Keywords:
island barrier dune
microbial mats
dune slack
succession
motor vehicles

Location:  The Netherlands, Province of Friesland, Schiermonnikoog

The Wadden Island Schiermonnikoog
Schiermonnikoog is a barrier island in the Dutch Wadden Sea. Dunes, salt marshes and mud flats are the main habitats. Since 1989, the island has been a National Park with Natuurmonumenten as the manager, the Dutch national society for nature conservation. All in all, the National Park covers an area of approximately 2500 ha, the village Schiermonnikoog and farmland excluded.
The sample area was an unvegetated sandy beach situated in a seepage zone of the main dune massive. Fresh water was regularly discharged at the surface, particularly in the wet season, and during high tides the beach was regularly flooded.

Observations
Primary succession in dune slacks can be roughly divided into four phases: (1) microbial and algal mats are dominant and accumulation of organic material is low, (2) colonization by phanerogams which are adapted to low nutrient availability, (3) development of a moss layer of pleurocarpic bryophites and invasion by tall grasses and shrubs and (4) rapid accumulation of organic matter, partly due to acidification of the top layer which leads to replacement of non-competitive plant species by shrubs and trees. Many endangered dune slack species are most abundant in the stages (2) and (3) of dune slack succession (e.g. Parnassia palustris, Liparis loeselii, Herminium monorchis).
Undisturbed not fully developed microbial mats were collected in 1994 to a depth of 6 cm using PVC tubes. Some material was sterilised by heat to disturb the mat as a functional entity and to stop all biological processes. All samples were placed in a temperature and light-controlled green house. Juveniles of plant species were added to study the influence of microbial mats on species of different successional stages. The selected plant species were Salomus valerandi (pioneer stages),  Calamagrostis epigejos (more productive, late successional stages) and Juncus alpinoarticulatus (wide range of successional stages). Different treatments were examined: (A) actively growing microbial mats under waterlogged conditions, (B) actively growing microbial mats under moist conditions, (C) heat-sterilised mat material under waterlogged conditions, (D) heated mat material under moist conditions and (E) actively growing mats in which the juveniles were planted by breaking the mats with a small pin (in all other cases the juveniles were placed on top of the mats without disturbing the surface). Oxygen and Sulphide concentrations were measured. The observation period took 26 weeks, after which the plants were harvested.

Results
Microbial and algal mats inhibit the growth of Juncus alpinoarticulatus and Calamagrostis epigejos in the case of intact mats as well as of heat sterilisation. This may be due to the apparent difficulty of juvenile roots to penetrate the mats as well as a low accumulation of organic matter, which is very favourable for pioneer species. The pioneer species Salomus valerandi was not affected negatively when grown under very wet conditions. Slightly lower water tables had a negative effect on the performance of all species while the relatively low oxygen content had no effect. No significant differences of pH could be measured. However, relatively high concentrations of sulphide in pots with poor growth of Juncus and Calamagrostis plants may exceed toxic levels for these species. Microbial mats may prevent a rapid acidification of the top soil which generally occurs in infiltration areas with a considerable precipitation surplus. This is due to the capability of the mats of promoting calcium carbonate precipitation.
The results 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. As a consequence, these mats may be used to select suitable sites for future nature conservation purposes. However, the mats are easily disrupted by car or motor tracks. Traffic on sandy beaches, which is still permitted in certain parts of the European coast, should be avoided in areas with extensive microbial mat formation.

References

• Grootjans, A.P., F.P. van den Ende & A.F. Walsweer (1997): The role of microbial mats during primary succession in calcareous dune slacks: an experimental approach. Journal of Coastal Conservation 3: 95-102. (KJc97).
• Grootjans, A.P., P. Jones, F. van der Meulen & R. Paskoff (1997): Ecology and restoration perspectives of soft coastal ecosystems – Introduction. Journal of Coastal Conservation 3: 5-8. (KJc97).