Coastal Habitats

The term coast has a much broader meaning than shoreline and includes many other habitats and ecosystems associated with terrestrial and marine processes.• The six major coastal settings are: estuary,

lagoon, salt marsh, mangrove swamp and coral reef.

• Shorelines are one of the most productive ecosystems and because they are shallow, they strongly respond to the effects of waves, tides and weather.

Estuaries are semi-enclosed bodies of water where fresh water from the land mixes with sea water.

• Estuaries originate as: drowned river valleys, fjords, bar-built estuaries, and tectonic estuaries.

• Salinity typically grades from normal marine salinity at the tidal inlet to fresh water at the mouth of the river.

12-1 Estuaries

Estuaries can be subdivided into three types based upon the relative importance of river inflow and tidal mixing.• Salt-wedge estuaries are dominated by the

outflow from rivers.

• Partially-mixed estuaries are dominated by neither river inflow nor tidal mixing.

• In well-mixed estuaries tidal turbulence destroys the halocline and water stratification.

• Because river discharge and tidal flow vary, conditions within an estuary can also change, being well-mixed when river flow decreases relative to tidal mixing, to becoming a salt-wedge estuary at times of maximum river discharge.

12-1 Estuaries

The widely fluctuating environmental conditions in estuaries make life stressful for organisms.• Estuaries are extremely fertile because nutrients are brought

in by rivers and recycled from the bottom because of the turbulence.

• Stressful conditions and abundant nutrients result in low species diversity, but great abundance of the species present.

• Despite abundance of nutrients, phytoplankton blooms are irregular and the base of the food chain is detritus washed in from adjacent salt marshes.

• The benthonic fauna strongly reflects the nature of the substrate and most fishes are juvenile forms living within the estuary until they mature and migrate to the ocean.

12-1 Estuaries

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

• Salt Marsh Succession

• A salt marsh is "born" by the arrival of a seed or the rafting of a plant of the cord grass Spartina alterniflora. The grass spreads asexually by means of a subterranean rhizome system. The grass becomes dense and forms a baffle, which encourages the deposition of fine particulate sediment, including organic matter (salt marsh peat). This, in effect, causes a rise of the sediment surface and makes the habitat more terrestrial. As this happens, other somewhat less salt-tolerant grasses are able to invade. Eventually, this series of invasions and takeovers leads to a vertical zonation of grasses and a spread of the entire marsh system.

• SAS = Spartina alterniflora - short form

• SP = Spartina patens, the next higher grass species

• Border of Spartina patens - Spartina alterniflora zone

• In the foreground we see the Spartina patens zone, which is higher (though that is hard to tell from the photo) than the S. alterniflora zone (taller grass in the background). S. patens is less salt tolerant than S. alterniflora.

• Marsh System Cross-Sectional View

• This is a schematic cross section of a marsh with creeks. Note the tall form of Spartina patens, which tends to live adjacent to creek edges, which are bathed in nutrient-rich water. The short form lives in higher sites away from creek edges. The ribbed mussel Geukenzia demissa often lives among Spartina spp. and its production of nitrogen-rich feces and pseudofeces enhances grass growth.

• Ribbed Mussel Geukenzia demissa

• The high intertidal marsh mussel Geukensia demissa is ubiqui-tous in Spartina salt marshes. The late Charles Lent showed that this mussel was capable of breathing air, and often got more of its oxygen from air than from water.

• The diagram at left shows that Geukensia demissa is more elongate than the mussel Mytilus edulis, and lives semi- infaunally in the sediment, with byssal threads attached to sedimentary grains or rhizomes of Spartina.

• Marsh mussels can be quite densely packed. Mark Bertness and colleagues demonstrated that this mussel enhances the growth of Spartina. The mechanism may relate to biodeposition of nitrogen-rich feces and pseudofeces. The byssal threads of this mussel also help bind the sediment and may retard erosion. Another common salt marsh resident, the fiddler crab Uca pugnax, also aids in Spartina growth by burrowing and aerating the sediment. Note the barnacles attached to mussel shells, which is possible owing to the mussel's semi-infaunal habit.

• The aerenchymal tissue allows Spartina to exchange gases, even when surrounded by an anoxic soil. The tissue in this photograph is visible as a series of circular passageways around the periphery. Plants cannot use nutrients efficiently without oxygen, so this tissue allows a connection between the aerobic leaves to the stems, that are surrounded usually by anoxic water.

• Salt Marsh in Fall

• In the fall, Spartina leaves senesce and eventually calve off at the base. This material then is deposited on the sediment surface. By the time the particulate organic material is deposited on the surface, nearly all nitrogen has been removed by microbes, so it is very carbon rich and nitrogen poor. The carbon is mainly in the form of cellulose, which is largely indigestible by invertebrates.

• The senesced leaves accumulate on the sediment surface as shown at left. This material will smother any grass beneath, but a high spring tide or a storm may remove it all quite rapidly and transport it out of the marsh system to the shelf.

• Several species of Salicornia can be found in eastern American Spartina salt marshes in the highest part of the intertidal zone. They are all very salt tolerant. This plant is only about 5 cm high.

Mud Flats

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

Adaptations

Vivipary

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.

12-5 Coral Reefs

• 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 benthonic filter feeders in nutrient-rich water where phytoplankton are abundant.

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.

12-5 Coral Reefs

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