Legacy Phosphorus and Ecosystem Memory Control Future Water Quality in a Eutrophic Lake

Abstract

Lake water clarity, phytoplankton biomass, and hypolimnetic oxygen concentration are metrics of water quality that are highly degraded in eutrophic systems. Eutrophication is linked to legacy nutrients stored in catchment soils and in lake sediments. Long lags in water quality improvement under scenarios of nutrient load reduction to lakes indicate an apparent ecosystem memory tied to the interactions between water biogeochemistry and lake sediment nutrients. To investigate how nutrient legacies and ecosystem memory control lake water quality dynamics, we coupled nutrient cycling and lake metabolism in a model to recreate long‐term water quality of a eutrophic lake (Lake Mendota, Wisconsin, USA). We modeled long‐term recovery of water quality under scenarios of nutrient load reduction and found that the rates and patterns of water quality improvement depended on changes in phosphorus (P) and organic carbon storage in the water column and sediments. Through scenarios of water quality improvement, we showed that water quality variables have distinct phases of change determined by the turnover rates of storage pools—an initial and rapid water quality improvement due to water column flushing, followed by a much longer and slower improvement as sediment P pools were slowly reduced. Water clarity, phytoplankton biomass, and hypolimnetic dissolved oxygen differed in their time responses. Water clarity and algal biomass improved within years of nutrient reductions, but hypolimnetic oxygen took decades to improve. Even with reduced catchment loading, recovery of Lake Mendota to a mesotrophic state may require decades due to nutrient legacies and long ecosystem memory.

Stochastic dynamics of phycocyanin in years of contrasting phosphorus load

“…Kamarainen, Penczykowski, et al (2009) and Kamarainen, Yuan, et al (2009) showed that P supplied by recycling could meet phytoplankton demand for growth, although both internal supply and phytoplankton demand varied from day to day and among locations in Lake Mendota. A model of lake physics and biogeochemistry calibrated to the long-term database for Lake Mendota evaluated the lake's response if the external P load was decreased to zero (Hanson et al, 2023). After elimination of external P load, model results indicated that internal recycling of P would maintain elevated phytoplankton production for nearly a century (Hanson et al, 2023).…”

“…A model of lake physics and biogeochemistry calibrated to the long-term database for Lake Mendota evaluated the lake's response if the external P load was decreased to zero (Hanson et al, 2023). After elimination of external P load, model results indicated that internal recycling of P would maintain elevated phytoplankton production for nearly a century (Hanson et al, 2023). Despite the low external P loads of 2021, it is plausible that cyanobacteria were sustained by recycling P from sediments and hypolimnetic water entrained across the thermocline.…”

A tale of two blooms: do ecological paradigms for algal bloom success and succession require revisiting?

“…In addition, numerous long-term monitoring programs exist in smaller lakes which offer a unique opportunity to investigate the conclusions derived here ( Kröger et al, 2023 ; Rhodes et al, 2017 ; Yang et al, 2016 ). Most recently, Lake Mendota (US) has been identified as a long-term water quality model ( Hanson, 2023 ) and was recently used to forecast how legacy phosphorus and ecosystem memory constrain future water quality ( Hanson et al, 2023 ). Moving forward, there is a need to interweave both smaller freshwater systems and the Great Lakes into future paradigm discussions.…”

Ecological memory of spring air temperature drives summer water quality dynamics in temperate lakes

“…Thus, thermal exchange prior to stratification, in combination with effects of spring air temperature on the phenology of ice thaw and/or stratification onset (Dugan 2021;Ladwig et al 2021;Jane et al 2023), may lead bottom-water conditions to be more correlated with spring, compared to summer, air temperatures (Figure 2a). Here, we refer to this phenomenon as seasonal ecological memory (following, e.g., Ogle et al 2015;Dugan 2021;Hanson et al 2023;.…”