Now the coast is dominated by Spartina marshes. In the late 1890's, Spartina was brought to a marsh in Poole, England. It spread and then crossed with a local species S.maritina to make a male sterile hybrid S.townsendii . The new species spread vegetatively and doubled its chromosomes to produce another species S.anglica which is fertile. All of these species have taken over England's marshes.
Positive estuaries have a river at the back and get less saline going away from the sea.
The true halophytes are found at the front, less salt tolerant wetland plants are found at the back. In a positive estuary the back zone is colonized by Juncus, Scirpus , and Phragmites . The middle zone is dominated by Spartina or mangroves, with patches of halophytes like Batis and Salicornia . The front is colonized by annual Salicornia before Spartina can get going, this is because Salicornia can easily colonize mud flats. Negative estuaries are like the Colorado River delta with no current river flow.