Ontario: A more than five decade experiment on a small Canadian lake has revealed how freshwater ecosystems can adapt when one nutrient is removed but another remains available. Scientists found that stopping nitrogen additions did not end recurring algal blooms in Lake 227 because phosphorus continued to fuel biological production.
Lake 227, located in northwestern Ontario’s Experimental Lakes Area, has been used since 1969 for one of the longest running whole lake experiments in the world. Researchers deliberately added measured quantities of nitrogen and phosphorus to study how nutrients influence algae and the wider aquatic ecosystem.
The lake covers about five hectares has an average depth of 4.4 metres and reaches a maximum depth of about 10 metres. Unlike laboratory experiments conducted in containers, the whole lake study allowed scientists to observe interactions among algae, nutrients, microbes, sediments, fish and zooplankton over several decades.
From 1969 to 1974 researchers added nitrogen and phosphorus at a ratio of about 12 to 1 by weight. The lake soon became highly eutrophic, with phytoplankton production increasing alongside phosphorus inputs.
In 1975, scientists reduced the nitrogen to phosphorus ratio to about 4 to 1. The change favoured nitrogen fixing cyanobacteria, which can obtain nitrogen from the atmosphere and convert it into forms usable for biological growth.
A major change came in 1990 when researchers stopped adding nitrogen completely while continuing phosphorus additions. The expectation was that removing nitrogen might substantially reduce algal production.
Instead, the lake remained highly productive. Nitrogen-fixing cyanobacteria became increasingly important and helped compensate for the loss of externally supplied nitrogen. Algal biomass continued to remain broadly linked to the amount of phosphorus entering the lake.
A peer reviewed study published in the Proceedings of the National Academy of Sciences examined 37 years of the experiment and found that eliminating added nitrogen did not control eutrophication while phosphorus remained available.
The findings do not mean nitrogen is unimportant. Nitrogen availability affected which organisms could thrive and changed the composition of the algal community. However, the experiment showed that organisms capable of fixing atmospheric nitrogen can partly compensate when dissolved nitrogen becomes scarce.
This biological response also changed the character of the lake’s algal community. Species such as Aphanizomenon schindlerii became important components of the summer phytoplankton community. After nitrogen fertilisation ended, nitrogen fixing cyanobacteria generally accounted for more than half of total phytoplankton biomass, except during a later food web experiment.
The Lake 227 experiment also showed that nutrients are not the only factor influencing algal blooms. Researchers conducted food web experiments in the 1990s involving northern pike and zooplankton.
After changes in the fish population large Daphnia became abundant. These zooplankton grazed heavily on algae, causing phytoplankton biomass to fall sharply in 1996. The reduction did not last. As the food web changed again, Daphnia populations declined, nitrogen fixing cyanobacteria returned to prominence and algal production increased.
The results highlighted the importance of studying an entire ecosystem rather than relying only on short term laboratory tests. Nutrient cycling, sediment interactions, nitrogen fixation and predator prey relationships can change over several years.
Eutrophication occurs when excessive nutrients stimulate unusually high plant and algal growth. Heavy algal production can reduce water clarity and when algae decompose, consume oxygen in deeper water. Some cyanobacteria can also produce toxins or form surface scums, creating risks for aquatic life, recreation and drinking water supplies.
The Lake 227 experiment strengthened the scientific case for controlling phosphorus inputs in freshwater systems. Sources of phosphorus can include agricultural runoff, wastewater, urban stormwater, leaking septic systems and soil erosion.
However, the researchers findings do not provide a universal formula for every lake. Different freshwater systems can respond differently depending on their chemistry, hydrology, biology and sources of nutrient pollution.
The long running experiment instead provides a detailed example of how an ecosystem can reorganise when one nutrient is reduced while another remains available. In Lake 227, eliminating added nitrogen did not stop eutrophication because phosphorus continued to support biological production and nitrogen fixing organisms helped supply the missing nitrogen.
More than five decades after the experiment began, Lake 227 remains an important example of why freshwater management may require sustained nutrient controls and long term monitoring rather than relying on short term changes in water quality.