Introduction
Seagrass beds are amongst the most productive marine habitats and their wide-ranging distribution. They provide homes and food sources for many marine animals, prevent erosion and provide nurseries for many commercial and recreational fish species. Despite their importance to the environment, seagrass beds are increasingly being decimated by the results of human activities. The destruction of seagrass beds may have serious consequences on the biodiversity of coastal regions and as a consequence, may negatively impact on commercial and recreational fishing, and ultimately all marine species.
Description of Issue
Seagrasses are flowering plants that live in marine and estuarine habitats. They are rooted in the sediments with the leaves appearing above the ground. Although often confused with seaweeds they are more closely related to terrestrial plants. They occur in most areas of the world, in shallow brackish and salty water. Seagrass beds change constantly and these changes can be caused by natural processes or human activities. As with most plant, there are marked seasonal changes in seagrass growth. Most species flower between mid-spring and mid-fall. Production of new shoots and rhizome growth occur mostly in the warmer months, although the timing of these events differs between species.
One important characteristic of seagrass beds is that they are periodically disturbed by a range of processes. Sometimes whole seagrass beds can be disturbed: at other times, small bare areas may be created inside established meadows by local disturbance. Some of these changes are natural, such as severe wave action. Others are caused by anthropogenic activities such as dredging, detonation of explosions and the influx of chemicals and other pollutants from land, as a result of mining, industry and agriculture.
Seagrasses are important because they provide food for many marine mammals and shelter for a large diversity and abundance of organisms. Seagrasses are very productive in relation to other habitats, and seagrass beds act as the nursery areas for many commercial species of fish. Microscopic organisms such as bacteria and single-celled plants colonize any exposed area of seagrass. Seagrass beds typically support a rich variety of these as well as sessile fauna, all of which contribute significantly to the overall primary production of the seagrass community. Other fauna associated with seagrass beds include a variety of small crustaceans and molluscs together with larger predatory fauna such as crabs prawns and fishes. Seagrass forms a major component of the diet of animals such as dugongs and green turtles.
When seagrasses die, they breakdown into fine particles known as detritus, some of which is carried away from the seabeds by ocean currents. Detritus is a very important source of food for some animals. It is a good source of carbon but lacks nitrogen. The fauna living in the seagrass beds are strongly influenced by the physical conditions of the environment in which the seagrass is found. These conditions include temperature, salinity, turbidity, oxygen concentration and movement of water and appear to be more important in determining the species of fauna present, than the seagrass species on which they are found.
Figure 1. Healthy Seagrass Bed
http://www.seagrasswatch.org/Info_centre/gallery/Cairns/2012Jul/Green_island_seagrass13.jpg
Evidence suggests that seagrass beds consistently have a greater abundance and diversity of animals than surrounding un-vegetated habitats, and the abundance and diversity of fauna are greater in areas of dense rather than sparse seagrass. The dense array of seagrass leaves acts as a baffle to slow down water movement, thereby slowing down the passage of larvae and spores, allowing them more time to settle, which partly explains the diversity and abundance of seagrass bed inhabitants.
The Importance of Seagrass Beds
The maintenance of seagrass beds is important for a number of reasons. Seagrass beds are one of the most threatened ecosystems on earth. Population growth resulting in intensive coastal development, deteriorating water quality, increasing water temperatures, dredging and unsustainable fishing practices all contribute to the destruction of seagrass habitats, leading to loss of species and ecosystem functionality and the invasion of opportunistic species such as sea urchins, which leads to further destruction. Seagrass beds are also an important global carbon sink, accounting for approximately fifteen percent of carbon dioxide uptake on a global scale. Seagrass beds also significantly contribute to the fish stock of coral reefs, and depletion of certain coral reef species has been directly linked to destruction of nursery stock originating in seagrass beds .
The economic impact of seagrass bed destruction is potentially enormous. Given that they act as a nursery for many fish species, the loss of such habitats for juveniles will have series financial impacts on commercial fisheries and those underdeveloped countries, already threatened by global warming, who rely on fishing as a major source of food. Loss of seagrass beds will also contribute to loss of large mammals such as dugongs, turtles and manatees, which will in turn lead to loss of species down the food chain.
Seagrass beds face three major threats: destruction of biodiversity, overfishing and eutrophication. Biodiversity increases resource use, stability of seagrass ecosystems and productivity. Loss of biodiversity therefore contributes to instability of the system, loss of productivity and the opening of niches for the invasion of opportunistic species such as sea urchins and the overgrowth of filamentous algae, which cause eutrophication. Baden et al (2012) found that, along with increased nutrient loads from agriculture and industry, overfishing was one of the largest contributors to eutrophication.
Figure 2. Seagrass Bed Denuded by Sea Urchins
http://www.petermacreadie.com/uploads/1/2/6/1/12619196/3454008_orig.jpg
Solutions to Seagrass Destruction
On a large scale, scientific research and monitoring and public awareness campaigns aimed at introducing the importance of seagrass beds are the best ways in which to assist in their protection. The mapping of coastal zones is one important approach to understanding the extent and composition of seagrass beds and their inhabitants. Restoration of seagrass beds is also another important aspect to ensuring their protection. By reducing nutrient load from industry, and ceasing to dredge in in vulnerable areas, seagrass beds will be able to regenerate naturally.
Conclusion
Seagrass beds are an essential component in the biodiversity of the marine environment, and although there has been a rapid global decline in the last fifty years, understanding of their importance to the environment is increasing. Together with our increased understanding of human influences on the planet, prompted most recently by the enormous attention given to the issue of global warming, humans have at last developed an appreciation for the vital importance of the marine environment and are taking steps, both globally, and at a local level, to reduce the amount of emissions and pollutants that enter our atmosphere and oceans and hopefully ensure their survival.
Works Cited
Baden, S. E.-J. (2012). Shift in seagrass food web structure over decades is linked to overfishing. Marine Ecology Progress Series, 451, 61–73.
Duffy, J. (2006). Biodiversity and the functioning of seagrass ecosystems. Marine Ecology Progress Series, 311, 233-250.
Keough, M. a. (1995). Seagrass Meadows and Their Inhabitants. In A. a. Underwood (Ed.), Coastal Marine Ecology of Temperate Australia (pp. 221-239). Sydney, Australia: USNW Press.
Nakamura, Y. (2010). Patterns in fish response to seagrass bed loss at the southern Ryukyu Islands, Japan. Marine Biology, 157(11), 2397-2406.
Pillay, D. B. (2010). Ecosystem change in a South African marine reserve (1960–2009): role of seagrass loss and anthropogenic disturbance. Marine Ecology Progress Series, 415, 35-48.
