Glacial Runoff: Bane or Boon for Aquatic Life?

As glaciers melt around the world, their waters carry high concentrations of sediments into glacial lakes and rivers. That glacial sediment brings some nutrients into the lakes, but also blocks sunlight– the energy source which organisms need to survive.

In a recently published paper in The Journal of Plankton Research, titled When glaciers and ice sheets melt: consequences for planktonic organisms, Dr. Ruben Sommaruga of  the University of Innsbruck, Austria, analyzed the relationship between sunlight, nutrients and organisms.

Glacial rivers and lakes often appear blue because of the particular mix of minerals and sediments, known as glacial flour, or glacial milk, that gets scraped up when glaciers expand and grind the bedrock surface over which they move. Glacial retreat releases that glacial flour into rivers and lakes at unprecedented rates. At the same time, new glacial lakes and rivers are forming, providing scientists with an opportunity to observe if and how life will thrive in these bodies of water.

Figure of turbid glacial lakes transitioning to clear oligotrophic lakes.
Figure of turbid glacial lakes transitioning to clear oligotrophic lakes. Figure by RUBEN SOMMARUGA 2015

High concentrations of glacial flour in young glacial lakes makes them  turbid, or cloudy, blocking sunlight. These lakes become clearer as they age, as glacial flour settles to the lakebed. This process increases sunlight penetration in the water. Combined with an increase in other forms of nutrients entering the lake over time, such as bird droppings, this process results in a clear blue glacial lake with a healthy ecosystem. These clear lakes, called oligotrophic lakes, can support plankton and small fish, but do not have many aquatic plants. Eventually, if nutrients keep increasing in oligotrophic lakes, they can develop into highly biologically active lakes, called eutrophic lakes, with abundant aquatic flora and fauna.

An image of Kurtkowiec Lake, an oligotrophic lake in the Tatra Mountains of southern Poland.
Kurtkowiec Lake, an oligotrophic lake in the Tatra Mountains of southern Poland, via Wikipedia.
An image of Lake Waahi, a eutrophic lake in Huntly, New Zealand.
Lake Waahi, a eutrophic lake in Huntly, New Zealand, via Flicker.

In order for this eutrophication to occur, organisms at the base of the food chain, particularly  plankton, need to survive in the water during its early phase with low sunlight penetration.

Once a lake loses its connection with its original glacier, either because the glacier completely melted or because its runoff ceased, it changes more rapidly from a cloudy glacial lake to a clear oligotrophic lake. The location and size of the lake and glacier influence the pace of this process and the potential for eutrophication.

Research on these processes–which integrate climatic, hydrological, chemical and biological components–contributes to a more general understanding of the ecological consequences of climate change.

In the article, Dr. Sommaruga states “ . . . estimates based on a scenario where all glacier ice disappears in the Swiss Alps, predict 500 new lakes, which represent 30% more lentic [stillwater] systems for Switzerland. In other regions, such as in northern Patagonia, total glacial lake area has increased by 65% from 1945 to 2011.”

This research shows that plankton and other small organisms survive, though not always thrive, in young cloudy glacial lakes and rivers. Future research will extend current understanding of these ecosystems, and trace the implications for our planet’s freshwater ecosystems.

Other posts at GlacierHub have described glacial lakes in Switzerland and Greenland.

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