Hidden phosphorus: the overlooked driver of algal blooms in global waters

The Phosphorus Problem

Phosphorus is a fundamental nutrient for life, influencing everything from aquatic primary productivity to the global carbon cycle. However, when present in excess, it leads to eutrophication—a process that fuels massive algal blooms, depletes oxygen, and degrades water quality. For decades, efforts to control this pollution have focused on inorganic forms of phosphorus. Despite these efforts, many lakes and rivers continue to suffer from frequent and severe algal blooms, suggesting a missing piece of the puzzle.

An Underestimated Culprit: Organic Phosphorus

A new review by researchers from Beihang University and the Chinese Research Academy of Environmental Sciences synthesizes 50 years of global research to identify that missing piece: organic phosphorus. This form of the nutrient, bound within organic molecules, is a substantial component of the total phosphorus in aquatic environments. The study reports that organic phosphorus can account for 21 to 60 percent of total phosphorus in sediments and more than 50 percent in particulate matter, making it a potentially large and unmanaged source of pollution.

Advanced Techniques Reveal Chemical Structures

Understanding the impact of organic phosphorus has been limited by the technical difficulty of measuring its various forms. The review examines the progress made in analytical techniques, such as phosphorus-31 nuclear magnetic resonance spectroscopy. A team led by academician Fengchang Wu has developed new methods for extracting and characterizing organic phosphorus from water, sediment, and algae. These technical improvements allow scientists to precisely identify different phosphorus compounds and assess their ability to feed algae.

From Algae to Algal Blooms

The research shows that much of the organic phosphorus in water systems is highly bioavailable, meaning it can be easily used by organisms. Decomposing algae and aquatic plants release large quantities of organic phosphorus back into the water. For example, the study notes that over 54 percent of the organic phosphorus in algae can be hydrolyzed by enzymes to release orthophosphate, a form directly usable by other algae. This creates a self-sustaining cycle where dead algae provide the nutrients for the next bloom.

Sediments as a Persistent Source

Lake sediments act as a vast reservoir of organic phosphorus. While some of this phosphorus gets buried permanently, a significant portion remains active. The review details how organic phosphorus in sediments, particularly in eutrophic lakes, can be degraded and released back into the overlying water. This process creates a long-term internal source of nutrients that can sustain eutrophication for years, even after external pollution sources are reduced. In some eutrophic lakes, over 70 percent of the organic phosphorus in sediment was found to be bioavailable.

A New Model for Phosphorus Cycling

Based on extensive data from Chinese lakes and global studies, the authors propose a new quantitative model for the biogeochemical cycling of organic phosphorus. This model offers a more complete picture by tracking the movement and transformation of organic phosphorus between water, sediments, algae, and aquatic plants. The model quantifies the contribution of each source to algal blooms, finding that in Lake Tai, China, algae-derived organic phosphorus contributed 76 percent to subsequent blooms, far outweighing other sources.

Rethinking Water Quality Management

The findings presented by lead author Weiying Feng and corresponding author Fengchang Wu have serious implications for water management. Current water quality standards and pollution control strategies primarily target total and inorganic phosphorus, largely ignoring the specific role of organic phosphorus. The authors argue that to effectively combat eutrophication, management strategies must be updated to account for this hidden nutrient source. This includes developing regional water quality standards and targeted methods to control the internal loading of bioavailable organic phosphorus.

Corresponding Author:

Fengchang Wu

Original Source:

https://doi.org/10.1007/s44246-023-00038-4

Contributions:

W.F. wrote the manuscript, T.W. and Y.Z. analyzed the data and developed the graphical representations, F.S. modified the manuscript, J.P.G. modified and improved the manuscript language, and F.W. modified the manuscript, provided experimental platform and constructive guidance. All authors discussed the results and reviewed the manuscript. The authors read and approved the final manuscript.