Alternative Stable States in Brackish and Marine Ecosystems—Where Are We After 50 Years of Investigations?

Abstract

About 50 years after the first publications on the existence of alternative stable states (ASS) in ecosystems, the concept has become widely accepted. However, there is still a diverse terminology and no consensus about which criteria must be fulfilled, and what kind of investigations or data are suitable for proving the existence of ASS. Most empirical observations report sudden shifts between stable states or equilibria. However, they usually fail to demonstrate that forward and backward shifts occur at different threshold levels of a gradually changing external driver. This is a key requirement for hysteresis and ecosystem states to be considered “alternative.” We suggest that evidence for ASS should be based on field observations or experimental field investigations, ideally combined with modeling and laboratory experiments on specific stabilizing mechanisms, and fulfill all four of the criteria: (1) sudden shifts between states, (2) the occurrence of a hysteresis, (3) sufficient spatial and temporal scales, and (4) the presence of efficient feedback mechanisms stabilizing each state. In marine and brackish ecosystems, a number of investigations, especially from shallow environments, seem to fulfill these criteria, but few come from large and deep bodies of water. Several studies have assumed the presence of ASS in shallow brackish lagoons along the Baltic Sea. Strong feedback is exhibited in the turbid state when it is dominated by cyanobacteria, while feedback mechanisms in the macrophyte-dominated state require further investigation. For successful management, understanding ASS and, especially, the most efficient stabilizing feedback mechanisms is essential to reach and maintain a good ecosystem status.