Estuarine Habitats: Where the River Meets the Sea
Mangrove Swamps, Salt Marshes, & Seagrasses
Salt Marshes
Salt Marshes are located in estuaries and along the coast. These are formed where gently-sloping shores combine with nutrient-rich sediments washed up by the tides. The salt marshes can be split into the upper marsh and lower marsh. The conditions in each of these areas varies which affects what types of organisms can live there.
Upper Marsh:
Most of the salt marsh plants are not food because they are too tough and salty; however, their root systems hold sediment which combined with the gathering of dead halophytes creates a mat of detritus. This decomposing matter allows for huge communities of invertebrates, water birds, juvenile fish, and other organisms.
Salt Marshes are located in estuaries and along the coast. These are formed where gently-sloping shores combine with nutrient-rich sediments washed up by the tides. The salt marshes can be split into the upper marsh and lower marsh. The conditions in each of these areas varies which affects what types of organisms can live there.
Upper Marsh:
- Area rarely flooded by tides
- Organisms must be tolerant of extreme conditions due to salts left behind from evaporation
- Plants rarely deal with seawater and it's easily diluted by river water; however, they must be able to deal with salt left behind
- Example: Pickleweed (Salicornia) handles excess salt by storing it in "sacrificial" leaves. When the salt level gets to high, the leaf will drop off of the plant.
- Area flooded regularly by tidal cycle (twice daily for most places)
- Halophytes thrive due to lack of competition from other plants
- Example: A hollow reed, Spartina, excludes salt from its tissues and moves oxygen produced during photosynthesis to its roots (since they are typically underwater)
Most of the salt marsh plants are not food because they are too tough and salty; however, their root systems hold sediment which combined with the gathering of dead halophytes creates a mat of detritus. This decomposing matter allows for huge communities of invertebrates, water birds, juvenile fish, and other organisms.
Mangrove Swamps
(Mangrove Forests) Mangrove swamps are found in tropical areas. Mangroves are similar to the plants of a salt marsh but they are bigger and tougher halophytes. The trees have strong, tangled root systems that make them an ideal habitat for juvenile fish and invertebrates as well as providing a nursery for nearby ecosystems. The roots hold soil which prevents erosion and also trap nutrients which help nearby coral reefs and other ecosystems from eutrophication. |
Mangrove trees are not a single species but actually more than 50 species of halophytic trees and shrubs! While all mangroves have adaptations that allow them to live in salt water, we will be looking at 3 specific species: red, black and white.
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Red mangroves (Rhizopora) grow on stilt-like roots which allows the roots to get oxygen. The roots will filter the salt water by active transport to exclude the uptake of salt.
Black mangroves (Avicennia) have roots that grow in the sediment below the water line. These roots have structures called pneumatophores ("dead man's fingers") which act as snorkels to get air. Some Black mangroves can produce sacrificial leaves while others have special glands in the leaves to excrete salt.
White mangroves (Languncularia) don't have specialized adaptations like the black and red species. They are very tolerant of saltwater but usually grow nigh enough above the tide line where the don't need special root adaptations.
Seagrasses
Seagrass communities are similar to other halophyte-based ecosystems. The roots are important in stabilizing sediments (prevent erosion) and provide shelter to many organisms, especially juvenile fish. Seagrasses are rooted, vascular (have "veins"), flowering plants that live entirely underwater except during rare, low tides. They must live in protected, shallow coastal areas where light can reach them. They act like underwater pastures providing food for organisms like sea turtles, urchins, & manatees. Many organisms don't eat the grass itself rather the algae (epiphytes) that grows on the grass. |
Seagrasses are important for water quality. When sediment in fast moving water hits seagrass, it slows down the water and allows the particles to settle.
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