Tadpole Shrimp, Arctic Charr, and Glacial Retreat in Svalbard

Popular images of the Arctic often feature a polar bear with its white fur matching the surrounding sea ice or a narwhal with its tusk piercing the ocean waves. You are less likely to consider the Arctic tadpole shrimp, a tiny crustacean that is vitally important to many food webs in harsh Arctic environments. A recent study in the journal Boreal Environment Research examined the tadpole shrimp and its contribution to the diet of the small salmon-related Arctic charr in a glacial-fed river and lake in Svalbard, Norway.

Arctic tadpole shrimp are found in lakes across the Arctic, from Siberia to Iceland. The size of the shrimp population in a lake reflects the density of the charr population. In deeper lakes, where Arctic charr are prevalent, the shrimp are rare or not found at all, but in shallow lakes with few or no charr, the shrimp are widespread. In lakes where the two species coexist, the shrimp are a key source of food for the charr.

Photo of the Arctic tadpole shirmp
The Arctic tadpole shrimp (Source: Reidar Borgstrøm).

Though the connection between charr and tadpole shrimp populations has been established, no one had ever studied the charr’s diet in Arctic streams, many of which flow into lakes inhabited by both the tadpole shrimp and charr. This study set out to fill this gap by examining the summertime diet of riverine charr on Spitsbergen, the largest of the islands of the Svalbard archipelago.

The study focused on the streams that feed the shallow lake Straumsjøen on Spitsbergen and its outlet river. The streams that empty into the lake from the south and west discharge clear water, while water flowing from the northern stream fed by the glacier Geabreen is cold and cloudy because of glacial meltwater and silt.

Map of Straumsjøen
Svalbard with the location of Straumsjøen and its outlet river (Source: Borgstrøm et al.).

To analyze the diets of the charr, the authors captured fish from the the lake’s outlet stream by utilizing electrofishing, a fish surveying method that stuns a fish when it swims near an electrode-generated electric field. The researchers then killed the captured fish and analyzed the contents of their stomachs.

The results were surprising. Charr caught in the outlet river had tadpole shrimp in their stomaches. This discovery was unexpected because young tadpole shrimp are planktonic, meaning they drift in the water instead of swimming, which is why they were previously thought to be unable to inhabit running waters. In fact, this was the first time the tadpole shrimp had ever been recorded in running waters and as a part of a charr’s diet on Spitsbergen.

One possible explanation for the tadpole shrimp’s presence in the outlet river is that the shrimp simply drifted from lake Straumsjøen and ponds connected to the river, according to the authors. However, this possibility was considered unlikely given the significant number of tadpole shrimp found in the diet of riverine charr.

Photo of the outlet river.
A section of the outlet river from Straumsjøen (Source: Borgstrøm et al.).

The more likely explanation takes three factors into account, one of which is the glacier. First, the eggs and larva of the tadpole shrimp are adhesive and able to attach to rocks and other objects within the rivers. This trait would allow the shrimp to avoid being washed away down the river. Secondly, the presence of the tadpole shrimp in the rivers could signal low fish density. A lower fish density would allow the tadpole shrimp population to remain steady and still contribute to the charr diets.

The third factor is the retreat of the glacier Geabreen which feeds lake Straumsjøen and its outlet river. The glacier’s retreat has caused a subsequent decrease in the discharge of cold, silty meltwater into the lake. Thus, the presence of the tadpole shrimp in the Straumsjøen watercourse may be a result of the upstream retreat of the Geabreen, as resultant river conditions are now more conducive to tadpole shrimp, lead author Reidar Borgstrøm told GlacierHub.

The changing climate driving the retreat of the Geabreen glacier is also likely to impact river conditions and in turn tadpole shrimp populations. Under future climate change scenarios, the Arctic is projected to get warmer and wetter. Rising temperatures in Svalbard during the summer months, however, are unlikely to negatively impact the tadpole shrimp as populations of this widely distributed species in southern Norway, where summers are already fairly warm, have remained stable, Borgstrøm said.

Photo of Spitsbergen
A glacier on Spitsbergen, the island where the study took place (Source: Fins and Fluke/Twitter)

Increased rainfall in conjunction with increased glacial meltwater, on the other hand, could have a negative effect on the tadpole shrimp, as the heightened streamflow could potentially flush the tadpole shrimp from the river. These changing conditions may cause riverine tadpole shrimp populations to fall, which would in turn have a cascading effect on the Arctic charr who rely on the shrimp as a major source of food in the Straumsjøen watercourse.

Future studies in both Svalbard and other places across the Arctic would help scientists better understand how glacial retreat and climate change will impact the tadpole shrimp and other species.

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