Coral Reefs and Estuaries

France Michael B. DayritIV - Photon

CORAL REEFS

Source : Grolier International Encyclopedia, “coral reefs”

Coral ReefsA coral reef is a wave-resistant underwater point built of

remains of coral, coral sands, and solid limestone at or slightly below sea level. They are aragonite structures produced by living animal colonies, found in marine waters containing few nutrients. In most reefs, stony corals predominant. Stony corals are built from colonial polyps that secrete an exoskeleton of calcium carbonate. Reefs grow best in shallow, clear, sunny and agitated waters. The accumulation of skeletal material, broken and piled up by wave action and bioeroders, produces formation that supports the living corals and a great variety of other animal and plant life. Coral reefs have three forms: fringing reefs, barrier reefs, and atolls. Often called “rainforests of the sea”, coral reefs form some of the richest and most diverse ecosystems on earth.

How Reefs Form

Most coral reefs were formed after the last glacial period when melting ice caused the sea level to rise and flood the continental shelves. This means that most coral reefs are less than 10,000 years old. Reefs that didn't keep pace could become drowned reefs, covered by so much water that there was insufficient light for further survival.

Coral reefs are also found in the deep sea away from the continental shelves, around oceanic islands and as atolls. The vast majority of these ocean coral islands are volcanic in origin. The few exceptions have tectonic origins where plate movements have lifted the deep ocean floor on the surface.

Climate Change

Any rise in the sea level due to climate change would effectively ask coral to grow faster to keep up. Also, water temperature changes can be very disturbing to the coral. High seas surface temperature (SSTs) coupled with high irradiance (light intensity), triggers the loss of zooxanthellae, a symbiotic algae, and its dinoflagellate pigmentation in corals causing coral bleaching. Reefs can often recover from bleaching if they are healthy to begin with and water temperatures cool. Warming may also be the basis of a new emerging problem: increasing coral diseases. Warming, thought to be the main cause of coral bleaching, weakens corals. In their weakened state, coral is much more prone to diseases. If global temperatures increase by 2 °C, coral may not be able to adapt quickly enough physiologically or genetically.

ESTUARY

Source: Grolier International Encyclopedia, “estuary”. Harold B. Wanless

EstuaryEstuaries are drowned coastal river valleys in which

fresh water and sea water mix. Drowning occurs as a result of either a worldwide rise in sea level or subsidence of the land. They are also the habitats of shell fish that live in brackish or moderately salt water. In the inner portions of estuarine embayments, water circulation is stratified, with less dense river water flowing out over a wedge of denser sea water. Some biologists and oceanographers restrict the term estuary to that portion of an embayment in which such circulation takes place.

• Sediments in an estuary have both a river and an ocean source. Sand carried into an estuary by tidal currents is moved farther inward by currents in the saline wedge. Coarser, riverborne sediments drop out near the head of the estuary as current velocities decrease, while finer river sediments accumulates in the upper riches of estuaries.