Abstract :
Spatial, temporal and successional patterns of biomass and species composition of microalgal assemblages were measured on a saltmarsh restoration site (Blackwater, Essex, U.K.) from December 1992 to July 1995. Classification analysis of the microalgae generated six distinct microbial assemblages (two diatom assemblages, D1 & D2; two mixed composition assemblages, M1 & M2; a green algal assemblage, G; and a cyanobacterial assemblage, C). Each of these assemblages differed significantly in the abundance of living and dead diatoms, cyanobacteria, filamentous bacteria and green algae. Microalgal biomass was highest at the beginning of the study and always significantly (P<0•001) higher on the upper and middle marsh stations than on the lower shore stations. Declines in microalgal biomass over time corresponded to increased macrophyte cover and decreasing organic content on the upper and middle marsh stations. Cyanobacterial mats occurred mainly on a region of compacted sediments with a low ash-free dry weight and water content. The succession from immature cyanobacterial mats dominated byOscillatoriaspp. andSpirulinato mats dominated byMicrocoleus chthonoplastestook 3 years. Thirteen of the 67 algal taxa identified during the study showed significant preference of occurrence, either by station, assemblage type or season, or with time. Principal component analysis showed a linear sequence between D1, D2, M1, M2 and G type assemblages, relating to the position on shore, macrophyte cover and microalgal biomass. Cyanobacterial assemblages were independent of this successional sequence. There was a significant relationship between the biomass of D1, D2 and M1 type assemblages and the ability to biostabilize sediments through the production of colloidal carbohydrate exopolymers. Assemblage composition needed a relative abundance of diatoms greater than 50% to possess a significant relationship between exopolymer concentration and biomass. It was concluded that diatom films would be the initial colonizers of deposited sediments and that M and G assemblages would develop with increasing sediment bed height in association with increasing macrophyte cover. Compaction of managed retreat sites during construction would lead to the establishment of cyanobacterial mats and delay the development of a typical saltmarsh macrophyte community.
