The open oceans around New Zealand are home to a great variety of plants and animals, from plankton to whales and sharks. Floating pastures of tiny plants with chlorophyll called phytoplankton sustain this ocean life. Where phytoplankton flourish, bigger creatures come to feed. When the sea is green and visibility is low, usually there are large quantities of phytoplankton. When the sea is deep blue and we can see a long way under water, phytoplankton is sparse. In recent years marine scientists have begun to understand some of the factors controlling the distribution and abundance of plant and animal plankton around New Zealand.
Seasons in the sea
The growth of plankton is governed by the seasons. Phytoplankton convert energy from sunlight into food for tiny animals called zooplankton. These in turn become food for larger animals. Phytoplankton need sunlight and nutrients, but for much of the year one or the other is in short supply.
In winter, nutrients are abundant, but light is limited. Around parts of New Zealand there is a small increase in phytoplankton growth at this time. In spring, more light penetrates the surface waters and phytoplankton grow rapidly, reaching a peak in New Zealand around September and October. However, as spring turns to summer and the sun warms surface waters, the cooler, denser depths are not stirred up so there is no mixing of the layers that could replenish nutrients. The bloom of phytoplankton dies away. In autumn and winter, surface waters begin to cool and, assisted by strong winds, mix with the nutrient-rich layers below. In this way surface waters are recharged with nutrients.
The vertical traveller
Neocalanus tonsus, a flea-sized zooplankton, is particularly plentiful in New Zealand waters in spring and early summer. At this time it is an important food for filter-feeding seabirds such as the broad-billed prion, and large animals such as basking sharks and sei whales. From late January until October Neocalanus tonsus completely disappears from waters down to 500 metres, moving to depths between 500 and over 1,300 metres.
In summer, Neocalanus tonsus feed and grow on smaller plankton. They put on weight and store extra food as a globule of red lipid (fat), which gives them energy to reproduce. In a diapause (resting stage) until September, they moult and become adult males or females, then mate and lay eggs. These hatch and the young probably migrate towards the surface without feeding. They arrive in time to eat from the spring phytoplankton bloom.
This remarkable life history is a strategy for getting by without food and escaping predation in winter.