Melting glaciers may reduce Greenland fishery

Glaciers that flow directly into the ocean provide additional nutrients to the surface water layer of fjords. This sustains a large fish stock. But when the glaciers melt and retreat on land, there is a great risk of a decline in fish stocks.

2017.09.11 | Peter Bondo Christensen

– Glaciers flowing into fjords release fresh water deep into the water column. The fresh water rises to the surface and thus drives deeper-lying nutrient-rich ocean water up to the phytoplankton in the upper water layers. This sustains a high production of fish. The photo shows a so-called ‘upwelling’ – the ice-free area near the glacier edge. (Photo: Lorenz Meire).

– Glaciers retreating on land release fresh water into the surface waters of the fjords. This prevents the water in the fjord from mixing and nutrients from the deeper-lying nutrient-rich water from reaching the phytoplankton in the upper water layers. This may lead to fewer fish, and researchers now warn that in the near future, many glaciers will retreat so far back that they end up on land. (Photo: Lorenz Meire).

Throughout the last two decades, the melting of the ice sheet has doubled and currently more than 1,000 cubic kilometres of fresh water flow to the fjords around Greenland every year.

Both the ocean and the atmosphere have become warmer and the release of the huge amounts of fresh water is mainly due to retreating glaciers.

Glaciers in West and South Greenland, which flow directly into fjords, retreated on average by 110 m per year during the period 2000-2010.

Researchers now warn of a tendency that glaciers that flow into the fjords, ‘marine-terminating glaciers’, melt and retreat on land. This has a profound effect on the Greenlandic fjords – both on the water flow and on the biological production – including fisheries.

Thermocline

The melt water from the glaciers and the ice sheet is poor in nutrients. How fresh water is transported to the fjords is, nevertheless, of vital importance for the fish stocks.

The bottom water of the fjords is rich in nutrients. But these need to be transported to the upper water layers before the microscopic phytoplankton can exploit them and the sunlight to perform the photosynthesis, which forms the basis of all life in the ocean.

The phytoplankton feeds the zooplankton, which in turn feeds small fish, which then feed larger fish, and so on – an important food web that sustains productive fisheries around Greenland – for example, the successful Greenland halibut fishery.

A glacier that retreats on land, a ‘land-terminating glacier’, transports large amounts of melt water to the surface water layers of the fjord. As fresh water is lighter than salt water, a ‘thermocline' is formed in the water column with the heavy ocean water at the bottom and the fresh water on top. The thermocline forms a physical barrier between the two bodies of water that makes it almost impossible for the two bodies to mix. The nutrient-rich bottom water therefore remains in the deep water to where the sun's rays do not penetrate.

Moreover, the fresh water from the glaciers brings with it suspended sediments that spread out in a fan shape into the fjord, thereby shading the phytoplankton.

Upwelling

The process is quite the opposite for the marine-terminating glaciers. Here, the melt water also flows to the surface of the fjord. But the crucial and important difference is the so-called subglacial outflow. This means that fresh melt water from the marine glaciers, as opposed to the land-based glaciers, is also to a large extent released deep below the ocean’s surface.

As melt water is lighter than the ambient fjord water, it rises to the surface and mixes with nutrient-rich fjord water on the way. The sun light at the water’s surface creates favourable conditions for the phytoplankton to multiply. This phenomenon where water rises to the ocean surface is called ‘upwelling’.

Fjords with marine-terminating glaciers often have a high primary production and thus a large fish stock. This is seen, for example, in Godthåbsfjorden, which is in contact with three glaciers of this type. And it applies not least to Ilulissat where 5-6,000 tonnes of halibut are caught each year over a few square kilometers. Far greater quantities than in the open sea.

Less nutrients, fewer fish
“We can clearly see that fjord systems with glaciers ending on land are much less rich in nutrients," says post doc Lorenz Meire from the Greenland Institute of Natural Resources and the Arctic Research Centre, Aarhus University, which has been at the forefront of the recent studies that have just been published.

The researchers have compared data on the Greenland halibut fishery in the West Greenland fjords and clearly see that the catch has become significantly smaller in areas with land-terminating glaciers.

Greenlanders have for many years referred to glaciers melting on land as ‘dead glaciers'. There used to be significant fishery for Greenland halibut in South-West Greenland. This fishery has now almost ceased and one of the likely reasons is the retreat of glaciers.

“We are working with complex systems. We do not know how long it takes for the glaciers to retreat and more research is needed before we can predict with certainty what this means for the Greenland fishery,” says senior researcher John Mortensen from the Greenland Institute of Natural Resources who participated in the work.

“In the years to come, when the glaciers melt, we may see an increased amount of fish, but in the long run fishery is likely to decrease when the glaciers move on land within a relatively short period of time,” says Lorenz Meire.

Fishery currently accounts for up to 88% of Greenland’s exports.

Additional Information

Lorenz Meire: Arctic Research Centre, Aarhus University, mail: lome@natur.gl; Phone: +299 361265

John Mortensen, Greenland Institute of Natural Resources, mail: jomo@natur.gl; Phone: +299 361239

Arctic Research Centre