Story: Open ocean
Life in the open ocean seems very remote to us because it is largely unseen. This mysterious world is home to a host of highly specialised creatures. The richness of ocean life is closely connected to the supply of nutrients. The weather brings nutrients to sunlit surface waters, where they fuel the growth of microscopic plants.
Full story by Janet Grieve
Main image: The tiny zooplankton Neocalanus tonsus
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Plankton are living organisms that drift in the ocean. They are the base of the food chain: plant plankton convert sunlight into energy and provide food for animal plankton and other creatures. Where plankton flourish, larger animals come to feed.
Winter is an important season for plant plankton growth. Helped by cold weather and the wind, surface waters mix with cooler water lower down, stirring up nutrients. In spring, plankton blooms because of the extra sunlight.
The tiniest plant plankton are eaten by larger animal plankton. Around New Zealand these include moon jellyfish, salps and arrow worms. These animals are highly specialised for making the most of the ocean’s food resources.
Moon jellyfish look like transparent bells. Tiny beating hairs direct food into their mouths.
Salps have hollow, tubular bodies and sometimes travel in chains, which can be as long as a whale. They have been called the vacuum cleaners of the ocean because they suck plankton from the water.
Arrow worms dart at their food. They can eat up to a third of their body weight each year.
Plankton provide food for ocean fish such as hoki, the most common commercially-fished species in New Zealand waters. A scientific study of hoki found that some release their eggs in the sea west of the South Island. Hoki larvae rely on a large supply of animal plankton and certain water conditions to survive.