Coastal Wetland Ecosystems

North America

Mitsch and Gosselink Ch. 9, 10, 11

 

Tidal Salt Marshes

 

Defining Characteristics

 

1.  Some salinity (2 ppt to 36 ppt)

2.  Most marshes form where there is river influence because sediment is there.

3.  East and Gulf coasts, extensive shallows

4.  West coast, limited mainly to enclosed estuaries

5.  North coast, extensive shallows; isostatic rebound

6.  An open system for nutrients and detritus

 

Geographic areas

 

1.  Gulf coast.

 

Spartina alterniflora, S. patens.  Hypersaline lagoons in S. Texas and Mexico.  Extensive low-salinity marsh in Louisiana, Mississippi, Alabama.

 

2.  East coast

 

Shallow coast with many rivers, productive bays.  Spartina alterniflora.

 

3.  West coast

 

Steep coast; limited salt marsh.  Spartina foliosa in California; Carex lyngbyei from N. California to Alaska.

 

4.  North coast

 

Puccinellia phryganodes, Carex aquatilis, C. paleacea.  Very flat with much fresh water in Hudson Bay Lowlands.  Low salinity in Alaska.

 

Hydrology

 

Salt marshes are salty, and they are affected by tides.  Tides can be as little as a foot on the Gulf of Mexico, to as much as 20 feet in Cook Inlet (Anchorage, Alaska).

 

When the tide is in, salt marsh plants cease or slow gas exchange and photosynthesis.

 

Coastal marshes depend on active sediment deposition to continue their existence.  If sediment is cut off, the marsh erodes away.

 

Nutrients

 

Salt marshes are primarily nitrogen-limited.  Because of anaerobic soils, it is generally available as ammonium.  It is least available where plants are actively growing.

 

Primary Productivity

 

1.  Vascular plants

 

Aboveground production of Carex lyngbyei and other meadows in Puget Sound varies from 600 to 2000 g m-2 y-1

 

2.  Algae

 

Algal productivity, though resulting in much less standing biomass, is about as great as that of vascular plants.

 

3.  Limits to productivity:  salinity, nutrients, soil redox potential, inundation, day length and time of high tides.

 

Fate of Primary Production

 

1.  Detritus

 

2.  Consumption:  (detritus), (micro and meiofauna), (crabs, shrimp, snails, oysters, mussels).

 

3.  Direct herbivory, less than 5-10%

 

Tidal Freshwater Marshes

 

Defining Characteristics

 

Tidal, but not saline (see Fig 10-1)

 

Geographic Areas

 

Occur primarily where major rivers empty into coastal waters where there is a flat coastal gradient and a significant tidal range.  This limits it to the Atlantic and northern Gulf coasts.

 

Hydrology

 

Tidal fluxes are dampened.  Movement of incoming salty tidewater is impeded.

 

Vegetation

 

Sagittaria, Nuphar, many grasses, Pontederia, Peltandra, Potamogeton

 

Consumers

 

Used heavily by wildlife

 

Food chain predominantly detrital, with benthic invertebrates a major decomposer.

 

Nekton, in form of juvenile fish, use system extensively.

 

Highest use by birds of all wetland types.

 

Primary Productivity

 

Generally high; between 1000 and 3000 g m-2 y-1

 

Nutrient cycling

 

Similar to that in salt marshes.  Flooding  provides the potential to move nutrients around, but much of it is recycled within a given marsh.  Still an open system.

 

 

Mangrove

 

Defining characteristics

 

Mangroves in North America are coastal wetlands that are made up of woody trees and shrubs that live in salt water.  The system is dominated by black mangrove, Avicennia germinans, red mangrove, Rhizophora mangle, and buttonwood, Conocarpus erecta.

 

There distribution latitudinally limits them to tropical and subtropical areas.  They are not tolerant of freezing weather.

 

Geographic extent

 

In the U.S., they are quite common in Florida, and exist as a fringe along the coasts of Louisiana and Texas.

 

Hydrology

 

Mangroves exist in four major kinds of ecosystems:

 

1.  Fringe mangroves

2.  Riverine mangroves

3.  Basin mangroves

4.  Dwarf or scrub mangroves

 

Riverine systems are most productive, followed by fringe mangroves.  Basin and dwarf mangroves tend to be in areas where water exchange is very limited, and salinity may concentrate.

 

Chemistry

 

Salinity may range from fresh to around seawater salinity; in basins the chemistry can be hypersaline.  Salt water is not necessary for mangrove survival, but does give them a competitive advantage.

 

Dissolved oxygen can be very low; reduced soil conditions exist when mangroves are flooded.

 

Mangrove soils are usually acidic, however in Florida there is a high carbonate presence and soil reaction is close to neutrality.

 

Ecosystem structure

 

Black mangrove and red mangrove both form dense canopies, often including both species and white mangrove (Laguncularia racemosa).

 

North American mangrove swamps have a conspicuous lack of understory vegetation.

 

Mangrove Adaptations

 

Salinity control:  mangroves can both restrict the entrance of salt into their tissues through micro-filtration in the roots, and  can excrete salt from leaves.

 

Black mangrove roots “breathe” using pneumatophores.  Red mangrove prop-roots are also covered with breathing lenticels.

 

Red mangroves have viviparous seedlings.  Black mangroves have seeds that float in salt water and control their buoyancy so that they sink to the bottom in fresh water and germinate.

 

Primary Productivity

 

Primary productivity is high, comparable with salt marshes, but can be quite low in areas of stagnant water and salt concentration.

 

Crabs and invertebrates play a major role in decomposition of mangrove leaves.

 

Mangrove forests are major exporters to adjacent estuaries, with riverine systems providing the greatest amount of exported biomass.

 

Mortality

 

Hurricanes have an interesting effect on mangrove swamps.  When trees are knocked over or uprooted, the subsequent lack of soil oxygenation by root systems results in an anoxic sediment that is resistant to immediate recolonization.