Tracking the ecological recovery of German lakes following eutrophication
When an aquatic ecosystem is loaded with excess nutrients – from detergents, fertilisers or sewage, for example – algal blooms often occur. High levels of nutrients such as nitrogen and phosphate in a water body can cause rapid growth of plants and algae, which can ‘choke’ the ecosystem of light and oxygen.
When algae die and decompose, the nutrients they contain are converted into inorganic forms by microorganisms, a process which consumes oxygen. This means that decomposing algal blooms can starve an aquatic ecosystem of dissolved oxygen, which is vital for fish populations. Waterbodies with low dissolved oxygen are described as anoxic, or in extreme cases as experiencing hypoxia.
These ecological responses to increased nutrient levels are often called eutrophication. Eutrophication can be a natural process, occurring over long timescales in response to climatic changes and geological weathering, but is vastly accelerated in both speed and impact by nutrient pollution from human activity.
Eutrophication has become a major environmental issue in lakes, canals and slow flowing rivers in Europe and North America since the mid-20th century. Its impacts can vary, but often include reductions in freshwater biodiversity, fish kills, increases in water toxicity and cloudiness, and the resulting need for increased water treatment to produce safe, clean drinking water.
In Europe, the adoption of the Water Framework Directive in 2000 has forced national governments to take steps to reduce nutrient pollution in lakes and rivers. Many lakes that became dominated by phytoplankton under eutrophication are now recovering, and supporting increasingly diverse and healthy ecosystems.
Ecological monitoring procedures using biological quality elements – such as phytoplankton or aquatic plant (or macrophyte) populations – are used to help scientists track the ecological recovery of freshwaters in response to reduced nutrient levels. Existing studies have found that macrophyte recovery may be delayed, whereas phytoplankton levels often showed almost immediate reductions.
Delayed macrophyte growth is likely to be due to longer generational time, dispersal limitations, and a lack of viable seed banks for regrowth. As such, macrophytes’ complex and variable responses to reduced nutrient pollution make them potentially unreliable biological indicators for surveying water quality, at least in the short term.
Over longer periods, the restoration and regrowth of a formerly eutrophic ecosystem is likely to be dominated by new plant communities. However in the short term (e.g. from season to season) phytoplankton levels are generally used to provide the best indicator for water quality changes, which are then reported to the Water Framework Directive monitoring program.
A new study, published in the journal Ecological Indicators, tests the responses of macrophyte and phytoplankton communities to reductions in nutrient levels in lakes. The study, led by Falk Eigemann from the Leibniz Institute of Freshwater Ecology and Inland Fisheries in Germany, assessed 263 German lakes recovering from historical nutrient pollution and eutrophication between 2003-2012.
The study found that a lake’s ecological status was recorded as lower when using macrophytes as a biological quality element, as compared to when phytoplankton. This is due to the lag in response of macrophyte populations to reduced nutrient levels in the ecosystem.
Longer-term data (beginning in the 1980s) from five lowland lakes where phosphorus levels had been reduced showed that phytoplankton levels indicated a constant improvement in ecological status which tracked decreases in nutrient levels. Macrophytes, on the other hand, showed a 10-20 year delay in their ecological recovery.
There are two important points to take from this study. First, the study’s authors confirm that the way ecological status is measured (i.e. the choice of indicator) affects monitoring results. However, the parallel reductions in phytoplankton levels in response to decreasing nutrient levels confirm its usefulness as a bioindicator.
Second, and more broadly, the study demonstrates how ecological recovery and restoration is a slow process, even when measures such as nutrient reduction are put in place. As the authors of this study put it, “each lake ecosystem is unique and thus responds differently. When sufficiently low nutrient concentrations have been achieved, still patience may be needed when anticipating an improved ecological lake status.”
The study suggests that in German lakes, aquatic plant communities may take decades to regrow after eutrophication. Even where political and public will is in place, ecological recovery is often a slow, and potentially unpredictable, process.
Falk Eigemann, Ute Mischke, Michael Hupfer, Jochen Schaumburg, Sabine Hilt, “Biological indicators track differential responses of pelagic and littoral areas to nutrient load reductions in German lakes”, Ecological Indicators, Volume 61, Part 2, February 2016, Pages 905-910