Introduction to Oceanography

Physical Oceanography-Coastlines 2-

Lagoons are isolated to semi-enclosed, shallow, coastal bodies of water that receive little if any fresh water inflow.

• Lagoons can occur at any latitude and their salinities vary from brackish to hypersaline depending upon climate and local hydrology.

• Bottom sediments are usually sand or mud eroded which was from the shoreline or swept in through the tidal inlet.

• In the tropics, the water column is typically isothermal.

• In the subtropics, salinity generally increases away from the inlet and the lagoon may display inverse flow.

12-2

Lagoons

Salt marshes are intertidal flats covered by grassy vegetation.• Marshes are most commonly found in protected areas with a

moderate tidal range, such as the landward side of barrier islands.

• Marshes flood daily at high tide and then drain through a series of channels with the ebb tide.

• They are one of the most productive environments.

• Marshes can be divided into two parts: Low salt marshes and High salt marshes.

• Distribution and density of organisms in salt marshes strongly reflects availability of food, need for protection, and frequency of flooding.

12-3 Salt Marshes

Mangroves are large woody trees with a dense, complex root system that grows downward from the branches.

• Mangroves are the dominant plant of the tropical and subtropical intertidal area.

• Distribution of the trees is largely controlled by air temperature, exposure to wave and current attack, tidal range, substrate and sea water chemistry.

• Detritus from the mangrove forms the base of the food chain.

12-4

Mangrove Swamps

A coral reef is an organically constructed, wave-resistant, rock-like structure created by carbonate-secreting organisms.• Most of the reef is composed of loose to well-cemented organic debris of

carbonate shells and skeletons.

• The living part of the reef is just a thin veneer on the surface.

• Corals belong to the Cnidara.

– The animal is the coral polyp.– The corallite is the exoskeleton formed by the polyp.

• Corals share a mutualistic relationship (mutually beneficial) with the algae zooxanthallae which lives within the skin of the polyp and can comprise up to 75% of the polyp’s body weight.

• Corals can be either solitary or colonial.

• Corals can not survive in fresh, brackish water or highly turbid water.

• Corals do best in nutrient poor water because they are easily out-competed by benthic filter feeders in nutrient-rich water where phytoplankton are abundant.

12-5Coral Reefs

Coral Reef Classifications

Fringing Reefs cling to the margin of land.

Barrier Reefs are separated from land by a lagoon.

Atolls are ring-shaped islands of coral reef enclosing lagoons.

As a result of corals growing continuously upward towards the sunlight as sea level rises and/or land subsides and, coral reefs pass through three stages of development.

• Fringe reefs form limestone shorelines around islands or along continents and are the earliest stage of reef development.

• As the land is progressively submerged and the coral grows upward, an expanding shallow lagoon begins to separate the fringe reef from the shoreline and the reef is called a barrier reef.

• In the final stage the land vanishes below the sea and the reef forms a ring of islands, called an atoll, around a shallow lagoon.

12-5

Coral Reefs

Coral reefs consist of several distinct parts developed in response to their exposure to waves.

• The algal ridge occurs on the windward side of the reef and endures the pounding waves.

• The butress zone is the reef slope extending down from the algal ridge.

• The reef face extends downward from the butress zone and usually is devoid of living colonial corals because insufficient light reaches this depth.

• The reef terrace is landward of the algal ridge and lies at mean water level.

• The shape of the colonial coral masses reflects the environment in which they live.

12-5Coral Reefs

Coastlines are desirable areas for human habitation, but human activity conflicts with the dynamic state of coastal systems.• Humans try to stabilize the coastline in two ways: by

interfering with longshore sand transport, and by redirecting wave energy to prevent erosion.

• Preventing of sand drift involves jetties and groins.

• Redirecting wave energy involves breakwaters and seawalls.

• Beach nourishment with sand is expensive and temporary.

• An increase in sea level from global warming will cause more land to be flooded and threaten more coastal buildings.

11-7 Impact of People on the Coastline

A map showing shore erosion by region. One example of shore erosion is the lighthouse on Cape Hatteras, which was moved during 1998 and 1999 to protect it from destruction. It was threatened by rising sea levels and a changing shoreline.