Seagrass: Life as a marine Angiosperm
Seagrasses are a unique group of flowering plants (angiosperms) that have adapted to survive fully submerged in marine water. They are often mistaken for seaweeds but are more closely related to grasses which are found on land. There are 78 species of seagrass globally that belong to 4 different groups. They are commonly found in shallow salty and brackish water but, one species has been found (Halophila decipiens) at depths of 85 meters. They range from tropics up to the arctic circle. Seagrasses can form large continuous meadows underwater (Figure 1) that are so large they can be visible from space. What makes these plants so successful in the marine world? As a habitat, they are one of the most productive in the world as they provide shelter and nursery for a variety of species, capture and store Carbon Dioxide (CO2) and stabilize the sediment. In terms of ecosystem services, seagrasses are estimated to be worth $15837 per hectare per year, which is almost double than that estimated for cropfields. But, globally seagrasses are in trouble. Up to 2 football pitches of seagrass is lost each hour. Whats causing the decline of these highly evolved species?
What are seagrasses?
Seagrasses are flowering plants that belong to the group monocotyledons, which is also the group that terrestrial grasses belong too. Like terrestrial grasses, seagrasses have chloroplasts, which are small organelles that use sunlight to convert into carbon dioxide and water into sugar through the process of photosynthesis. But seagrasses do differ from terrestrial flowering plants as they don’t have stomata. These are small pores on the leaves that allow for the control of water in and out of the plant and for gaseous exchange e.g. Co2 in and Oxygen out. Leaves have a thin surface which allows for the diffusion of gases and nutrients in and out. Another seagrass adaption is the presence of ‘lacunae’ which are pockets of air situated within the veins of the leaves helping to keep them buoyant. Roots and additionally rhizomes (horizontal growing roots) systems in seagrasses are important as they: a) absorb and store nutrients in the same way that terrestrial species do, and b) anchor the plant to the substrate.
Reproduction in seagrasses can occur in two ways, one is sexual reproduction which is the same as terrestrial plants and the other is asexually where it forms clones. Asexual reproduction occurs as the roots and rhizomes spread horizontally under the sediment and then new identical shoots grow. This is the most common form of reproduction and seagrasses and is important for the expansion of the meadow. Due to this method of reproduction, a patch of Mediterranean seagrass (Posidonia oceanica) in the Mediterranean is the oldest known living plant, with an estimated age of 200 000 years old. The other form of reproduction is sexual reproduction by flowering, where male and female parts of the plant are required. This form is important to increase the genetic diversity. To reproduce sexually most plants release the pollen (from their stamen; Figure 2) into the water column where it then disperses. Finally, fertilization occurs when the pollen lands on the pistil (Figure 2)
Why are seagrasses important?
Seagrasses are one of the most valuable and productive ecosystems on the planet. They are commonly referred to as the ‘lungs of the sea‘ as 1 square meter of seagrass can produce up to 10 litres of oxygen a day through photosynthesis. Seagrasses also provide a huge variety of other functions:
Storage of Carbon Dioxide known as ‘Blue Carbon’
Seagrasses remove CO2 from the surrounding water and store it within their leaves and roots. As the plants die and fall to the seafloor, they are buried and trapped in the sediment. Annually it’s estimated that 27 million tonnes of CO2 is stored within the sediment through this process.
Stabilization of the sediment and coastal protection
The horizontal and vertical root systems help to stabilize the sediment and prevent erosion of sediment. Stable sediment and the presence of seagrass leaves also helps to reduce the wave energy following storms.
Dense leave canopy provides an excellent shelter for a variety of small invertebrates, fish species and juvenile fish (Figure 3). Many of the juvenile fish species associated are commercially important species such as Cod and haddock.
However, seagrasses are in trouble and are being lost at a rate equivalent to 2 football pitches per hour. According to the IUCN Redlist 15 species are classed as near threatened to endangered, which nearly quarter of all seagrasses but numerous species are data deficient. Numerous threats have lead to a decline in seagrasses from environmental to human impacts (anthropogenic).
- Increased nutrients: The greatest threat is the increase of nutrients such as fertilizers and pollution which are washed off the land. High nutrients lead to the increase of algal blooms and epiphytes growing on the seagrass leaves. These limit the light that reaches the seagrass leaves which prevents them from photosynthesising effectively.
- Natural physical disturbances: Seagrass are vulnerable to physical disturbances such as storms leaving to wind-driven waves. Species such as skates and rays often dig (bioturbation) for clams which are buried within the sediment which leads to seagrass being uprooted. Erosion of the sediment causes the seabed to destabilize causing the seagrass to be uprooted easily.
- Human physical disturbances: Impacts such as boat propellers, anchors and moorings uproot seagrass and leave scars within the meadow which fragment the meadow (Figure 4).