Seagrasses are flowering plants that live underwater. Unlike algae, they have roots, leaves and flowers for reproduction and are entirely adapted to life in the ocean. They are found in sheltered bays and coastal areas in the lagoon and are mostly found within a maximum of 20 meters’ depth, as they require light for photosynthesis. Seagrasses help stabilize lagoon sediment and sand and help prevent beach erosion. Seagrass beds are an important habitat for small crustaceans, sea cucumbers, herbivorous fish and provide feeding grounds for green turtles.
Seagrasses may resemble seaweeds, but they are very different. Seagrasses are part of the monocotyledons group of plants that include grasses, lilies and palms. Like other terrestrial plants, seagrasses have leaves, roots and veins, and produce flowers and seeds. They photosynthesise i.e. they use the sun’s energy through chloroplasts in their leaves to convert carbon dioxide and water into sugar and oxygen for growth, while their veins transport nutrients and water throughout the plant. Like other flowering plants, their roots and rhizomes can absorb and store nutrients and hold the plant to the seabed. Unlike terrestrial flowering plants, they have little air pockets called lacunae that help keep the leaves buoyant. They have a thin cuticle layer to enable gasses and nutrients to diffuse directly in and out of the leaves instead of the pores which terrestrial plants have to enable gas exchange. Seagrasses grow both vertically and horizontally—their blades reach upwards and their roots down and sideways—to capture sunlight and nutrients from the water and sediment. They spread by two methods: asexual clonal growth and sexual reproduction. There are eight species of seagrasses in Mauritius, four of which are present in Bel Ombre, they form mixed meadows in the sheltered parts of the lagoon.
Seagrasses provide a myriad of ecosystem benefits. They contribute to water clarity and quality by generating oxygen every day through photosynthesis and capturing sand, dirt and silt particles. Their roots traps and stabilize the sediment and reduce erosion. Seagrasses play an important role in nutrient cycling in the lagoon: they absorb nutrients in runoff from the land but also release nutrients through their leaves.
Their leaves and roots provide shelter to many marine creatures, ranging from small invertebrates like crabs and shrimp, to small fish and juveniles of larger species, and sponges, clams, polychaete worms and anemones may be nestled between the leaves or in the sediment. These smaller animals attract larger ones such as octopus, squid, cuttlefish, among many others. Seagrasses are also an important habitat for green turtles, which feed on their leaves.
Seagrasses are threatened by coastal development, where the establishment of swimming zones has often led to their destruction. The gradual degradation of the reef, rising sea levels and stronger storms also threaten these plants which thrive in calmer waters. Excessive suspended sediments can smother the seagrasses and the organisms that live in them. Protecting seagrass beds is vital to keeping the lagoon ecosystem healthy. Where seagrasses have been removed, restoration of seagrass beds may be an option.
Patch Reefs are quite common in shallow lagoons and are relatively small reef outcrops, often surrounded by sand or near seagrasses. Corals are unique animals made of tiny polyps which live in symbiosis with a microscopic algae called zooxanthellae. The latter give the coral their colour and provide the polyps with the majority of the energy they require through photosynthesis. Like other invertebrates, the skeleton of corals is found outside of their body. Corals grow in warm, clear and shallow waters. They protect the shoreline from erosion and strong wave action during extreme weather events. Corals are considered as the rainforests of the sea, being host to 25% of marine life on the planet. Local commercial fishes such as Capitaine and lobsters depend on coral reefs for their survival. Patch reefs host a variety of marine life including, echinoderms such as starfish and sea cucumbers and many reef fish including parrotfish, surgeonfish and damselfish.
Coral patches found in the lagoon are typically different than those found on the reef crest and the outer reef slope. The corals found in the lagoon thrive in calmer waters and are less tolerant to strong wave action. Some of the most common corals found in the lagoon are Acropora species, which grow faster but may also break more easily. The lagoon outcrops are home to smaller species of fish and invertebrates as well as juvenile fish – which may migrate to the reef crest and external reef slope as they become adults.
Although most of the wave energy from the open ocean has been attenuated by the reef crest, coral patches play a role in further slowing down wave action before it reaches the shoreline. Like all corals, lagoon coral patches provide important spawning, nursery, breeding and feeding areas for a multitude of organisms. Patch reefs are also used for many coastal leisure and recreational activities such as snorkelling and glass-bottom boat tours.
Lagoon patch reefs have been affected by excessive nutrients from inland runoff and pollution, coastal boating and recreational activities. They are increasingly vulnerable to the effects of climate change with rising sea temperatures leading to coral bleaching. Management of lagoon activities and reduction of direct human impacts
through best practices and codes of conduct can ensure increased resilience of the corals to the effects of climate change. Marine Protected Areas are part of the tools used to better manage and protect lagoon patch reefs.
Fringing reefs are one of the four main types of coral reefs, along with barrier reefs, atolls and patch reefs. Based on a theory developed by Darwin, scientists believe that on volcanic islands, fringing reefs are the first type of reef to develop when a volcano becomes extinct and the island and ocean floor subside, allowing reefs to build a shallow lagoon between the land and the main reef. While some fringing reefs grow directly from the shore, many fringing reefs are a few hundred meters or more from the shore and contain an extensive backreef (lagoon) area which provides ideal conditions for patch reefs, seagrasses and mangroves to grow in the sheltered lagoon. In the Bel Ombre lagoon the corals can be seen growing from the shore or just after a small passage used by boats. Towards the south of the lagoon, the water depth is extremely shallow, and boats cannot pass over the area.
Fringing reefs are the most common reefs found around Mauritius. They are highly productive marine ecosystems with thousands of organisms living within and around them. They are usually associated and interconnected with seagrass beds and mangrove ecosystems for nutrient cycling and biodiversity exchange. Coral reefs – unlike rocky reefs – are structures created by biological processes, namely the growth and death of reef-building corals, sponges, and other immobile marine animals. Coral reefs can be zoned into three parts; the back reef, reef crest and reef slope. The back reef, which includes the shallower, shoreward side of the reef and lagoon. This habitat includes small patches of coral, seagrasses, algae and sand. The reef crest is the highest part of the reef and is a high energy area that can be exposed to the air, it demarcates the lagoon and open ocean. The fore reef or reef slope is ideal for coral growth. The highest abundance of corals can be found between 15-20 m depth and decreases with increasing depth and lower light availability.
Coral reefs are tremendously valuable ecosystems, for humans and non-humans alike. As a structure and habitat, they provide shelter, hiding places, feeding, reproductive and nursery grounds for thousands of different marine species throughout their life cycles. Coral reefs are the first line of defence during tropical storms, as they dissipate wave energy and protect coastal beaches and shorelines from erosion and protect coastal infrastructure. They are home to many commercial fish species which coastal communities rely on for income and food and are important recreation sites for the tourism industry. Corals and marine organisms are also important for scientific research and finding new medicines and cures for disease.
Coral reefs face challenges that threaten their very survival. Direct impacts from human activities include; destructive fishing and boating practices, land and sea-based pollution and the introduction of invasive species. A bigger threat to the survival of corals worldwide is climate change: rising sea temperatures lead to coral bleaching, a process by which coral polyps expel their symbiotic zooxanthellae and can cause coral to die. Increased carbon dioxide in the ocean also leads to ocean acidification, which weakens the exoskeletons of corals and other invertebrates. Management of coral reefs towards climate change and human impacts includes establishing good practices, protecting vast areas of corals through Marine Protected Areas, and identifying corals that are resilient to rising sea temperatures, reproducing them and replanting them on affected sites. .