Question: Are there hot spots of algal mat deposition in space and time at the marsh scale and, if so, how does this affect the coexistence of a dominant (Spartina anglica) and gap dependent (Salicornia europaea) species?

Location: The Rattekaai salt marsh in the Scheldt estuary in the southwestern Netherlands (NW Europe).

Methods: Mat cover and the abundance of the gap dependent species Salicornia europaea were monitored at the scale of a marsh. The effects of mat cover on the vegetation structure were studied by applying three mat removal treatments over three growing seasons.

Results: The low marsh border was found to be a hot spot of algal mat deposition during the growing season, which had a correlated spatial pattern between two successive years at a 20 m × 20 m scale. The combination of duration, timing and repetition of mat cover determined growth inhibition of the competitive dominant Spartina anglica, and thereby the abundance of subordinates such as Salicornia europaea. Mat cover reduced the storage of carbon reserves in Spartina and our results imply that repetition of non-lethal mat cover can lead to ‘gap creation’. Gaps gave only temporary habitat to less dominant species since Spartina quickly re-invaded them. The gap dependent annual Salicornia was most abundant at intermediate levels of disturbance measured as a function of both space and time.

Conclusions: In addition to disturbance level, the spatial and temporal distribution of disturbance are important in creating and maintaining habitat for gap dependent species. Relatively small disturbances will have a large effect on diversity if the spatial and temporal distribution of the disturbances leads to ‘disturbance hot spots’.