OPERATION - Zooplankton and micronekton sampling on the Walters Shoal
Zooplankton and micronekton are key components in the transfer of energy from lower trophic levels (i.e. phytoplankton, bacteria) to higher trophic levels (i.e. pelagic fish, whales) and also in the energy transfer to deeper part of the oceans through respiration, release of faecal pellet and natural mortality as zooplankton and micronekton are able to perform diel vertical migration (vertical migration from depths to the surface at dusk and downward migration to deeper layers at dawn).
The aim of this oceanographic mission is to learn about the productivity and biodiversity of the pelagic community over the Walters Shoal, as well as about the trophic interactions.
What are zooplankton and micronekton?
Zooplankton and micronekton encompass diverse animal. The size range of these animals varies from tens of micrometers (unicellular organisms such as ciliates) to several meters in diameter (jellyfish).
By definition, zooplankton can move but are unable to swim against the current, whereas micronekton are characterized by larger size organisms (2 to 20cm), which can swim against the current and feed on plankton and other preys.
Zooplankton and micronekton can be herbivores (consume plants), carnivores (capable of eating only other animals) or detritivores (consume dead organic material). Many animals, however, are omnivores consuming a mixed diet of plants and animal material.
In addition to size and positions in food chains, zooplankton can be categorized by the length of residency in the pelagic environment: holoplankton (or permanent plankton) spend their entire life cycle in the water column, whereas meroplankton are temporary residents of the plankton community like fish eggs and fish larvae (adults fish are nektonic), as well as many benthic invertebrates such as clams or starfish.
Collection methods
All along this mission, microzooplankton to micronekton sampling is done using different collection methods by deploying small mesh size nets and trawl.
Microzooplankton (25 µm to 200 µm), are collected using Niskin bottle located on the CTD rosette from the surface and the fluorescence maximum depth. Twelve L of seawater is sieved through 200 µm to retain mesozooplankton (> 200 µm). The filtrate is again sieved over a 25 µm mesh and then conserved in a solution of buffered seawater: formalin solution (final concentration 4%).
A mini-bongo fitted with two 63 µm mesh size nets is used to collect vertically zooplankton within the water column from 100m depth to the surface. Larger organisms are collected using either a bongo fitted with two 200 µm mesh size nets or with 300 and 500 µm mesh net towed obliquely from 200m or 500m respectively. A neuston net (900 µm mesh size) is towed for horizontally at the surface behind the ship, to collect neuston organisms (i.e. organisms living at the ocean/atmosphere interface).
Based on the echo-sounder observations, water layers with a high density of organisms are sampled with a IKMT (Isaacs-Kidd Midwater Trawl) for 30minutes. The IKMT is a selective trawl to collect micronekton. All sampled organisms are measured, identified, counted and conserved at -20°C for species identification and stable isotopes analysis (δ15N et δ13C). Stable Isotopes analysis allows the determination of the trophic level of the organisms or taxon (δ15N) and the different sources of primary production (δ13C). The samples from the mini-bongo and the 300 µm bongo are also used for the stable isotopes analysis.
This work is completing a larger study on seamounts influence on zooplankton and micronekton abundance and diversity in the South-West Indian Ocean. The comparison will be made between three seamounts: MadRidge, La Perouse and Walters Shoal located at 200m, 60m and 18m below the sea surface, respectively.