Consequences of alternative stable states for short-term model-based control of cyanobacterial blooms

Cyanobacteria can form dense blooms in eutrophic lakes that can be toxic to humans and other animals and harmful to the ecosystem’s functioning. While better nutrient management is generally considered the long-term solution to this problem, short-term mitigation efforts (e.g., flushing, algaecides, flocculants) are becoming increasingly necessary to safeguard water quality and the ecosystem services it provides. Here, we explore potential model-based management strategies for these short-term mitigation efforts. We focus on the case where blooms are linked to the existence of alternative stable states, such that, under the same conditions but depending on the past, a lake may be dominated either by cyanobacteria (causing a harmful algal bloom) or by green algae and macrophytes in a clear water state. Changing conditions may cause the favourable clear water state to disappear through a tipping point, causing the lake to switch rapidly to the turbid cyanobacteria state. At the same time, it may take considerable effort to undo this tipping and return to the favourable state. We identify four different strategies for bloom mitigation in this scenario: Doing nothing, reacting to a bloom, resetting the lake at a later point, and preventing the bloom. We explore the different requirements for these strategies along with their associated cost profiles. We also investigate the effect of transition times from one state to another on the efficacy and costs of different strategies.