Exploring the influence of hydrology on the threshold phosphorus-loading rate in shallow lakes

The Alternative Stable States Model describes the two contrasting conditions of shallow lakes that occur in response to external phosphorus (P) loading: 1) a clear-water, macrophyte-dominated state during times of lesser external P loading, and 2) a turbid, phytoplankton-dominated state during periods of greater P loading. Determination of the P loading rate that distinguishes the two stable states, i.e. the threshold phosphorus loading rate (TPL), is fundamental to shallow-lake science and management. We used data from nine shallow subtropical lakes in Florida (USA) to generate a parsimonious model that predicts the TPL from the hydraulic detention time (HDT). The model reveals that the TPL is more sensitive to changes in HDT in lakes with short (<1 year), as opposed to long (>10 years) HDTs. We compared our results with published data from 54 geographically distributed shallow aquatic systems and found that the Florida-based HDT-TPL model has general applicability. Given many lakes worldwide are shallow, our findings have the potential to improve understanding and management of numerous aquatic ecosystems around the globe.