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Zooplankton

The fascinating oceanic zooplankton: conveyors of energy from the surface to the deep sea


At the surface

The sun is the ultimate energy source of photosynthesis (primary production) in the sea. The sun's energy is however absorbed as it passes downwards, decreasing exponentially with depth. Photosynthesis is thus restricted to the upper layers of the ocean, performed by small microscopic algae. 

These algae may be consumed by different kinds of small animals (e.g. copepods, euphausids) that are too small to swim against water currents. Animals that are drifting passively with the movements of the water-masses of the oceans are called zooplankton.

In the deep sea

The deep-sea water column is usually thought to be an environment with extremely harsh conditions and ill-suited for supporting life. It is cold, the pressure is high (1 atm pressure for each 10m of depth), and because there is no light for photosynthesis so far beneath the surface, animals depend on food particles from the productive upper ocean.

Problem

How do food particles reach the deep-sea water fauna, several 1000 of meters below the productive sea surface? There are to major processes by which organic material can be transferred to deeper layers: (i) by sinking of aggregates and the carcasses of large pelagic animals, and (ii) by the diurnal migration of animals.

The role of zooplankton

Sinking of aggregates: By feeding on algae in the surface layers, zooplankton produce faecal pellets. Faecal pellets of high density, will sink at a high rate, thus increasing the transport of particles from the upper layers to the deep.


Krill
Vertical migration: Zooplankton exhibit a range of behaviours depending on both biological and physical forcing. Diel vertical migration is observed in most species, most typical in the form of a "nocturnal migration" descending to the deep at dawn and rising to the surface again at dusk. During their descent the zooplankton may be eaten by animals in the deep, and thus contributing to the vertical transport of food from the surface to the deep. 

Aim

MAR-ECO researchers aim tol map the vertical distribution, abundance and biomass of zooplankton during diel cycles at different locations above the MAR. The vertical distribution will be related to hydrography and topography. The aim is to characterise the diversity of zooplankton, and to reveal vertical zonations of species assemblages.

Why?

The oceans have a significant role in biospheric carbon fluxes and the fate of CO2. Zooplankton play a key role in oceanic carbon fluxes and are one of the primary mechanisms for vertical transfer of carbon from surface waters to the deeper waters and sediments. Zooplankton therefore can have profound effects on the carbon cycle and sustainable resources in the oceans.

How?

Sampling zooplankton in the deep sea is a troublesome an time consuming task. Most investigations at large depths therefore involve only single observations, which do not allow statistical analysis of spatial or temporal variability. Acoustic and optical recording methods allow continuous, high-resolution studies of biological systems to be carried out rapidly and comparatively cheaply.

Researchers will use a towed and verically profiling sampling gear, equipped with optical and acoustical instruments and sampling nets. This will provide a continuous observation of the vertical distribution of zooplankton. Additional net samples of zooplankton for species identification will be taken at restricted depths.
 Images by Astrid Saugestad

Learn more about zooplankton

What is plankton?

Why learn about plankton?

Read a Nature review about marine microbiology

Nature review on microbial food webs

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