Bimodality and alternative equilibria do not help explain long-term patterns in shallow lake chlorophyll-a

Area of Science:

• Ecology
• Limnology
• Environmental Science

Background:

• The theory of alternative equilibria suggests shallow lakes exist in either clear, plant-dominated or turbid, phytoplankton-dominated states.
• This theory has broad applications in ecological and socioecological systems.

Purpose of the Study:

• To examine the relationship between phytoplankton biomass (chlorophyll-a) and nutrient concentrations in shallow lakes.
• To assess the validity of the alternative equilibria theory across different timescales using simulations and real-world data.

Main Methods:

• Utilized data simulations and real-world datasets from Denmark and the northeastern USA (902 lakes).
• Applied three diagnostic tests to identify the presence or absence of alternative equilibria.
• Analyzed chlorophyll-a and nutrient concentrations across various temporal scales.

Main Results:

• Data simulations confirmed the reliability of diagnostic tests for identifying alternative equilibria.
• Real-world data aligned with simulations where alternative equilibria were absent.
• A predictable linear relationship between nutrient concentration and chlorophyll-a emerged only at longer observation scales (>3 years).

Conclusions:

• The concept of alternative equilibria may not be required to explain shallow lake responses to nutrient enrichment over longer timescales.
• The study questions the utility of the alternative equilibria theory for understanding and managing shallow lake eutrophication and recovery.