Halophila stipulacea

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

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

Diet and growth of juvenile queen conch Lobatus gigas (Gastropoda: Strombidae) in native, mixed and invasive seagrass habitats

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.

Date
2019
Data type
Media
Theme
Education and outreach
Research and monitoring
Geographic location
St. Eustatius
St. Maarten