Monday, June 18, 2012

Tropical Marine Ecosystems: Mangroves and Sea Grass

Mangroves

SE Asian Mangroves (here are links to some blog posts about SE Asian Mangroves from my sabatical).

Malaysia


Thailand



Seagrass Meadows

More Info About Seagrass (McG's lecture)

            Most people head to tropical marine environments to dive or snorkel on beautiful coral reefs.  Divers who eagerly carry their gear to a boat that carries them out to sea and eventually drops them on top of the reef might be surprised to learn that the coral reef is the not the only interesting, and important, tropical marine community.  In fact, if you put on your mask, fins, and snorkel and head off of the beach in most tropical areas, then the first community you come into contact with is not the reef, but instead a large “meadow” filled with dense stands of “grass-like” plants that are approximately 14 inches long.  I am sad to admit that on my first tropical snorkeling trip that I kicked as hard as I could to carry me past this “prairie” get out to the reef where all of the good stuff lives.  However, a little more experience has taught me that there is plenty of interesting stuff going on in, on, and around these grasses.

            Let’s start by taking a close look at these plants.  Because they are growing in the ocean you might conclude that you are looking at algae.  However, if you did a little bit of digging around in the base of each plant you would observe that these plants produce roots (in fact, they make a lot of roots).  Because algae lack true roots, you would be wise to decide that you are not looking at algae.  The linear shape of the leaves might make you think that you are looking at a grass (that’s why they are called sea grasses).  However, although these plants are monocots, they are not in the same family as true grasses.

Who are the sea grasses?
Worldwide there are 45 species, in 12 genera spread across 4 monocot families.  In the Caribbean there are three main species- Thalassia testudinum, Syringodium filliforme, and Halodule wrightii.

Where are they found?
            Sea grasses are not found on high energy shores, so they tend to be located in lagoons that are protected by the reef.  Sea grasses are most abundant where sediment transport is low  and where there is significant tidal flow.  They are limited to relatively shallow water.  Typically, they are found in water shallower than 10 meters but they may be found as deep as 40 meters.

Characteristics of Sea Grass
            Sea grasses invaded the ocean from land about 75 million years ago so they have had to evolve characteristics to allow them to live in the aquatic environment.  Flowering plants invading the sea have to overcome a number of problems including (1) they must be adapted to salt water (problems with losing water to the saline environment), (2) must be able to grow while they are completely submerged (problem with oxygen uptake from the water), (3) they must be securely anchored in order to withstand wave action and tidal current, and (40 they must be able to pollinate underwater.

            Sea grasses have several vegetative adaptations for surviving in the sea. 
1.      They are clonal and produce extensive underground rhizomes.  Genetic analysis shows that a sea grass meadow is made up of a number of smaller individuals rather than one giant.  Although the leaves are relatively short lived, shoots can be quite old (8 – 12 years). 
2.      The linear shape of the leaves limits the damage caused by wave action.
3.      The leaves lack stomates, yet the leaves are covered with a waxy cuticle that is porous enough to allow carbon dioxide to enter so that photosynthesis can occur in the epidermis.
4.      The plants have greatly reduced xylem because they do not need to move water from the soil to the leaves.
5.      The roots live in an anoxic environment, yet still require oxygen for cellular respiration.  Roots use oxygen released during photosynthesis which is stored in air spaces in the roofs.  Oxygen produced in the leaves diffuses to the roots through air spaces.
6.      Sea grasses avoid problems with water loss because they maintain high internal salt concentrations so that water diffuses into the plants.

Sea grasses also have a number of interesting reproductive adaptations.
1.       Sea grasses reproduce asexually by producing underground rhizomes.  Vegetative spread may be adaptive in environments where the probability of seedling establishment is low.
2.      Sea grasses also reproduce asexually.  They produce small white flowers which are produced near the base of the leaves.  Flowering occurs from late April through August and the timing is affected by water temperature and salinity.  Some species produce unisexual flowers, but other species produce both sexes.

Sea Grasses of the Caribbean.
            There are three main species of sea grass in the Caribbean.
1.       The most common species in Thalassia testudinum (Turtle Grass).  Their leaves are long (about 14 inches and wide (4 – 12 mm).  Their rhizomes and roots fill the zone from 5 – 24 cm beneath the surface.  They may produce roots that are 4 – 5 meters long.  There is an order of magnitude more biomass below ground than above ground.
2.      Syringodium filiforme (Manatee Grass) produces long cylindrical leaves that may reach 45 cm in length.

Sea Grass Community Ecology
            What factors determine the distribution and abundance of sea grasses.  There is no strong evidence for allelopathy in sea grasses, so interference competition is probably not too important.  However, a number of studies have documented the influence of exploitative competition. 

Effect of sea grass on the environment.
            Sea grasses affect the abiotic conditions. 
1.      The root system stabilizes the sediment and slows erosion.  There are many species that live around the roots of sea grasses. 
2.      The leaves slow the rate of water flow which allows sedimentation.  As particles settle, the water becomes clearer.
3.      The leaves provide attachment points for epibionts such as hydroids and algae.  Epibionts are an important component of the productivity of these communities.  Thirty percent of the productivity and 30% of above ground biomass can be epibionts.
4.      Leaves provide protection for animals from their predators.  Thus, sea grass communities can be important nursery areas.

Sea grasses also influence biotic conditions.
1.       These are very productive communitities.  A single acre can produce 10 tons of leaves in a year.
2.      Leaves grow very quickly. Tt 2 – 5 mm per day and Sf 8.5 mm per day but are replaced rapidly (Tt – 50 days, Sf 18days) so lots of detritus is produced. 
3.      Sea grasses serve as the base of 2 distinct food chains. 
a.       The grazing food chain depends on leaf biomass that enters the food chain via grazing (<50% of plant biomass).  Leaves have a high C/N ratio and contain phenolics so they are not as good of a food choice as algae.  Below ground biomass can also be an important source of energy.  The importance of grazing in this community is poorly understood because the important grazers in this community (green turtles and manatees) are mostly gone.
b.      The detrital food chain supports a variety of species that feed on detritus or on the bacteria that feed on detritus.  Because leaves contain air spaces they float.  Thus, detritus is exported from the sea grass beds.  Sea grass detritus is found in many deep sea samples.
4.      There is also the epibiont food chain based mostly on the diatoms and filamentous algae that live on the leaves.  The production of epibiomts may equal that of the sea grasses and the algae are a better food source. 
5.      The diversity of food chains allows a number of similar species to coexist in the community.  For example, Tripneustes ventricosus and Lytechinus variegates are two very similar sea urchins that are able to coexist because Tripneustes feeds on the leave blades and Lytechinus feeds on the epibionts that live on the leaves.

Sea Grass Communities
            Thalassia beds may have 30,000 animals per square meter in addition to the 100 species of epiphytic algae found there.  A single acre may contain 40.000 fish and 50 million small invertebrates. The abundance of animals in sea grass communities is an order of magnitude greater than the surrounding unvegetated areas.

            Why are there so many species found in such high abundance in the sea grass community?  Sea grass adds structure to both the above and below ground habitats.

            Corals are not abundant in sea grass communities because there is too much sediment.  Corals found in the sea grass tend to be very small.

Role as Nurseries
            Nursery areas are places where juvenile inverts and fishes survive better and grow faster.  Many people have suggested that sea grasses act as important nurseries so that they should be protected. 

            The data suggests that sea grasses provide protection for inverts from actively foraging fish species because the vegetation provides places for them to hide.  However, ambush predators such as sea horses might be more successful.  It appears that the roots also provide protection from predators for species living below ground.  There is little evidence suggesting that organisms grow faster in the sea grass.

Links to Other Communities 
            The sea grass community can be linked to other communities through the movement of animals and the export of detritus. 
            Fish from coral reefs feed in the sea grass beds at night (e.g., grunts) so may of the species that we will see during the day time are just hanging out waiting to feed at night.  Fishes take nutrients back to the reef and release them when they defecate.  These nutrients can increase coral growth rates.  Some fishes living in the mangroves leave to feed in the sea grass.
           
Sea grass provides great benefits to humans.  In Florida estimates $20,500/acre/year for a total of $55 billion per year.  Sea grasses are at risk from human activities such as
1.      Eutrohication
2.      Outboard motors
3.      Commercial shellfish harvesting
4.      Dredging








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