Productivity

New Zealand’s marine environment is influenced by its geological history, its isolation, and the action of major ocean currents. Longer-term weather cycles affect New Zealand’s oceans. Together, the atmosphere and the rotation of the earth drive major ocean circulation patterns and climate systems across the world, affecting deep-water and surface currents, sea temperature, sea levels, seawater chemistry, and the productivity of our oceans. ²⁴³¹

Productivity is a measure of the amount of life that can be supported in a given area. It is driven by the availability of nutrients and light for phytoplankton growth and reproduction. ²⁴³²  There are a number of key processes that are responsible for marine productivity levels. Upwelling occurs where the deep, cold ocean water rises to the surface, both in the open ocean and along coastlines. This water is replacing ocean surface water pushed away from the land by off-shore winds. There is also a reverse of the process called downwelling, where the wind pushes surface water towards the coast where it eventually sinks down. The water coming from the deeper ocean is naturally rich in nutrients, primarily nitrate and phosphate. This helps to fertilise the surface water and promotes phytoplankton growth, ultimately resulting in high biological productivity in these areas. ²⁴³³

Natural inflows of nutrients and sediments into coastal ecosystems are an important part of biological, physical and chemical cycles in New Zealand’s inshore area. These are derived both from deep water upwellings and inputs from the land. Such cycles are the basis for productivity in these areas. The offshore ocean area is primarily influenced by major oceanic currents. ²⁴³⁴

As discussed previously, habitats contribute to marine productivity in different ways and to differing degrees. For example, estuaries are highly productive environments. They have a continuous source of nutrients from their freshwater inputs and are important sites of nutrient recycling in the coastal environment. Nutrients within the estuary are used by plants such as algae, seagrasses, seaweeds and mangroves and they enter other parts of the food web when these plants are grazed on by invertebrates. When the plants, or parts of them die, nutrients are re-cycled within the estuary by the actions of invertebrates and bacteria. ²⁴³⁵

Different species contribute to marine productivity in various ways. For example, seagrass and kelp store nutrients in their biomass for a long time, including nitrogen and phosphorous compounds transported by rivers from agricultural areas to the sea. This means that they function as a kind of biological purification system in coastal ecosystems. ²⁴³⁶

Food webs

Food webs are complex networks through which energy and materials move within and among trophic levels in an environment, from microbes all the way through to top predators. ²⁴³⁷  Marine food webs or chains refer to a group of organisms linked through feeding relationships. All of the plants and animals living in the sea, from algae to whales, form part of the marine food web. In addition, marine life is intricately linked to the physical processes within the ocean, including factors such as temperature and sunlight.

Some species will spend all their lives in one habitat whereas others will use a range of different habitats during their lifecycle. For example, adult snapper generally spawn their eggs in highly productive areas close to estuaries and harbours, such as the Hauraki Gulf. The juvenile snapper move into sheltered coastal areas to mature, and then once they have grown to adult size, they often move further out to sea. Studies have shown that the larvae of longfin eels actually hatch far away from New Zealand, possibly near Tonga, and one tagged female longfin eel took 161 days to swim from Canterbury’s Lake Ellesmere to a point 160 kilometres north-east of New Caledonia. ²⁴³⁸

The wide variety of lifecycles for different species, from spawning and pupping, to migrations over vast distances, means that the taking of an ecosystem approach to understanding food webs is critical. This looks at the relationships between all species using the oceans, rather than considering species as independent entities. ²⁴³⁹  An ecosystem approach recognises that all species are interlinked, so that a change to one species will have an impact on other species in the food web to some degree. The population of a single species through time is dependent on its life history and the interactions with other species and habitats throughout this life history. For example many rocky reef species are dependent on a heterogeneous habitat during recruitment phases. 

Last updated at 2:41PM on February 6, 2018