D341: Porcupine Abyssal Plain cruise 2009


Cruise blog

Sunday 9 August 2009

How big is a plankton's appetite?

‘Aren’t they cute?’ – The answers I get to this question vary strongly when I let the people on this ship have a glance through my microscope – from ‘Yuck, they look horrible’ to ‘Aww look, it’s fighting the other one’. I never got a ‘They look really tasty!’ as an answer. But that may be as I never asked a fish, whale or other carnivorous zooplankton. What I am looking at are copepods: small shrimp-like animals that occupy a key role in the marine food web.

To get my sample, I get up at horribly early hours in the morning, because these little shrimp-like creatures know quite well how tasty they are. To avoid predation, they migrate into the deeper waters during daytime, when the sunlight is shining through the upper water layers and fuels the phytoplanktonic photosynthesis. At night time however, in the safety of darkness, copepods and other zooplankton migrate upwards into the food-rich surface layer to feed. They haven’t expected me!

So there I stand with a 200 micron-mesh plankton net at 3 am. After I got my sample, I choose the animals I am interested in and sort them according to size and species. This sounds rather simple. In reality though, I pick each animal individually using a microscope and tweezers. The copepods I am looking at are sometimes as small as 0.4 mm and are just about visible with the naked eye. On a ship that is constantly moving, and in a crowded lab with a table that is vibrating from the filtration rig of my neighbour scientist, this can be a rather tough call. But I am young and patient, and so I count myself lucky when I finally got all the animals needed at about 7 am: It is breakfast time!

It is breakfast time for me, and for the copepods, too! I am about to set up my feeding experiment. I am interested in how much and what these little animals eat. As I cannot watch them actively eat like my fellow scientists at the breakfast table, I need to use an indirect approach. Imagine two identical meadows, and assume that the grass is growing at the same rate on both meadows. Now a farmer brings 10 cows onto one of the meadows and leaves them for 24h. If you mow both meadows after those 24 h and weigh the grass, you can see from the difference how much the 10 cows ate in 24 h. This is exactly what I am doing. I put 10 copepods in 2.2 L glass bottles containing phytoplankton-rich water that has been collected for me from about 25m depth using a CTD rosette. Simultaneously, I set up three control bottles without animals. The bottles are screwed onto a plankton wheel, which is slowly rotating the bottles to keep animals and food in suspension. After 24 h, I take water samples from experiment and control bottles, and fix the sample with the preservative Lugol’s iodine. When I am home again, I will count the phytoplankton in each sample and calculate how much these small copepods munched. This information will help us to understand the role of mesozooplankton in the carbon cycle. Even though a single copepod is tiny, the high abundance of these little grazers results in an enormous turn-over of carbon. Because of their high grazing rates and daily vertical migration, copepods may transport high amounts of particles through the water column. Together with the zooplankton sampler ARIES, my feeding experiments will give us a better idea of how much these wee marine beasties contribute to the biological carbon pump.

So, what happens to the animals after the experiment and all the other animals that I do not use? Yes, you are right: they get flushed down the sink. And yes, I do feel horrible about it even though they are minute. But dear fellow scientists: Imagine how much zooplankton a single 20 tonnes baleen whale eats every day!!!

The plankton wheel

Sari and her plankton net

Zooplankton samples

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