A seagrass bed is an ecosystem which often occur alongside coral reefs. Although they are much less charismatic and frequently overlooked, seagrass beds play a vital role in maintaining the health and diversity of adjacent coral reefs. Seagrasses are not one but 50 species of flowering marine plants which usually form dense stands in the shallow, sandy bottom environments of back reef lagoons and semi-enclosed bays. They are bright sunlit fields of waving green fronds and are the favoured habitat of the globally endangered Green Turtle (Chelonia mydas) and Queen Conch (Lobatus gigas). Other common inhabitants are cushion stars, urchins and sea cucumbers. Seagrasses are quite literally a world apart. Scientists have found over 113 species of algae (marine plant) growing on seagrass leaves, not to mention sponges, hydroids, flatworms and tunicates.
One of the most remarkable things about seagrasses is their ability to spread and grow by sending out underground rhizomes (roots). It is also their greatest weakness as it makes them phenomenally vulnerable to trampling and physical damage. Seagrasses trap sediment and bind soil thereby preserving water quality and protecting coastal environments.
Similar to mangroves systems, seagrass beds provide an ideal hiding place for the young of many species of reef fish and other marine life. The Queen Conch (Lobatus gigas) hide for the first year of their life in the sand surrounding seagrass beds before emerging there to forage and live. Adult parrotfish (Scaridae), surgeonfish (Acanthuridae) and porgies (Sparidae) are also frequent visitors, migrating into seagrass environments to feed.
- Seagrasses are flowering plants, sometimes with extensive root systems strongly attached to the sediment (unlike algae)
- 60 species have been described around the world, on Bonaire we have 4 naturally occurring species (turtle grass most common) and 1 invasive species (Halophila stipulacea)
- Seagrass beds are important habitats and nurseries for many animals, including seaturtles, conch, many fish, crabs, anemones, and even sharks; also a hiding place for the young of many species
- They offer coastal protection by sediment stabilization (turtlegrass) – protect our coasts by holding the sediments in place with their root systems and also help maintain water quality
- Help us keep the planet thriving by removing CO2 from the atmosphere (decrease greenhouse gasses and help combat climate change) and storing it in their roots (carbon sequestration) and nutrient cycling (make the nutrients other animals such as corals, fish, turtles, etc need to survive available again for them – multivitamin pills for turtles)
- Seagrass beds are disappearing faster than tropical forests (like the Amazon), maybe just as fast as coral reefs
Research in Bonaire
- In Bonaire the invasive species is increasing in abundance – this one is less tasty and healthy for the turtles; also not as good at providing a home for all the animals that live in seagrasses (lower number of species in invasive seagrass beds than in native seagrass beds); not good at protecting our coast line
- In 2011 the invasive could only be found at certain places in Lac (6% of the area), in 2017 it was found at 27% of locations
- However, there is now a little more total seagrass in Lac (due to expansion of invasive) than there was in 2011
- Invasive species, Halophila, is however very good at taking up any space that is freed up – when seagrass is eaten by turtles, when people trample and kill the existing seagrass beds, sargassum events that end up killing seagrass
- Unlike the native seagrasses, the invasive seagrass floats in the water as a complete plant (with leaves, stems, and roots) making it easier for it to reattach at new locations
- Sea turtles prefer turtle grass, but this might also be helping Halophila to expand into new territories
- The numbers of other species are going down while the numbers of the invasive are going up; in 2013 it was the second most abundant seagrass
- Lac Bay largest sea grass area on Bonaire (200 ha), but we also have Lagoen and Pekelmeer; invasive seagrass has been found at all these places
- Turtle food preference experiments done in Lac bay showed that turtles would much rather eat the turtle grass; Halophila was their least favourite seagrass to eat
- testudinum much more nutritious in grazed patches – (turtles maintain little seagrass gardens. The places where they graze have more nutritious leaves than the places they don’t graze)
- If turtlegrass becomes less abundant the turtles would have to eat the less nutritious seagrass which could impact their ability to grow and reproduce