Inland waters increasingly low in oxygen

Inland waters – lakes, rivers and artificial reservoirs – are a vital component of the global biogeochemical system, playing a key role in regulating element cycles, greenhouse gas dynamics and the functioning of terrestrial and marine ecosystems. However, recent studies show that their functioning is undergoing profound changes. Over the past century, the oxygen cycle in inland waters has been severely disrupted, and the consequences of this process may extend far beyond the aquatic environments themselves.

World model shows how rivers and lakes breathe

A team of scientists from Utrecht University, led by Junjie Wang and Jack Middelburg, has developed the first-ever global model describing the oxygen cycle in inland waters. The IMAGE-DGNM model covered the years 1900-2010 and allowed for a precise estimation of how the production, consumption and exchange of oxygen with the atmosphere changed under the influence of climatic factors, human activities and biogeochemical processes.

The results of the analysis are alarming. In a century, the production of oxygen in waters has increased from 0.16 to 0.94 billion tons per year, while its consumption has increased from 0.44 to 1.47 billion tons per year. This means that while inland waters are producing more and more oxygen, they are also consuming more of it, and are therefore unable to meet their own demand. As a result, the net balance of production and consumption is becoming increasingly negative. The deficit has increased from -0.3 to -0.5 billion tons of oxygen per year.

Where does the oxygen in the water come from?

Oxygen in inland waters is produced mainly by photosynthesis, carried out by algae and higher plants. At the beginning of the 20th century, production near the bottom (so-called benthic) was dominant, but over time photosynthesis in the depths of the water (pelagic) began to play an increasingly important role. The breakthrough came in the 1970s, when much larger amounts of nutrients began to enter the waters, mainly as a result of agricultural and municipal activities.

The geography of oxygen production has also changed over time. In 1900, the most active water systems in this regard were those in the tropics, including the Amazon, Congo and Orinoco river basins. By 2010, the center of activity had shifted to more urbanized areas: the southeastern United States, Western Europe and Southeast Asia. At the same time, the role of water bodies increased. Their share of global oxygen production increased from 53 to as much as 85 percent, a result of the massive construction of dams that turned natural rivers into reservoirs with long retention times.

Oxygen is depleting faster than it is arriving

Oxygen demand in inland waters is increasing due to the intensification of many biological and chemical processes. Oxygen is consumed during, among other things, respiration of aquatic organisms, mineralization of organic matter and chemical processes such as nitrification. Although algae and plants produce oxygen through photosynthesis, they also consume it themselves, especially at night, when respiration processes take precedence over photosynthesis. Also problematic are the increasing amounts of organic matter coming in from the land and generated locally and decomposed by microorganisms, generating a high demand for oxygen. Particularly intensive consumption occurs in bottom sediments, which, as the model results show, absorb more oxygen than can be replenished by photosynthetic production or exchange with the atmosphere.

As a result, the demand for oxygen in inland waters continually exceeds its local production, leading to a permanent deficit. To maintain the biogeochemical balance, ecosystems take oxygen from the atmosphere. According to the analysis, in 1900 atmospheric oxygen uptake was 0.66 billion tons per year, and in 2010 it was already 0.95 billion tons. Although the surface area of inland waters is only 0.2 percent of that of the oceans, they absorb almost half the amount of oxygen that the Allocean gives off to the atmosphere. Most of the gas exchange occurs in rivers and streams, but the most intensive processes of oxygen production and consumption occur in water bodies. Inland waters, despite their important role, are still not included in global climate models and IPCC reports, which is a major gap in estimates of the global oxygen cycle.

Why are inland waters losing oxygen faster and faster?

The model identifies three main causes:

• Excess nutrients (nitrogen, phosphorus) from fertilizers and wastewater;
• Hydrological transformations – dams and reservoirs that extend water retention times;
• Global warming, which reduces the solubility of oxygen and accelerates the decomposition of matter.

Significantly, rising temperatures alone are responsible for only 10-20 percent of changes in the oxygen cycle. This means that it is not the climate, but primarily direct human activity (fertilization, wastewater, river regulation) that is responsible for the current state.

Simulations have shown that without the historical increase in nutrient input, oxygen production in inland waters would be lower by as much as 56 percent, and oxygen consumption by 67 percent. This means that eutrophication actually drives the intensification of oxygen-forming processes, but increases the demand for this gas even more strongly. As a result, the balance remains negative – and it is the excess of nutrients, combined with hydrological transformations (which account for more than 80 percent of the increase in oxygen production), that leads to profound and adverse changes in the functioning of aquatic ecosystems.

Impacts on climate and ecosystems

Disruption of the oxygen cycle in inland waters has serious consequences not only for aquatic organisms. Oxygen affects the cycling of many other elements, such as carbon, nitrogen and phosphorus. Oxygen deficiency can lead to:

• more frequent and more intense algal blooms, whose decomposition after death exacerbates hypoxia;
• hypoxia (known as suffocation) and massive fish die-offs;
• The deterioration of drinking water quality;
• biodiversity loss;
• Increased greenhouse gas emissions.

All of this could have implications for climate, public health and food security. The study’s authors emphasize that unless action is taken to reduce nutrient inputs and greenhouse gas emissions, inland waters will become even larger sinks of atmospheric oxygen, with further consequences for the entire Earth.