Week 8: Food web of the Ross Sea
The Ross Sea in Antarctica is home to a huge number of different organisms. Tiny plants called phytoplankton are found floating in the water or frozen in sea ice. They get eaten by herbivores such as krill and zooplankton. Carnivores in the Ross Sea include whales, penguins and seals, as well as less well known carnivores like the giant squid or the 2 metre long toothfish.
Adelie penguin colony in the Ross Sea
Adélie penguins are the smallest of the Antarctic penguins with a very health population in the millions. They breed on land and forage for food in the sea – where they are able to dive to depths of around 500 metres, although most feeding expeditions see them diving only a couple hundred metres.
The scientists will try to understand how the food web in the Ross Sea works. The arrows in the diagram go from prey species (these get eaten) to predators (the hunters).
The food web in the Ross Sea
This diagram shows how the food web in the Ross Sea works. The arrows go from prey species (these get eaten) to predators (the hunters).
This work will help to show how fishing affects other animals in the Ross Sea and try to answer two important questions:
How many of each kind of animal are in the Ross Sea? For example, if there are lots of fish, we know that they will be eating lots of a particular type of prey – bad news if you are their favourite snack!
What is the favourite food of the different animals in the Ross Sea? For example, do silverfish prefer to eat krill or zooplankton? Do fish living at the bottom of the sea eat mainly squid, fish or worms living in the mud?
IPY blogs week 8
The bottom of the food web Algae or phytoplankton form the base of the ocean’s food web. The sea’s ‘grass’ undertakes photosynthesis, turning sunlight and inorganic nutrients into food that can be eaten by organisms like zooplankton. Phytoplankton are microscopic but occur in large numbers in the water and are also attached to sea ice. Zooplankton that eat the phytoplankton vary in size from microscopic to those we can see with the naked eye such as krill. Larger zooplankton eat smaller zooplankton, which eat even smaller zooplankton, with sometimes up to 4 links in the food web between the phytoplankton and krill in Antarctic waters. Larger zooplankton are then eaten by fish like the Antarctic silverfish.
Written by Julie Hall
See the video The bottom of the food chain for more information.
Phytoplankton on ice
Iceberg with a band of ice algae (yellow/brown band) attached close to the water surface.
Phytoplankton concentrations in the Ross Sea
A satellite ocean colour image of the Ross Sea showing a range of chlorophyll and hence phytoplankton concentrations.
Benthic food webs Animals that live on the seabed depend on food produced in the surface waters where there is enough light to support photosynthesis by phytoplankton. In other seas, much of this plankton is eaten by zooplankton or broken down by bacteria in mid water. The Ross Sea is remarkable in that most of the production by phytoplankton sinks directly onto the seabed, supporting rich communities of larger benthic animals, such as sponges and sea cucumbers that filter plankton from the water or from the seabed. This feeding strategy means finding food throughout the year rather than going hungry in the long winter. Despite the high biomass of animals, we can see on the seabed (megafauna) a large part of all the primary production in the Ross Sea is broken down by microbial action in seabed sediments.
Written by Dave Bowden
Benthic communities
The summer phytoplankton bloom supports a rich community of benthic suspension-feeders.
Sea urchin eating a crinoid
Sea urchins graze on phytoplankton deposits in summer but in winter will eat anything they can find; this one is eating a stalked crinoid.
Food webs and fish The link between phytoplankton and top predators like whales, seals, penguins and toothfish is a community of fish and invertebrates referred to as mid-trophic level animals. More unkindly, these are sometimes called ‘forage fish’ because it seems that their main purpose in life is to be eaten by something bigger! The main forage fish in the Ross Sea is the Antarctic silverfish. This small silvery fish (5–15 centimetres long), looks like a sprat or pilchard and is thought to be very abundant in the Ross Sea. However, one of the main objectives of this voyage has been to measure the specific abundance and distribution of silverfish, and for this, we are using specialised echo-sounders. This will help us to understand how many big predators the ecosystem can support.
Written by Richard O’Driscoll
Antarctic silverfish
A catch of Antarctic silverfish (Pleuragramma antarcticum).
Silverfish on the echosounder
An acoustic recording showing how schools of silverfish appear on a scientific echosounder. The green line shows the path of a trawl that went through this school and caught 200 kg of silverfish.
Squid and demersal fish in the Antarctic Ecosystem Demersal (bottom-living) fish play a complex role in the Antarctic ecosystem. To understand their role, we examine the ‘feeding guild’ – all of the species that feed in the same way and compete directly with each other for exactly the same resource. Most demersal fish species are either fish feeders, krill and zooplankton feeders, benthic invertebrate browsers or a combination of these. By understanding diet, abundance, feeding rate and energy content of prey items, we can get a better understanding of the fish’s role in the ecosystem. Squid provide an important link between krill, mid-water fish that eat small plankton, and larger predators such as penguins, seals, albatross, toothfish and toothed whales, but it is difficult to study what a squid eats as they mash up their prey before they eat it.
Written by Chris Jones and Darren Stevens
Find out more about this in the video To eat and be eaten.
The icefish species, Cryodraco myersi
The icefish species Cryodraco myersi, a member of the krill and zooplankton feeding guild, observed using the DTIS.
The glacial squid
The glacial squid, Galiteuthis glacialis, an abundant mid-water Antarctic species, and an important prey item for larger predators.
Top predators Marine mammals like birds and large predatory fish are called top predators. Within the Ross Sea, they fall into two groups. The first are the baleen whales (minke, blue, humpback), penguins and crabeater seals, which mainly feed on krill, and the second are the toothed whales (killer, sperm), leopard seals, Weddell seals and Antarctic toothfish, which feed on fish and squid. Sometimes these turn prey on each other (i.e. leopard seals eat penguins). Worldwide populations of top predators (whales, seals and large fish) have been decimated through overfishing. However, the Ross Sea has been less affected than other ecosystems by human exploitation, and apart from the loss of whales, much of the Ross Sea ecosystem is still intact. Increasing human activities including fishing for the Antarctic toothfish in the Ross Sea, harvesting of whales and effects of climate change make it crucial to understand the potential effect on the Ross Sea ecosystem.
Written by Stu Hanchet
In the video The top predators, NIWA scientist Stu Hanchet explains what types of top predators can be found in the Ross Sea.
A leopard seal
A leopard seal lying in the pack ice digesting its latest meal.
Large Antarctic toothfish
Scientists with a large Antarctic toothfish – weighing 57 kilorgrams and measuring over 1.5 metres long – caught during the survey.
Blue whales
A pair of large blue whales seen in the north of the Ross Sea.
