely due to increased herbivory on nutrient enriched seagrass leaves, as we found that excluding large herbivorous fish (e.g. parrotfish) doubled invasive expansion rates in bare patches on Bonaire. On Aruba, H. stipulacea expansion rates were higher overall, which coincided with lower abundances and diversity of native fishes, and were limited by mimicked fish grazing. We suggest that top-down control by the native fish community may counteract eutrophication effects by increased grazing pressure on nutrient-rich invasive seagrass leaves. We conclude that diverse and abundant herbivore communities likely play an important role in limiting invasion success and their conservation and restoration may serve as a tool to slow down seagrass invasions.

ABSTRACT: Juvenile queen conch are primarily associated with native seagrass such as Thalassia
testudinum in large parts of their range in the Caribbean and the southern Gulf of Mexico. Here,
a number of non-native seagrass species have been introduced including Halophila stipulacea,
which is natural to the Red Sea and the Indo-Pacific. In the Caribbean, H. stipulacea often creates
dense continuous mats with little or no sediment exposed, compared to native seagrass, which
grows much less dense. We examined the diet and growth of juvenile conch in both native, mixed,
and invasive seagrass beds using stable isotope analysis and an in situ growth enclosure experiment.
Organic material in the sediment (i.e. benthic diatoms and particulate organic matter) was
found to be the most important source of carbon and nitrogen for juvenile queen conch in all 3
habitats investigated, and there was a significantly higher probability of positive growth in the
native seagrass compared to the invasive seagrass. Due to the importance of the organic material
in the sediment as a source of nutrition for juvenile conch, limited access to the sediment in the
invasive seagrass can potentially cause inadequate nutritional conditions to sustain high growth
rates. Thus, it is likely that there is a negative effect on juvenile queen conch growth currently
inhabiting invasive seagrass beds, compared to native seagrass beds, when other potential
sources of nutrition are not available.