F. Montserrat

Spatial Self-Organization on Intertidal Mudflats through Biophysical Stress Divergence

In this study, we investigated the emergence of spatial self‐organized patterns on intertidal flats, resulting from the interaction between biological and geomorphological processes. Autocorrelation analysis of aerial photographs revealed that diatoms occur in regularly spaced patterns consisting of elevated hummocks alternating with water‐filled hollows. Hummocks were characterized by high diatom content and a high sediment erosion threshold, while both were low in hollows. These results highlight the interaction between diatom growth and sedimentary processes as a potential mechanism for spatial patterning. Several alternative mechanisms could be excluded as important mechanisms in the formation of spatial patterns. We developed a spatially explicit mathematical model that revealed that scale‐dependent interactions between sedimentation, diatom growth, and water redistribution explain the observed patterns. The model predicts that areas exhibiting spatially self‐organized patterns have increased sediment accretion and diatom biomass compared with areas lacking spatial patterns, a prediction confirmed by empirical evidence. Our study on intertidal mudflats provides a simple but clear‐cut example of how the interaction between biological and sedimentary processes, through the process of self‐organization, induces spatial patterns at a landscape level.

Organism-sediment interactions govern post-hypoxia recovery of ecosystem functioning

Hypoxia represents one of the major causes of biodiversity and ecosystem functioning loss for coastal waters. Since eutrophication-induced hypoxic events are becoming increasingly frequent and intense, understanding the response of ecosystems to hypoxia is of primary importance to understand and predict the stability of ecosystem functioning. Such ecological stability may greatly depend on the recovery patterns of communities and the return time of the system properties associated to these patterns. Here, we have examined how the reassembly of a benthic community contributed to the recovery of ecosystem functioning following experimentally-induced hypoxia in a tidal flat. We demonstrate that organism-sediment interactions that depend on organism size and relate to mobility traits and sediment reworking capacities are generally more important than recovering species richness to set the return time of the measured sediment processes and properties. Specifically, increasing macrofauna bioturbation potential during community reassembly significantly contributed to the recovery of sediment processes and properties such as denitrification, bedload sediment transport, primary production and deep pore water ammonium concentration. Such bioturbation potential was due to the replacement of the small-sized organisms that recolonised at early stages by large-sized bioturbating organisms, which had a disproportionately stronger influence on sediment. This study suggests that the complete recovery of organism-sediment interactions is a necessary condition for ecosystem functioning recovery, and that such process requires long periods after disturbance due to the slow growth of juveniles into adult stages involved in these interactions. Consequently, repeated episodes of disturbance at intervals smaller than the time needed for the system to fully recover organism-sediment interactions may greatly impair the resilience of ecosystem functioning.

Long-term divergent tidal flat benthic community recovery following hypoxia-induced mortality.

Macrobenthos recovery after hypoxia-induced mass mortality was assessed in an estuarine tidal mudflat during 3 years. During the first 2 years, a Pearson-Rosenberg type of community recovery took place along with the improving bottom water oxygen conditions. After 3 months, spionid polychaetes became superabundant (i.e. opportunistic peak), followed rapidly by a steep decline (i.e. ecotone point). Subsequently, a moderate increase in species richness and a steep increase in biomass, related to the growth of long-lived species occurred (i.e. transition region). Afterwards, however, the recovering community diverged again from the ambient, undisturbed, sediments due to enhanced recruitment success of long-lived species presumably resulting from the lowered interference from bioturbation during early recovery stages in the disturbed plots. Hence, despite early community recovery may be more or less deterministic, lagged divergent community reassembling may occur at the longer-term, thereby contributing to benthos patchiness in areas which are frequently subjected to disturbances.