Impacts of phosphorus loading temporal pattern on benthic algae growth in Lake Ontario

Abstract

Nuisance growth of the alga Cladophora has been a critical issue in the Laurentian Great Lakes for decades. Although loading of total phosphorus (TP) has been below target values since the 1990s, the nearshore of the lakes still experience Cladophora blooms. This study explores the effects of TP loading temporal pattern on Cladophora growth in Rochester Embayment in Lake Ontario, where Cladophora blooms are frequently observed. Long-term TP loadings from the Genesee River, which is the primary tributary to the embayment, are calculated with a hydrological model, and Cladophora indicators based on beach closures and satellite data are collected for 2008–2017 to analyze the TP loading temporal patterns and their relation with Cladophora abundance. Using time series clustering for both TP loading and Cladophora indicators, it is found that besides total annual TP loading, the timing of high TP loading also plays an important role for Cladophora growth. High TP loading in mid-March to mid-June, compared to that in January to mid-March, is found to produce higher Cladophora abundance in summer. This finding is supported and explained using an integrated hydrodynamic and ecological modeling framework that includes a state-of-the-art Cladophora module. This study suggests that future Cladophora control should consider not only the total annual TP loading mass, but also the temporal loading patterns.