COMPARATIVE STRUCTURE, PRIMARY PRODUCTION AND BIOGENIC STABILIZATION OF COHESIVE AND NONCOHESIVE MARINE-SEDIMENTS INHABITED BY MICROPHYTOBENTHOS

The microstructure, rate of primary production and erodibility of noncohesive and cohesive marine sediments colonized by autotrophic microbial assemblages were investigated. Microbial development on non-cohesive sediments (Island of Texel, The Netherlands) formed millimetre-thick stratified mars. A surface sheath of extracellular polymeric substances (EPS) was visible on these mats; diatoms dominated the surface 500 mu m with an underlying layer of cyanobacteria, mainly Microcoleus chthonoplastes. In comparison, biofilms on cohesive sediment (Bristol Channel, U.K.) were relatively thin (100 mu m) and unstratified. The microspatial distribution of the algal biomass was confirmed by measurements of oxygen evolution and rates of primary production using oxygen micro-electrodes. While gross primary production was greater in the non-cohesive sediments, peak rates of photosynthetic activity and algal biomass were similar for the two sites. The erodibility of colonized areas was measured and the mechanisms of biogenic stabilization were examined by low-temperature scanning electron microscopy (LTSEM). Cohesive sediment stabilization was purely by the secretion of EPS while both EPS binding and network formation by cyanobacteria were visualized within the non-cohesive sediments. Areas where mat and biofilm systems were visible were more resistant to sediment erosion than adjacent areas without biofilm development. Direct comparison between biogenic effects at the two sites must be treated with caution because of the different nature of the sediments but the index of biogenic stabilization for both sites is reported. The stability of the non-cohesive sediments was greatly increased by the presence of a microbial mat. The hydration state of the EPS matrix was considered to have a potential effect on the critical erosion threshold of the sediments.