Sea Ice Ecology

Background

When you freeze pure water you get a solid block of ice. Freezing sea water is very different because it is full of dissolved salt. When the sea water gets cold enough to freeze, the sea ice forms a semi-solid block of ice that is full of little channels of very salty water called brine. These channels can be anywhere between one millionth of a meter to several millimetres wide. The brine that collects within the channels can get very salty. Normal seawater (that you have probably swum in at the beach) is usually 35 parts per thousand (ppt) of dissolved salt. The brine found in sea ice channels can be up to 150 ppt – that’s four times as salty as the water at the beach!

The really interesting thing about these channels is that lots of different types of organisms like to live in them. Things like bacteria, algae, copepods and tiny worms have been found in them so far and scientists refer to them as 'sympagic organisms'. These organisms must be pretty tough because the environment they are living in is very harsh and is always changing.

These organisms grow together in groups called communities. These communities change depending on who is living there and at what depth they are at. Particularly ‘busy’ communities (where there are lots of organisms living) are referred to as having a 'high biomass' by scientists. The busiest communities with the highest biomass are usually found in two places in the sea ice. The first is a place called the 'free-board layer' – the level where the weight of the snow pushes the sea ice below it to the same level as the surrounding sea water. The second place is at the ice-water interface – the exact place where the bottom of the sea ice meets the sea water it is floating on. Some of the sympagic organisms, especially the algae living at the bottom of the ice, are food for other animals that live in the Southern Ocean including krill.

Dr Klaus Meiners and others scientists are responsible for investigating these sea ice communities during the SIPEX voyage. Below is an outline of some of the BIG questions they are investigating. As the Voyage progresses we will keep you up to date with any progress or breakthroughs they make.

Question 1. Who is living in the sea ice and what are they doing there?

Question 2. Who is living directly under the sea ice and what are they doing there?

Question 3. How many things are living directly under the sea ice?

Question 4. How does the thickness of sea ice change what is growing inside & underneath it?

Question 5. What types of plankton are currently living in the Southern Ocean at this time of year?

Read on to find out more about how scientists aboard the SIPEX voyage are trying to answer these questions....

Question 1. Who is living in the sea ice and what are they doing there?

Scientists on the SIPEX voyage are trying to answer this on two different levels. At the moment they are not interested in the big animals that you can easily see on top of the ice and swimming underneath it like penguins and seals. Instead, they are interested in the smaller things that are a lot harder to find. At the moment, very little is known about these tiny organisms – and what is know is mostly restricted to observations using microscopes.

On a medium scale, some scientists are searching through the ice cores they take for zooplankton and other invertebrates. At every ice station these scientists are collecting ice cores and bringing them back on board the ship. When their ice cores melt, they sort through the samples under a microscope. At the moment they are trying to identify the species and the amount of biomass present in the ice (that is, the different types of animals there and how much they collectively weigh).

On an even smaller scale, some scientists on board SIPEX are looking for biodiversity in sea ice using molecular techniques. What this means is rather than looking down a microscope to work out what is there, they are taking DNA “fingerprints” of microscopic communities growing in the sea ice. From this, they can work out the level of biodiversity in the sea ice (meaning they can work out the number of different species of micro-organisms living in the ice at the time of the sample).

To do this, scientists take samples of sea ice at each station using an ice corer. They slowly melt the ice and then get to work extracting and sequencing a particular part of the DNA. Once they have worked out the different sequences present in their samples, they will record them in a type of library. It is a bit like taking fingerprints of a group of people and trying to figure out their identity of the people in the group. Instead of fingerprints with unique features, these DNA libraries record codes that are unique to different types of organisms.

At the same time the scientists are extracting part of the DNA, they are also extracting RNA from each sample. The RNA extracted from the samples can be used to work out what the organisms are doing metabolically at the time they were sampled. It is a bit like being a forensic detective and searching for clues to tell you what your suspects have been doing at the time their fingerprints were taken.

Combining this information and relating it to the work other scientists have done allows the scientists involved in SIPEX to figure out what types of organisms are actually living in the sea ice and what they are doing there.

In the final part of the project, the scientists plan to match the micro-organisms they find in the sea ice with where they are grouping together (community structure, and physical & chemical nature of the ice environment) and what they are doing there (their metabolic pathways). Understanding this will bring us a big step closer toward unlocking the secrets of the sea ice.

The scientists involved in answering this question are Dr Klaus Meiners, Dr Andreas Krell, Dr Kerry Swaddling, Dr Christine Crawford, and Annette Scheltz.

Question 2. Who is living directly under the sea ice and what are they doing there?

The underside of Antarctic sea ice is an important environment for sea ice algae to grow and is also known to be a habitat for krill – the little crustaceans that form an important link in the food chain of the Southern Ocean.

There is a close relationship between the extent of winter sea ice and the numbers of Antarctic krill in certain areas. The theory suggested for this relationship is based on the fact that krill (particularly baby krill) eat the algae that grows on the bottom of the sea ice.

Previously, krill have been observed feeding off the bottom of sea ice, particularly in late winter and early spring. What scientists want to know is whether or not the sea ice algae is major food source for most of the krill population of the Southern Ocean.

During SIPEX scientists are using two pieces of equipment to study the under ice environment; 1) a Remotely Operated Vehicle (ROV), and 2) a Surface and Under Ice Trawl (SUIT).

The Remotely Operated Vehicle (ROV)is put down a hole in the sea ice to help investigate the under-ice environment. It has a 350m long tether and is piloted from a control unit on the ship. The ROV has a lot of interesting features that make it perfect for investigating the under ice environment: a video-camera, a depth sensor, an upward-looking sonar, and a hyperspectral radiometer.

The video camera films the under-ice environment and the footage will be used to estimate the number of krill. Information from the depth logger combined with the upward-looking sonar will allow scientists to estimate the depth of the ice floes, which is important for determining how the thickness of ice affects the algal communities underneath it. The hyperspectral radiometer is used to measure the spectral composition of the light that reaches under the ice. In simple terms it means it can measure what different visible light waves are travelling through the sea ice and lighting up the under ice environment. The types of light that are able to travel through the sea ice are influenced by ice-thickness, snow thickness and by the concentration of algae growing on the ice. The data collected from this will be used to estimate the concentration of algae that live in the sea ice, which is in turn a food source for krill.

The ROV allows the scientists to take the measurements quickly in a way that causes very little disturbance to the environment and helps us to get valuable information on the distribution of physical and biologial sea ice properties. The results from the Hyperspectral radiometer will be checked against the results of ice algae concentrations in ice core samples to make sure it works properly.

The Surface and Under Ice Trawl (SUIT) is a special trawl net that is used to collect organisms from the under surface of the sea ice. The original design for the SUIT was invented in the Netherlands and has been successfully used to catch under-ice organisms in the northern hemisphere.

The SIPEX krill team will use an Australian-built SUIT trawl net to collect krill from under the sea ice. After a SUIT trawl is conducted, everyone involved in studying krill help sort through the samples that have been collected. Everything caught in the net is kept and counted and measured.

The types of measurements carried out on the krill collected include how much oxygen the krill are using and what is in their stomachs. They will also be measuring stable isotopes which can also be used to help scientists figure out who is eating whom in the sea ice-associated food web.

Scientists also hope to collect some krill that are still alive so that the can do special experiments on them. The Aurora Australis has been fitted out with special tank laboratories for conducting experiments on live krill during the SIPEX voyage. When the live krill are collected, they are placed in individual containers filled with very cold sea water. They are examined every 2 hours at exactly the same time and the moults they shed are collected and measured. The scientists are particularly interested in how young krill survive in the Southern Ocean during winter when there is very little food available for them. After preserving samples of juvenile krill, the scientists will continue to study their digestive systems and will hopefully unlock the secrets of their amazing survival in such a harsh environment.

Scientists involved: Dr Klaus Meiners, Dr So Kawaguchi, Dr Patti Virtue, Dr Kerrie Swading, and Dr Iris Werner.