Nitrogen is an overlooked component of lake ecosystems

A recent study led by Dr. Daniel Graeber from the UFZ has brought a paradigm shift in our understanding of nitrogen and phytoplankton growth in lakes, challenging the long-held belief that phosphorus is the sole nutrient limiting this growth.

Traditionally, lake management strategies worldwide have been focused on controlling phosphorus inputs to prevent eutrophication — a process that leads to excessive algae growth and deteriorates water quality.

Phosphorus: The long-standing villain in lake health

However, this approach has repeatedly fallen short in combating eutrophication, prompting scientists to question the existing model.

Dr. Graeber explains, “it was previously assumed that phytoplankton growth in lakes is mostly limited and driven by the availability of phosphorus. In this explanatory model, nitrogen plays no role. This is based on the fact that specific cyanobacteria in the water can bind the nitrogen contained in the air and introduce it into the lake. This would therefore preclude a long-term nitrogen deficiency in lakes.”

This discovery points to a more complex interaction between nutrients than previously thought, with both nitrogen and phosphorus influencing the growth of phytoplankton.

New perspective on lake nutrients

The study, which also involved limnologist Dr. Thomas A. Davidson from Aarhus University, analyzed long-term monitoring data from 159 shallow lakes across North America, Europe, and New Zealand.

“This model forms the basis for lake management worldwide, where the emphasis has been on controlling phosphorus inputs to counteract lake eutrophication,” explains Dr. Davidson.

“Reducing phosphorus inputs repeatedly fails to prevent eutrophication. This therefore gave rise to the question of whether the water equation included yet another unknown,” Davidson concluded.

Shallow lakes, which constitute about 89 percent of the world’s lakes, provided a comprehensive dataset for examining the relationship between nutrient ratios and phytoplankton biomass, measured by the concentration of chlorophyll-a.

“We wanted to determine the long-term relationships between the ratio of the two nutrients and phytoplankton growth,” explains Dr. Graeber.

“The idea for our study originated with several outliers in a prior study of the ecology of shallow lakes. In some of the lakes, we did not see a linear correspondence between the measured nutrient and a respective increased or decreased chlorophyll-a concentration,” he said.

Surprising impact of nitrogen in lake ecosystems

The research revealed that in 60 percent of the studied lakes, both nitrogen and phosphorus were limiting factors for phytoplankton growth, a finding that significantly deviates from traditional limnological consensus.

The implications of this study are profound for the field of limnology and lake management practices.

“The relationships were so clear that I couldn’t even believe it at first. They corresponded exactly to what was known from laboratory experiments — this was truly astounding,” says Graeber.

“The result withstood thorough testing of the statistics: The data were robust. Our results confirm the hypothesis that nitrogen is not a passive participant in lake ecology. And we were able to prove with a broad database for the first time that this clearly applies to lakes worldwide,” Graeber continued.

This dual-nutrient limitation model not only challenges existing theories but also emphasizes the necessity of incorporating nitrogen into lake management strategies.

Integrating nitrogen into lake management strategies

Dr. Graeber and his team urge for a reevaluation of lake management practices, suggesting a more comprehensive approach that considers the impacts of both nitrogen and phosphorus.

“We need a long-term investigation of the nutrient ratios to ensure the success of efficient and effective eutrophication management. This requires a greater focus on inputs from agriculture, which generally have a high nitrogen content,” Graeber recommends.

In summary, the study led by Dr. Daniel Graeber and his team marks a significant turning point in our understanding of lake ecosystems. They demonstrated that both nitrogen and phosphorus play critical roles in phytoplankton growth and, consequently, in the eutrophication process.

This revelation challenges the traditional phosphorus-centric approach to lake management and provides new recommendations for research and policy-making that encompass a more holistic view of nutrient impacts.

By advocating for the inclusion of nitrogen in management strategies, this finding paves the way for more effective and sustainable solutions to combat eutrophication, ensuring the long-term health and balance of lake ecosystems worldwide.

The full study was published in the journal Nature Communications.

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