The role of microbial mats during primary succession in calcareous dune slacks: an experimental approach

Abstract

Laminated microbial mats from a sandy beach plain were grown in water-saturated pots in a glass house for six months and then used to assess their effect on the establishment of juveniles of three plant species representing different successional stages in dune slack development. The selected species wereSamolus valerandi, characteristic of pioneer stages,Calamagrostis epigejos, characteristic of more productive, late successional stages, andJuncus alpinoarticulatus, which occurs in a wide range of successional stages. Juveniles of all three species that were placed on top of intact living microbial mats established themselves in the mat.C. epigejos andJ. alpinoarticulatus survived for several weeks but later on their numbers decreased and the total biomass production of the species after six months was poor.S. valerandi, in contrast, grew profusely in intact microbial and algal mats. Heating of the microbial mat by heat sterilization, prior to the experiment, did not improve the performance of the species. When the juveniles were planted in the microbial mats after breaking the surface of the mat, the survival of juveniles ofC. epigejos andJ. alpinoarticulatus was much higher and so was the biomass of surviving plants after six months. Planting ofSamolus in the mats had some positive effect on the survival percentage of the juveniles, but not on the total biomass at the end of the experiment. Slightly lower water tables had a negative effect on the performance of all species. Measurements of the pH in the pots revealed that there were no significant differences in the top layer. Sulphide concentrations were very low in all the pots where juveniles had been planted and also in the pots withS. valerandi. Relatively high concentrations (30–50 μmol/l) were found in pots with poor growth ofJuncus andCalamagrostis plants. These values may exceed toxic levels for these species. Although oxygen concentrations in the pots were generally low, no relation existed between plant biomass and oxygen content, indicating that plant growth was not primarily limited by oxygen stress. These experiments support the idea that microbial mats may assist in extending the life span of early pioneer stages during dune slack succession by inhibiting the growth of species of later successional stages.