Estuaries

Semi-enclosed areas where freshwater and seawater mix
Sediment is organic-rich mud with high levels of anaerobic bacteria
Salinity fluctuates dramatically (0 – 35 ppt) based on ratio of mixture, depth of estuary, and water circulationUnique among marine environments because it is regularly exposed to the air (emersion/immersion)

Most organisms are euryhaline and can tolerate a wide range of salinities
Soft-bodied organisms (molluscs, worms) are osmoconformers and allow their body fluids to change with the water
Fish, crabs, others are osmoregulators and use active transport to pump out excess water and absorb more solutes
Some plants have salt glands to remove excess salt or they accumulate large amounts of water to dilute salt

Murky water restricts photosynthesis by phytoplankton
Rich supply of fish and shellfish led to commercialization and urban development by humans
Used as nurseries by many marine fish species

Grassy areas bordering estuary that are partially flooded at high tide
Same environmental pressure as mudflats, but mud is held together by marsh plant roots
Dominant plant is cordgrass, pickle weed – tolerant to salt and flooding during high tide
Bacteria are crucial for decomposition of dead plant materials and nitrogen fixing to enrich sediment
Animals include crabs, snails, mussels, fish, birds

Tropical equivalent of salt marshes (lots located in Florida and Indonesia)
Mangrove trees are land plants adapted to intertidal environment – tolerance to immersion at high tide, adaptations to remove excess salt
Crabs very common, mudskippers unique to Indonesian mangrove, lots of burrowing suspension and deposit feeders, birds and reptiles as predators
Again, bacteria extremely important in breaking down detritus, but results in oxygen-deficient sediment