Quantile regression reveals phosphorous overwhelms nitrogen in controlling high chlorophyll-a concentration in freshwater lakes

Abstract

High chlorophyll-a (Chl-a) concentration, driven by nitrogen (N) and phosphorus (P), is a growing global concern that has more severe and far-reaching effects on freshwater lakes compared to general Chl-a concentration. Understanding the pivotal roles of N and P in regulating high Chl-a levels is imperative. Although the traditional approach relied on the total nitrogen (TN): total phosphorus (TP) ratio offers a brief framework for assessing the importance of N and P in controlling Chl-a, the efficiency of this method is increasingly being questioned in high Chl-a freshwater lakes where nutrient limitation may differ from less impacted systems. This study aims to address this knowledge gap by examining the relative contributions of TN and TP to Chl-a concentrations across a wide range of Chl-a levels. Utilizing a global dataset of 30,844 data points from 2542 freshwater lakes, we employed both ordinary least squares regression (OLSR) and Quantile Regression (QR) methods to elucidate the impacts of TN and TP on Chl-a. Our findings revealed that the TN: TP ratio may not always be a reliable indicator of nutrient limitation, especially in high Chl-a conditions where TP often exerts a more significant influence on Chl-a levels. This insight suggests that current management strategies, which heavily rely on the TN: TP ratio, may not be as effective as required in high Chl-a concentration. The study underscores the need for a reassessment of nutrient management practices, advocating for a more nuanced approach that considers the specific Chl-a concentrations and nutrient dynamics of individual freshwater lake systems. The implications of this research are crucial for developing more targeted and effective strategies to mitigate Chl-a and safeguard freshwater resources.