SiPCA-expedition: on ice, fish and sunlight

Fishing under the ice

During the SiPCA expedition a lot of fishing took place. With the indispensable help of Polarstern’s crew, 20 SUIT (Surface and Under Ice Trawl) hauls were conducted to sample organisms from underneath the sea-ice, plus 15 RMT (Rectangular Midwater Trawl) hauls to sample deeper water layers. The nets contained animals such as krill, amphipods, gastropods (or sea snails) and copepods. All material was preserved for later analyses back home. Unexpectedly, in the northern part of our research area the catches from both nets contained only low numbers of animals. The bird and mammal observers also reported that there seemed to be little life in this region, which was a lot different from the southern part of our research area. Only few Polar cod were caught with the SUIT, but colleagues fishing with a bottom trawl were more successful in catching fish and were happy to share these with less fortunate but interested researchers.

More than just a net

The SUIT not only catches animals, but is also used to investigate properties of the sea ice environment. “The SUIT has an advanced sensor array measuring e.g. water temperature, sea-ice thickness and chlorophyll-a, which can be used to link physical habitat properties with under-ice organisms”, explains Iceflux physicist Benjamin Lange from the Alfred Wegener Institute (AWI). Chlorophyll-a is a pigment used by algae to extract energy from sunlight. The quantity of this pigment is used by researchers as an indicator for algal abundance. Light transmission and sea-ice thickness measurements done by the SUIT sensors will be used to estimate the amount of chlorophyll-a, and thus the amount of ice algae, in the sea-ice. This information can help to study the importance of sea-ice algae as a food source. To be able to estimate the chlorophyll-a from the SUIT sensors, measurements were also done on the sea-ice itself by, firstly, measuring light transmission using a sensor that is attached to an ‘L-arm’ which is put through a hole in the ice, and, secondly, by taking ice cores and measure the amount of chlorophyll-a inside after they have melted.

A mini-SUIT

Measurements on the properties of sea-ice similar to those made by SUIT were done with a Remotely Operated Vehicle (ROV) which can be deployed though a hole in the ice. “If we compare the SUIT and ROV measurements with ice-thickness measurements using a helicopter-borne electromagnetic instrument (known as the EM-bird) we can quantify sea-ice properties on different scales”, says Lange. “This information can then be combined to classify and model regional, and perhaps even the overall Arctic sea-ice habitats”. AWI biologists and physicists also developed a fishing net that can be attached to the ROV. This ‘mini-SUIT’ was successfully used to sample for instance copepods and amphipods from directly underneath the ice.