Sargassum: Algae, Not Seagrass
Many confuse "seaweed" and "seagrass", assuming they are different names for the same organism as the two sound similar. On top of that, they resemble one another in appearance, and both grow underwater. However, they are actually classified as two completely different organisms. Hong Kong is home to over 200 species of seaweed, roughly categorised into three main groups based on colour: brown algae, red algae, and green algae. Among these, Sargassum (Sargassum spp.) belongs to the brown algae group. It is capable of withstanding strong waves, providing refuge for many marine creatures from turbulent waters and thus playing a significant role in marine ecology.
Sargassum mostly grows on rocks between the mid to low tide zones, down to depths of one to two metres below the surface of water. Some species can grow as deep as ten metres below the sub-tidal zone. They are widely distributed and more commonly found in the eastern waters of Hong Kong.
Sargassum may resemble plants in appearance and obtain nutrients through photosynthesis, but they are not plants and lack roots, stems, and leaves. Its body, called the "thallus", is brown and composed of blades, stipes, and holdfasts. The blades are long and slender, resembling horse tails, hence the name; the stipes are also elongated, similar to plant stems, while the holdfasts secure the body to substrates such as rocks.
Miniature Underwater Forests
Many Sargassum species have dense, round air bladders at the base of their blades, which allow their entire body to float in water and maintain a vertical posture. Sargassum can grow up to two to three metres long, resembling trees standing upright in the water. When a large number of Sargassum grows together and forms Sargassum beds, they create a miniature underwater forest.
Due to the dense and year-round growth of its blades, Sargassum provides a habitat for many small coastal animals. In particular, it provides refuge for the young of some marine animals from predators and turbulent currents, thus playing a significant role in the ecological restoration and sustainable development of nearshore fisheries.
Research conducted by a local university on the ecology of Sargassum beds documented up to 46 species of animals living within them, including gammarid amphipods, isopods, gastropods, and polychaetes, all of which are present year-round. Additionally, copepods and caprellid amphipods are known to appear in large numbers in Sargassum beds during specific seasons. The different animals that inhabit the Sargassum beds, use them for shelter, food, or as a place for spawning or nursing their young.
Beneficial to Harmful
Many brown algae species prefer temperate waters with lower temperatures, but Sargassum is unique in that it can thrive in warmer tropical waters. Consequently, with climate change exacerbating, scientists are concerned that the Sargassum may expand its range and proliferate abundantly in new coastal habitats, becoming an invasive species.
When Sargassum spreads to other water bodies, there is a possibility of massive outbreaks leading to harmful algal blooms, rendering what is originally beneficial to the ecosystem a menace that clogs beaches and causes marine organisms to suffocate and die.