“We must have patience”

Nutrient pollution “diets” work, but they take time

Reductions in external nutrient inputs in eutrophic aquatic ecosystems can lead to improved conditions, but other factors also play a role.

Text: Henrik Hamrén

A new study, led by Baltic Eye researcher Michelle McCrackin, examines how eutrophic aquatic ecosystems respond to nutrient management. The results show that reductions in external nutrient inputs can lead to improved conditions, but that other factors also play a role.

– As a scientist, I wanted to understand how quickly and completely aquatic ecosystems could improve after reduced nutrient inputs, says Baltic Eye researcher Michelle McCrackin.

She led an international team that synthesized the results of 89 scientific studies from around the world to explore how aquatic ecosystems respond to eutrophication management and how fast they recover from eutrophication.

89 scientific studies from around the world were syntesised to a report on how 
aquatic ecosystems respond to eutrophication management and how fast they
recover from eutrophication.  The blue dots represents the locations of the studies. 

In a new study, she and her team report that 20 years after management actions were taken, lakes an coastal areas recovered less than half of the so-called baseline conditions.

In this case, the baseline conditions refer to conditions prior to eutrophication, in a nearby undisturbed site, or a restoration target.

– We estimated that if human nutrient inputs are ceased, algae could recover in 7 to 25 years, submerged grasses could recover in 6 to 100 years, and invertebrates could recover in 7 to 30 years, says Michelle McCrackin.

Other aspects of the ecosystem, like water clarity, could recover in 4 to 52 years.

The causes and consequences of eutrophication in lakes and coastal areas have been studied for decades. It is well known that nutrient inputs from fossil fuel combustion, sewage wastewater, aquaculture, and agriculture can reduce water clarity, increase toxic and nuisance algal blooms, alter fish diversity, and cause dead zones, which are bottom areas with insufficient oxygen to support life.

A number of policy measures have sought to reduce nutrient inputs to surface waters and there have been noticeable reductions in many areas.

– We saw a need for better understanding the extent of recovery by aquatic ecosystems. Such knowledge is helpful to determine if different measures are effective enough, says Michelle McCrackin. 

She and her team weren’t able to understand why recovery progress was so variable across the studies. However, they conclude that there are many other factors that affect recovery, such as climate change, invasive species, and nutrient inputs from other, diffuse sources. The interactions of all these stressors aren’t well understood.

– While our study confirms that recovery from eutrophication takes time, we also found that some aspects of the ecosystems don’t show improvement, even decades after management actions, says Michelle McCrackin.

Around the world, significant amounts of money are spent to combat eutrophication by reducing nutrient inputs. The long recovery times found in this study – and the importance of other factors than external nutrient inputs – reinforce the need for environmental monitoring, says Michelle McCrackin.

Unfortunately, many of the national-level monitoring programs in for example the EU and in the US have suffered from budget cut backs.

– In order to protect aquatic ecosystems and investments made to reduce eutrophication, we need to keep collecting data, says Michelle McCrackin.

For the Baltic Sea, these findings are not new. Previous computer modeling studies show that once the nutrient loads of the Baltic Sea Action Plan are met, it will still take decades for the sea to achieve good ecological status.

According to a recent study by Danish, Finnish, and Swedish researchers, eutrophication in the Baltic Sea is decreasing in response to actions taken on land to improve sewage treatment and farming practices.

– For those concerned with eutrophication, our study reminds us that we must have patience while the Baltic Sea and other water bodies recover after nutrient management, says Michelle McCrackin. 

Michelle McCrackin



Henrik Hamrén

science journalist