Less and less oxygen in European lakes

Lakes in Europe and other temperate regions have become increasingly deprived of oxygen in recent decades. This is probably due to the warming of the climate and clouding of the water, writes an international research team in Nature. The oxygen depletion in fresh water is many times greater than that in the oceans and can pose a risk to the species that live in it.

The amount of dissolved oxygen in water plays an important role in water quality. For example, with a low oxygen concentration in deep water, phosphate is released from the sediment more quickly, which in turn can lead to the growth of harmful algae on the surface. An oxygen deficiency also leads to the death of underwater life. The reduced amount of oxygen can have various causes, including increasing stratification or ‘stratification’: if the surface water heats up more and more, this has adverse consequences for the circulation to deeper parts.

Huge diversity

Although relatively much is already known about oxygen availability in oceans and coastal waters, knowledge about freshwater environments has lagged behind. Co-author Wim Thiery of the Vrije Universiteit Brussel: “Oceans occupy about two-thirds of the earth’s surface and in that respect are a more important component of the climate system than lakes, which occupy only 2 percent of the surface. But lakes are often home to enormous diversity, and the species that live there are often much more vulnerable to the effects of climate change. After all, they cannot easily adapt to the effects of global warming by migrating.”

To increase knowledge about lakes, the research team, led by American and Swedish scientists, decided to analyze changes in water quality in 393 lakes over the past few decades – each lake had at least 15 years of data. The emphasis was on lakes in temperate regions, between latitudes 23.5 and 60 degrees north and south. Thiery: “The biggest bottleneck for this type of study is the availability of measurements. Historically, lakes in the temperate latitudes have been monitored much better because the research institutes are typically located there and such monitoring often requires regular maintenance of the measurement instruments. For example, only two lakes in Africa are included in this study, simply because hardly any measurements of oxygen concentrations are performed in lakes.”

On average, the oxygen concentration at the water surface in the lakes studied decreased by 5.5 percent between 1980 and 2017. Deeper in the water column, the decrease was even 18.6 percent. By way of comparison: the concentration of dissolved oxygen in the oceans has decreased by an average of 2 percent over the entire water column since the 1960s. The temperature in the deeper water layers of the lakes remained almost the same during the measurement period; on the surface there was a clear warming. This is consistent with increasing stratification in the water.

Algae provide oxygen

Although oxygen generally dissolves less well in water at higher temperatures, this was not the case for all lakes. Remarkably, the concentration of dissolved oxygen actually increased in one in three lakes with a surface water temperature above 25 degrees and at least 50 percent cultivated soil in the area. The influx of nutrients ensures algal blooms in warm weather, and those algae provide extra oxygen. That may seem favorable for the oxygen concentration, but it is not. Because here too there is talk of stratification: the oxygen availability on the surface is high, but not in deeper water – where many fish live.

Algae blooms also cause decreasing water clarity. As a result, sunlight is less able to reach the deeper water layers, which limits the growth of oxygen-producing aquatic plants.

It is not possible to conclude from this study to what extent the reduced oxygen concentration already affects the species richness in the temperate lakes. However, the authors warn that especially cold-water-loving, deeper-living species may have a hard time, and that further warming and eutrophication will only make the problems worse.