Mangrove Retreat with Rising Sea-level, Bermuda

Low island mangroves keep up with slow sea-level rise by peat accumulation. Holocene stratigraphic records show that they maintain the same pace as sea-level rise at rates up to 9 cm/100 years. Tide gauge records from Bermuda since 1932 show sea-level rise at a rate of 28 cm/100 years. The largest mangrove area (6•26 acres) at Hungry Bay has for the last 2000 years been building peat at a rate of 8•5 to 10•6 cm/100 years. Retreat of the seaward edge has caused loss of 2•24 acres of mangroves, commencing in the last few hundred years, with a second dieback between 1900 and 1947, and a third dieback in the last decade. The substrate elevation of the seaward margin of mangroves is below mean sea-level, the normal lower limit for mangroves. Present dieback shows problems of erosion indicating that the Bruun Rule of beach erosion with sea-level rise is also appropriate for mangrove swamps. Stratigraphy shows that before 4000 BP sea-level rose at a rate of 25 cm/100 years, from 4000 to 1000 years BP the rate of sea-level rise declined to 6 cm/100 years during which time mangroves established, and in the last 1000 years there was an increase to 14•3 cm/100 years, during which time the mangroves died back. This study indicates that low island mangroves will experience problems with the rates of sea-level rise predicted for the next 50 years