2018 Cruise - JC165

Zooplankton sampling in the flAtlantic

At noon today we deployed our zooplankton nets into the flattest water I have ever experienced at the PAP-SO. We recovered an amazingly large number of zooplankton (microscopic animals).

zoopOur moorings at the PAP-SO provide year round data on primary production (growth of microscopic algae), nutrient cycling and carbon export (see sediment trap post), but do not yet give a full picture of secondary production – the feeding and growth of zooplankton. We use zooplankton nets to evaluate secondary production and fill in some of the missing pieces of our open ocean jigsaw puzzle. On land such animals are called herbivores (plant eaters) but in the ocean the food web is incredibly complex and our nets capture zooplankton that feed on microscopic  algae and on each other.

We lower our net to 200 metres and bring it back to the surface collecting an amazing zoo of animals along the way. Some of the samples are sub-divided into different sizes. Back at NOC we will run sub-samples through a FlowCam system to count and measure the animals and build up a picture library.

Below are two samples collected using exactly the same methods and at similar times, but on different days. It is really important for us to make sure that every time we take a sample, we follow exactly the same methods- we even make sure that the net is brought up at the same speed each time. This is because when we find differences we need to know that it is real variation that we are seeing and not experimental error. There is great example below.

Thank you to Chris Cardwell for the pictures!

The two samples are different in space and time and illustrate patchiness, the huge variation of zooplankton abundance on quite small scales, that makes it really difficult to quantify how much zooplankton are where and what sorts are present. We know that we treated these two sampling events in exactly the same way so what we are seeing is real. Perhaps the windy days at the beginning of the week mixed nutrients and phytoplankton together. Then with the calm we now see, that allowed the phytoplankton to stay in the warm water near the surface and grow super-fast.

We care about such changes because the types of zooplankton have an important impact on the sinking marine snow (see the Sediment Trap blog for marine snow). Zooplankton influence the carbon cycle. So although this is really difficult to investigate, we must try to measure it so that we can more fully understand the carbon cycle and ocean carbon sequestration (storage at depth).

This is something that is also being investigated by other scientists from the NOC who are now on board the RRS Discovery, sampling off Namibia as part of the COMICS program. Here is a link to that project: http://comics.ac.uk. We look forward to building up such perspectives for the Global Ocean.


Written by Corinne Pebody