THE INFLUENCE OF VEGETATION PRE-DREDGING ON THE POST-DREDGING COMMUNITY

Vegetation present in a channel immediately before it is dredged was f ound to have a significant influence on the post-dredging vegetation i n terms of species composition. Approximately 60% of the species recor ded prior to dredging were found in the first 2 yr after dredging. Mea n cover values of species indicative of the latter stages of channel h ydroseral succession (e.g. great pond-sedge (Carer riparia Curtis) and common reed (Phragmites australis (Cav.) Trin. ex Steudel) are much l ess in the post- than in the pre-dredging channel. The most important elements of post-dredging vegetation were filamentous algae and floati ng species such as common duckweed (Lemna minor L.), fat duckweed (L. gibba L.) and frogbit (Hydrocharis morsus-ranae L.). These species wer e present before management but only with small percentage cover. Alth ough no submerged species was recorded predredging, a number of such s pecies did appear in channels post-dredging but they did not persist.