Plankton, which means floating, are microscopic organisms that drift in either saltwater or freshwater. These organisms can be animals, plants or bacteria. Despite their minute size, they play an important role in the lives of a wide variety of other life forms.

Plant plankton – phytoplankton – is the ‘grass’ of the sea. Like their cousins on land, in order to be able to grow, they need light (to provide the energy for growth), water, carbon dioxide and nutrients. Phytoplankton range in size from extremely small – less than 0.001 mm – to substantially bigger (on a phytoplankton scale) – about 0.5 mm in length. Phytoplankton are at the bottom of the food chain in the sea, providing the food for animal plankton. As such, they can be regarded as one of the producers of the ocean.

Animal plankton or zooplankton are typically classified based on their size into:

  • microzooplankton (less than 0.2 mm)
  • mesozooplankton (0.2 to 20 mm in length)
  • macrozooplankton (longer than 20 mm).

The size of the zooplankton influences what they eat.

  • Microzooplankton tend to eat the very small phytoplankton and the bacterioplankton.
  • Mesozooplankton eat the larger phytoplankton, microzooplankton and each other.
  • Macrozooplankton eat mesozooplankton and also other macrozooplankton smaller than themselves.

One of the best known zooplankton in the Southern Ocean is krill. These are macrozooplankton and are the food for a wide range of organisms including whales.

Get video: Collecting and measuring krill

Bacterioplankton are free-living bacteria in the water. These bacteria are very important in the food web as they break down organic material and make nutrients available for the phytoplankton.

IPY blogs week 5

The Continuous Plankton Recorder
Since the start of the voyage, we have used a continuous plankton recorder (CPR) to collect plankton samples from a wide range of environments between Wellington and the Ross Sea. The CPR technology has been used worldwide since the 1950s whereby a roll of filtering silk in a special container is wound very slowly forward as the CPR continuously moves through the water trapping plankton. During this voyage, we have seen distinct areas with high concentrations of zooplankton in the water, typically coinciding with high concentrations of phytoplankton. Our work forms part of a much larger international programme that is collecting samples from the oceans all around Antarctica.

Written by Julie Hall

Catching krill – where bigger is better
Krill or euphausiids are an important food source for seals, whales, penguins, fish and even people. Although they are classed as zooplankton, krill can swim – and swim fast! This makes them very hard to catch. When we use the regular fine-mesh plankton nets like the MOCNESS (Multiple Opening and Closing Net and Environmental Sampling), krill are able to get out of the way. Therefore, we fit the MOCNESS with bright strobe lights, which temporarily blinds them (like a possum in the headlights of a car) and allows us to scoop them up. On this survey, we also use a midwater fish trawl to catch the krill. These work so well that not even the speedy krill can get away!

Written by Richard O’Driscoll

Marine bacteria
With 10 to 20 thousand bacterial species in 1 litre of seawater, microbes in the ocean represent the greatest biomass on the planet. Bacterial numbers can be even as high as a billion per litre (that’s 1 with nine zeros). Marine bacteria come in all different colours, shapes and sizes. During this voyage, we get a chance to determine what bacteria are in the Ross Sea, possibly discovering new species along the way. We are sampling both the water and sediments for bacteria. On board, we plate them on agar and filter water so that we can extract DNA. The DNA collected will be analysed in the laboratory and the species determined through cloning and sequencing of the DNA.

Written by Els Maas

Get video: Sampling bacteria

Get video: Sampling zooplankton

The MOCNESS (Multiple Opening and Closing Net and Environmental Sampling), also referred to as the ‘octopus’, has 9 trigger nets that open and close at set depths to collect zooplankton from different depths. Some animals we catch look like aliens, most being big enough to be visible to the naked eye – up to 10 centimetres long – including jellyfish, predatory swimming worms, salps and more. Interestingly, salps and their cousins – sea squirts – are more closely related to humans than any other invertebrate! At shallow depths, we collect lots of phytoplankton, which looks like a green soup. This is an important food source for zooplankton species, which, in turn, are food for larger animals further up the food chain.

Written by Lisa Bryant

Water sampling
Water sampling is being carried out using a CTD (Conductivity, Temperature, Depth) rosette. The CTD has 24 10-litre sampling bottles. The top and bottom of each bottle, connected by bungee cord, are stretched open and latched prior to the CTD being lowered into the ocean. Sensors in the frame record salinity, conductivity, temperature and the amount of phytoplankton in the water. This data helps researchers decide what depths they need water samples collected from. During this voyage, extracting the water from the CTD bottles has not always been easy. On several occasions, the -12ºC air temperature on deck has meant the taps on the bottles and sometimes the water samples have frozen up as soon as the CTD came out of the water.

Written by Stu Pickmere

Get video: Collecting samples with the CTD

    Published 3 December 2007