[3][4] A definition five percent larger forms the Benfleet and Southend Marshes Ramsar site[5] and Special Protection Area.

[3] The SSSI and NNR include the eastern half of Two Tree Island, in Leigh on Sea which is managed by the Essex Wildlife Trust.

[n 2][10][11] The marshes and mudflats have internationally important numbers of wildfowl and wading birds, including the dark-bellied brent goose, grey plover, redshank and red knot.

Scarce invertebrates, such as the white-letter hairstreak and marbled white butterfly, have adapted to specific habitats in the marshes.

[3] Major landowners within Benfleet and Southend Marshes SSSI include the Ministry of Defence, Network Rail and Crown Estate.