Proximate response of fish, conch, and sea turtles to the presence of the invasive seagrass Halophila stipulacea in Bonaire

In this report we examined the proximate response of fish assemblages, queen conch, and sea turtles onH. stipulacea meadows in Lac Bay, Bonaire, Caribbean Netherlands. Here we primarily focused on the differences between the invasive species H. stipulacea and the principal species of native sea grass in Lac Bay, namely turtle grass Thalassia testudinum .

We addressed the following questions:

  • Has H. stipulacea expanded in area since 2011 and what effect is observed on T. testudinum cover?
  • How does the structural complexity, in terms of average height and density of vegetation, differ between meadows dominated by T. testudinum and those by H. stipulacea?
  • Do monospecific fields of H. stipulacea differ from monospecific fields of native T. testudinum in terms of fish assemblages and abundances?
  • Does the queen conch avoid H. stipulacea meadows?
  • Will green sea turtles in Lac Bay graze on H. stipulacea?

    In 2011 H. stipulacea was present in 7 of the 45 stations across the bay where the seagrass cover was recorded, while this increased to presence in 12 quadrats in the year 2013. From 2011 compared to 2013 the cover of H. stipulacea had generally increased, while the cover of the native T. testudinum had generally decreased in these quadrats. These results indicate that the invasive species is expanding in area in Lac Bay. It is unclear whether H. stipulacea is actively pushing out the native seagrass species, or whether the native seagrass cover is declining due to other causes and H. stipulacea is rapidly taking over areas that are left open.

    The two species of seagrass differed significantly in habitat complexity, reflected by a difference in the number of shoots and the length of shoots. H. stipulacea had significantly shorter shoots compared to T. testudinum. Higher habitat complexity due to the seagrass canopy (i.e. higher seagrass density, leaf surface or aboveground biomass) is assumed to result in higher faunal abundance due to reduced predation risk and enhanced food supply. Invasive macrophytes can impose changes on native communities via mechanisms that modify the habitat and cause variation in indigenous faunal composition.

    There was a large and significant difference in fish abundance between meadows dominated by T. testudiunum and those dominated by H. stipulacea; the abundance of fish was almost half in the H. stipulacea meadows. This result may be due to the reduced complexity of the invasive seagrass meadows, but also be due to underlying factors that were not measured. There was also a significant difference in the composition of fish species assemblage between T. testudiunum and H. stipulaceameadows. No Pomacentridae, Mullidae, and Sphyraenidae were recorded in the transects placed in H. stipulacea meadows, while these were present in T. testudinum meadows. If this is a long-term trend, the expansion in H. stipulacea may possibly result in a diminished nursery function of certain fish species in Lac Bay.

    Queen conch was equally present on H. stipulacea, native seagrass meadows, and sandy patches. Hence, in the area of observation of the present study, it did not appear to avoid the invasive seagrass. These results only refer to part of Lac Bay, therefore we are cautious with making general conclusions.

    We establish that Caribbean green turtles can feed on the invasive H. stipulacea. This was documented using a cafeteria experimental set-up. All three seagrass species were selected at least once.Thus the present study indicates that the green sea turtle does not necessarily avoid H. stipulacea as a food source. While H. stipulacea is new to the Caribbean, H. stipulacea forms an important food species in its native distributional range for the green turtle in the Red Sea. H. stipulacea is increasing in cover in Lac Bay and may become the main food source for green sea turtles. What is more, when sea turtles consume seagrass the associated invertebrates are an important component of the diet. Invertebrates that are associated with seagrass differ per seagrass species (e.g. Willete & Ambrose 2012) and this could result in a different nutritional uptake for the turtles. It is important to understand the nutritional difference this shift in diet may cause, before it can be concluded what the effect of long term consumption of H. stipulacea on green turtle health may be.

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