Seagrass

Superior biological traits of invaded (Caribbean) versus native (Red Sea) populations of the seagrass Halophila stipulacea

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.

Date
2022
Data type
Scientific article
Theme
Research and monitoring
Journal
Geographic location
St. Eustatius

The Pros and Cons of non-native Seagrass Explosive Expansion

A new study from the Caribbean Netherlands Science Institute and Utrecht University investigated the sediment stabilizing ability of non-native seagrass species Halophila stipulacea, found off the coast of St. Eustatius.  This new fast-growing seagrass has rapidly outpaced native species, leaving many to wonder if its explosive growth will be to the benefit or detriment to the island.

Halophila stipulacea growing into a sand patch. Photo credit: Francine M. van Hee

From anchoring sediments to creating fields of foraging and nursery areas, seagrass meadows play an important role in near shore environments.  Alarmingly, research from St. Eustatius has revealed that the seagrass meadows which once encompassed the entire island are now limited to a small area along the northern shelf. Furthermore, where native species such as turtle grass (Thalassia testudinum) and manatee grass (Syringodium filiforme) have disappeared the non-native species Halophila stipulacea has rapidly spread.

A new study conducted by the Caribbean Netherlands Science Institute and Utrecht University worked to explore the impacts of this population shift.  Namely, the study hoped to determine if this new species could stabilize sediments and help prevent coastal erosion as effectively as native species historically found in this area.

Sand Stability

After 14 weeks, researchers found that sediment within the H. stipulacea meadow had eroded, but this was mostly within the surface sediment layer.  The subsurface layer, however, was believed to be more stable due to the root and rhizome system of the seagrass and saw overall smaller variances when compared to bare sand areas, even during more extreme weather conditions.  Overall, there was less resuspension of sediment in non-native seagrass patches and more long-term sediment level stability when compared to open sand areas.

Habitat Forming

Seagrass meadow with sponge at Double Wreck. Photo credit: Francine M. van Hee

In addition to stabilizing sediments, this new habitat was found to support diverse invertebrate and fish populations.  More specifically, H. stipulacea seems to be popular among filter feeders such as sponges and bivalves, which then in-turn lures in predatory fish, improving overall biodiversity within the area.  Furthermore, since seagrass tends to grow adjacent to coral reefs, algae eating fish which are attracted to the seagrass then work to clean the neighboring corals. Finally, limited resuspension of the sediment coupled with the high update rates of nutrients by seagrass also means clearer waters for the neighboring corals

Implications

Although perhaps not as efficient as native seagrasses, the non-native seagrass H. stipulacea does appear to have some advantages.  Its fast-growing nature means that it can recover more rapidly after extreme storm events than native species. Its rapid expansion could help counter the massive seagrass losses seen around the island, contributing to more sand stability and improved habitat for a wide variety of marine life.

To learn more, the full report can be found on the Dutch Caribbean Biodiversity Database using the link below.

More info in the Dutch Caribbean Biodiversity Database

 

 

Published in BioNews 58.

 

Date
2022
Data type
Media
Theme
Research and monitoring
Geographic location
St. Eustatius
Author

Range expansion of Marinomyxa marina, a phytomyxid parasite of the invasive seagrass Halophila stipulacea, to the Caribbean

Abstract

Halophila stipulacea, a small seagrass species native to the Indo-Pacific, is a Lessepsian migrant and a high-profile invader that has successfully colonized two exotic regions, the Mediterranean (first observed in 1894) and the Caribbean (2002). In 1961, an intracellular phytomyxid parasite, Marinomyxa marina (SAR: Rhizaria: Endomyxa: Phytomyxea) was discovered in the petioles of H. stipulacea in the Red Sea, and three decades later, it was reported off the coast of Sicily (Mediterranean), suggesting parallel migration of the two organisms. In 2018, infected petioles of H. stipulacea were also observed in St. Eustatius (Caribbean), but the identity of the causative agent remained unresolved. Here, we provide information on four new localities of phytomyxid-infested populations of H. stipulacea in Greece (Mediterranean), and Bonaire and Martinique (Caribbean), including notes on infection prevalence and seasonal dynamics. Using the 18S rRNA barcoding gene, we bring molecular evidence that the disease is caused by a genetically uniform variant of M. marina at all the examined sites. We conclude that the parasite is now widespread throughout both invaded regions and has been present in the Caribbean since 2013 at the latest. For the first time, the production of fruits in infected plants is observed, indicating a non-lethal nature of the symbiosis. While the arrival of M. marina to the Caribbean is unlikely to alleviate the current invasiveness of H. stipulacea, we emphasize the need for its further monitoring since the host-specificity and general biology of seagrass-associated phytomyxids are still poorly understood.

 

 

View full text https://www.sciencedirect.com/science/article/abs/pii/S0304377022000663

Date
2022
Data type
Scientific article
Journal
Geographic location
Bonaire

Protecting Fish Proven Key in Slowing Down Invasive Seagrass

New research from Wageningen University, University of Amsterdam and Florida International University highlighted the role herbivorous fish species plays in staving off nonnative seagrass invasions.  A healthy and diverse fish population can provide top-down control by grazing on invasive seagrass species, minimizing its overall invasion.

Invasive species can pose a direct threat to native species through competition and hybridization. Species which evolved to reproduce and spread rapidly generally have a greater chance at survival, and when introduced to a new environment, can out compete slower growing native species. This is certainly the case for Halophila stipulacea, a seagrass native to the Red Sea, Persian Gulf and Indian Ocean which has been rapidly gaining habitat within the Caribbean since its first reported sighting in 2002. This species is quickly outpacing native Thalassia testudinum (turtle grass) and Syringodium filiforme (manatee grass), both of which provide critical habitat, coastal protection and foraging grounds. 

 

Mixed area of Halophila stipulacea (shorter blades) and Thalassia testudinum (longer blades). Photo Credit: Fee Smulders

The Study

A new study by Wageningen University and Research, the University of Amsterdam and Florida International University worked to improve overall understanding of the controlling factors in the spread of invasive seagrasses.  Researchers investigated the influences of local nutrient enrichment (nitrogen and phosphorus) as well as the impact of large herbivorous fish on the growth and expansion rates of Halophila stipulacea. The study took place between 2018 and 2019 within two seagrass meadows, Lac Bay on Bonaire and Barcadera on Aruba.

Seagrass Meadow. Photo Credit: Fee Smulders

The Results

At both sites, nutrients were added to selected seagrass plots by using slow-release fertilizer. Interestingly, only on Bonaire did these excess nutrients actually result in a reduction of H. stipulacea’s expansion into the turtle grass meadows, while native seagrass was unaffected.  This is believed to be because on Bonaire, herbivore fish abundance is 7 times greater and diversity is 4.5 times higher than on Aruba, therefore excess nutrients likely enticed more fish to graze therefore limiting the spread of the invasive seagrass.  Native seagrass is more adapted to high grazing pressures, during this study grazing pressure increased after nutrient enrichment but only the invasive species showed lower expansion rates.  In fact, the exclusion of large herbivorous fish (like parrotfish) doubled the invasive expansion rates within sandy patches on Bonaire, further strengthening this theory.

Top-Down Approach

This study highlights the importance of holistic approaches to ecosystem management.  Healthy and diverse fish communities can provide top-down control to invasive species expansion Increasing grazing pressures can help reduce the competitive advantage of fast-growing species, slowing down invasion of non-native species. The key to seagrass restoration and conservation could lie in protecting the biodiversity of these fragile areas.

To learn more, please find the full report on the Dutch Caribbean Biodiversity Database using the button below.

https://www.dcbd.nl/document/fish-grazing-enhanced-nutrient-enrichment-m...

 

Article published in BioNews 49

Date
2021
Data type
Media
Theme
Education and outreach
Research and monitoring
Geographic location
Aruba
Bonaire
Author

New field guide on invasive seagrass flowers

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

 

Date
2021
Data type
Media
Theme
Education and outreach
Research and monitoring
Geographic location
Aruba
Bonaire
Curacao
Saba
Saba bank
St. Eustatius
St. Maarten
Author

Seagrasses and coastal protection: The role of Halophila stipulacea in sediment stability at the coast of St. Eustatius, Dutch Caribbean

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.

Date
2022
Data type
Research report
Theme
Research and monitoring
Report number
MSc Thesis
Geographic location
St. Eustatius

Fish grazing enhanced by nutrient enrichment may limit invasive seagrass expansion

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.

Date
2021
Data type
Scientific article
Theme
Research and monitoring
Journal
Geographic location
Aruba
Bonaire

A comparison of fish communities of subtidal seagrass beds and sandy seabeds in 13 marine embayments of a Caribbean island, based on species, families, size distribution and functional groups

Differences in fish community structure between different estuaries, lagoons and bays can be very large, and generalisations are complicated by the use of a wide variety of sampling methods. In the present study, fish communities of subtidal seagrass beds and sandy seabeds in 13 marine embayments of a single Caribbean island were therefore sampled using a uniform method. The objective of the study was to determine whether the seagrass and sandy seabed habitats of various embayments are characterised by typical fish assemblages which differ in terms of taxa (species, families), size classes (life stages) and functional groups (ecological species groups, feeding time and diet). This was linked to the hypothesis that differences in fish assemblages between habitats in different embayments are larger at taxonomic levels than at the level of functional groups. A second objective was to determine the most useful discriminating features between the two habitat types. The above hypothesis was rejected, since differences in fish assemblages from different seagrass and sandy seabed sites did not increase from functional to taxonomic level, but from size class to diet/species to family/feeding time to ecological species group. However, the seagrass and sandy seabed habitats could each be characterised by typical fish assemblages which differed in taxonomical and functional group composition, irrespective of differences in environmental and biotic variables between the embayments in which these habitats were situated. The two habitat types could be best characterised on the basis of fish family, ecological species group, feeding time and size distribution. Seagrass beds mainly harboured nocturnally active nursery species (Haemulidae, Lutjanidae, etc.), whose relative abundance was related to vegetation (mainly seagrass) cover. Sandy seabeds mainly harboured diurnally active bay species (Gerreidae, etc.) whose relative abundance was related to cover of bare sand. Similarities in taxonomical and functional traits of fish species predicted whether they occurred more abundantly in seagrass beds or in sandy seabeds.

Date
2003
Data type
Scientific article
Theme
Research and monitoring
Geographic location
Curacao

Distinct Microbiomes in three Tropical seagrasses around the island of Curaçao: Halophila stipulacea, Halodule wrightii and Thalassia testudinum

Seagrasses represent the unique re-colonization of the marine ecosystem by angiosperms. As their terrestrial relatives, seagrasses are important habitat providers but in contrast, their microbiomes are still poorly known. The microbial community associated with terrestrial plants is intensively studied and plays an important role in plant fitness. The close relation of seagrasses to terrestrial plants suggests a resemblance in survival strategies, including the creation of a microbiome distinct of the surrounding environment. To obtain more knowledge regarding seagrass microbiomes and their intra- and interspecies differentiation, samples of three tropical seagrass species occurring around the island of Curaçao, the invasive Halophila stipulacea and the natives Halodule wrightii and Thalassia testudinum, were collected. Root and leaf-associated microbes were separately analyzed using high throughput Illumina sequencing of the region V5-V7 of the 16S rRNA gene. Sequences were aligned and clustered into Operational Taxonomic Units (OTUs). Results displayed the occurrence of a seagrass-specific microbiome, distinct from that of the surrounding seawater and sediment. The existence of a species and tissue (root/leaf) specific bacterial community and structure was detected, along with a bacterial community that was shared among the seagrasses. OTUs belonging to the shared seagrass community were mostly of the orders rhizobiales. Desulfobacterales was the most abundant order associated with the roots and Rhodobacterales with the leaves of the three seagrass species. Species-specific bacteria are represented mostly by OTUs of the same orders as the common OTUs, along with a few species-specific orders. The high abundant and widespread bacterial OTUs were identified to be mostly associated with sulfur and nitrogen cycling, which point towards the importance of these processes in seagrass fitness.

Date
2018
Data type
Research report
Theme
Research and monitoring
Report number
Student Thesis
Geographic location
Curacao
Author