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Marine Studies Up Close

Understanding Marine Habitats by Victoria Kruse, Graduate Marine Studies 1

Estuaries are a variable coastal environment due to a range of forces which acts upon them. External forces come from the land and sea, as well as the sky. Some factors which impact upon an estuarine environment are waves, tides, weather, seasonal fluctuations and pollution.

Wave movement from the sea brings nutrients into the estuaries that were not there before while washing away sediments at the same time. Debris is both brought in and taken out by waves and plants can be shifted about and seeds distributed into different parts of the estuary. Tidal movement bring numerous changes to the environment with as there are both dry and wet periods during a 24 hour period. When the water is higher, marine species that normally live in open water will enter the estuary both for breeding and protection. This means that there is a constantly changing population of species within the habitat. Juvenile fish may spend the first part of their lives in the estuary waters before heading out to open water.

Tides also cause a change of salinity in the water. Weather patterns also cause variability in the estuarine environment. Heavy rains will cause run off from the higher lying land surrounding the estuary bringing in more sediment and debris. Run off from rain can reshape the estuaries natural barriers and borders and during storms can score out deeper channels in some places. This can reshape the river or stream beds completely. Wind also plays a part in the environment changes that occur. On shore winds will bring salt and debris from the ocean, creating larger waves that stir up nutrients and sediment mixes as well as eroding shoreline and altering sand bars. In addition organisms that would not normally enter an estuary can be swept inside. Off shore winds would bring sediment and airborne pollution.

Seasonal fluctuations also affect estuaries; climate changes will bring temperature variables, differences in the amount and intensity of sunlight. Extreme cold may cause plant life to lie dormant and marine life to retreat from the area as well as birds populations changing with migratory pattens whilst warmer temperatures can create bountiful growth. Rainfall and wind levels will also be affected by seasons and cause changes as mentioned above. A change in seasons will also bring different levels of human interaction or inhabitation which has an impact upon the environment with boat wash and turbulence, fishing, shoreline restructuring and building and the introduction of foreign objects.

Pollution has an effect on estuaries. Industrialisation of nearby land and river banks will cause a change of nutrient mix in the run off water. Air pollution is absorbed by the estuaries causing changes in growth and health of inhabitants and plant life. Marine species that have been using the same estuarine space for many years can suddenly find it uninhabitable and may disappear which creates unbalance in the cycle.

In short estuaries live in a constant state of flux. Many of these variables, such as seasonal, water movement and weather, are natural necessary parts of the chain of life within the environment, however as human interaction increases there is increased likelihood of detrimental changes unless the environment is protected.

Coral Reef Formation and Demise by Victoria Kruse, Graduate Marine Studies 1

Coral reefs are hugely important marine environments. Although they only make up less than 1% of the worlds ocean surface they are home to up 25% of all marine species. Corals grow in warm, stable marine climates with most of them existing in a zone spanning 30o on either side of the equator. The majority of tropical corals grow in relatively shallow waters from 0 to 50 metres however some can grow in deep water and other have adapted to cooler temperature extremes. Most corals thrive where there is a thermocline, a layer of water that has been warmed by the sun and floats on top of cooler water. These waters are usually nutrient deficient as the upwelling that brings rich nutrients from decomposing organisms to the surface are absent in these environments.

Coral reefs come in three main forms - Fringing, Barrier and Atolls. Most of the modern coral reefs were formed when the ice melted after the last glacial period raising the water level of the ocean and flooding land. The reefs formed on the sinking land mounds, starting out as fringing reefs as the waters rose to encircle the peaks. Then as the water levels rose further Barrier reefs were created with a lagoon between the reef and the land remaining above the water line. Atolls, which are ring shaped reefs enclosing a open lagoon, are formed when a high land mass sinks completely below the surface. Other reef forms include patch, ribbon, table and sand cays.

The development of coral reefs is a slow process and an atoll can take up to 30 million years to form. Coral reefs are made up of live coral polyps that excrete hard calcium carbonate shells; these are known as hard or stony corals. As the corals die and are replaced the new corals grow on the shell left by the dead polyp. This continuous upward growth creates coral colonies that form the reefs such as staghorn and table corals. However corals but can also be solitary. Other organisms that make up the reefs we know include Hydrozas of which some, hydrocorals, are also able to form calcium carbonate skeletons similar to stony corals, Black Corals, Soft Corals, Sea Anenomes, Zoanthids, Corrallimorps and Sea Squirts.

Stony Corals, which are the main reef builders, live in a symbiotic relationship with Dinoflagellates called Zooxanthellae. These single celled organisms carry out photosynthesis which provides up to 90% of the nutrients that the stony coral polyps need to live. As the Zooxanthellae need sunlight to photosynthesise, corals thrive in clear water through which the sun can penetrate. The Zooxanthellae are also responsible for the colourful appearance of the corals. The remainder of the nutrients needed by the polyps are obtained by the polyps themselves nocturnally by extending their tentacles to catch passing food. This food may include zooplankton or organic debris. This process also allows oxygen to be obtained by the polyp.

Corals reproduce both sexually and asexually and may use both methods within its lifetime. Some corals fertilise eggs internally and then expel larva. Others reproduce by external spawning whereby the eggs and sperm are synchronistically released into the water, usually around sunset.

These spawning events depend on four factors, time of year, water temperature, tidal and lunar cycles. After fertilization the larva float until they can attach themselves to the surface of the reef and grow. Coral spawning evens are incredible things and amazing to watch happen. Unfortunately coral reefs have a number of direct and indirect threats to their growth and survival. It is estimated that 10% of the world’s reefs are already dead and up to 60% of the remaining are at risk.

Reefs do have some natural predators including the Crown of Thorns starfish which can eat the reef at a rate of up to 6 sq.m. per year. The numbers of these starfish have been increasing steadily and it is thought to be because of human impacts on fishing and chemical runoff that this is occurring. Other predators can also impact spawning events with coral spawn consumed before it can settle. Other causes for the demise of coral reefs include coral bleaching, diseases, pollution, coastal developments, overfishing, coral harvesting, careless recreational users, and a warming plant destructive fishing practices.

To discuss a few of these; coral bleaching occurs when a rise in water temperatures, pollution or exposure to the air causes the polyps to expel their Zooxanthellae. This corals will lose their colour and become white as well as losing their main source of nutrients that are necessary to their survival. This can occur within a region causing whole reefs to bleach. Recovery from bleaching can take decades.

Destructive fishing practices such as cyanide poisoning in order to harvest live (for a while anyway) fish for aquariums or dynamite fishing whereby dynamite is used to kill or stun fish within an area is still unfortunately widespread throughout parts of South East Asia. These practices are hugely destructive to the coral reefs either poisoning them or literally blowing them up and in these places here are now vast areas of coral rubble and dead coral structures that do not support the diverse marine life they once did.

Careless recreational users of the reefs can cause major damage to live reefs. Sadly it is often people who actually owe their survival to the existence of the reef such as the inhabitants of popular diving and holiday spots or the people using the area for vacations that are at fault. Careless swimmers, snorkel users and divers can break off parts of the reef and other may actually collect parts of the reef as souvenirs. Improper boat anchoring or management of boat propellers in shallow waters above reefs can also cause great damage. Local people often uneducated or below the poverty line can be careless of chemical runoff, overfishing, pollution, dredging and specimen collection and trade.

Global warming is causing a rise in sea temperatures which can result in coral bleaching as discussed above, a rise in sea levels which can mean that the Zooxanthellae does not receives enough sunlight and an increase in tropical storms which can strip reefs of corals and damage the reef among many other effects.

Most of these threats are human related and many are able to be stopped through education.

Reversing global climate change is not as simple, but a little conservation each day from every person could save corals reefs for the future generations.