DO ALTERNATE STABLE STATES OCCUR IN NATURAL ECOSYSTEMS? EVIDENCE FROM A TIDAL FLAT

Abstract

Studies from a wide variety of ecosystems indicate that primary producers may protect their environment against degrading processes such as erosion by water current or wind. Theoretical analyses showed that the dynamics of these systems are governed by positive feedback. We investigated the implications of a positive feedback between growth of benthic diatoms and erosion of silt in tidal flat systems. A simple mathematical model shows that alternate stable states may occur in systems with positive feedback between diatom growth and silt accumulation, particularly in sediments with intermediate bottom shear stress. High diatom cover, high silt content, and low levels of erosion characterize one state. The other state is dominated by erosion, and hence both diatom cover and silt content are low. In an experimental study, we tested the critical model assumption that the growth rate of diatoms increases with the silt content of the sediment. Net growth of diatoms was significantly higher on silt than on sandy sediment after nine days of incubation, supporting the premise that diatom-silt interactions are governed by positive feedback. Furthermore, we compared model predictions to data on the physical and biological prop- erties of sediments of a tidal flat. In accordance with our model, the silt content of sediments with intermediate to high bottom shear stress showed a clear and significant bimodal dis- tribution, which may reflect the existence of alternate stable states. At low bottom shear stress, silt content was better explained by a unimodal distribution, as was predicted by our model. Patterns in chlorophyll a content were less clear. Nevertheless, chlorophyll a content was best explained by a bimodal distribution at high bottom shear stress, and in two of the three periods at low bottom shear stress. Our study indicates that the positive feedback between enhanced production of diatoms and decreased erosion of sediment sig- nificantly affects the dynamics of intertidal flat systems.