Stories from a gelatinous mesocosm – part 2

Blog post from Tuesday 14th November! 

So, we are a week in to our mesocosm tank experiments. Yesterday we added 9 Mnemiopsis leidyi (3 into each tank) to our tanks and left 3 without. They have a tendency to sink right to the bottom without any circulation so we had to set up an air lift using compressed air tap in the room. It took some fidgeting with various valves and silicone tubing to get it right, but we finally sorted it so there is equal bubbling in all 6 of the tanks (replication is the law of science).

Sorting out the air pumps into the mesocosm tanks, we need to ensure we have enough air for circulation but not too much to damage the organisms. We also want the same flow rate into each tank.

The other difficulty with this experiment is that we want to check we can see the M. leidyi each day, so we know they are happy and alive. However, they are about 3cm in size, and mostly transparent… so trying to find them in a 30L bucket is a little tricky! Today we managed to find most of them (with some good spotting from my colleague Janice).

Looking down into the tank, can you spot the Mnemiopsis? 

What has been happening in the tanks over the last week? After we added the nutrients (nitrogen, phosphorus and silica) we began to see an increase in algae. Particularly a type of algae known as diatoms. Diatoms have a very rapid growth rate and use the silica to make a case around themselves known as a frustrule. The smallest glass factories in the ocean! They come in a variety of beautiful shapes as you can see below.

Different shapes and forms of the diatom algae (image from

Along with this, we have also seen incredibly high numbers of small algae (known as “picoeukaryotes”), with up to 90,000 cells per mL of water! However, in the last few days their numbers have dramatically declined, whether this is due to them being eaten by grazers or killed by a viral infection remains to be seen. We have counted both grazers and viruses so can try to correlate these data sets afterward!

We have been collecting (and preserving) a lot of samples from this experiment! For algal biomass, we can look at chlorophyll abundance. Chlorophyll is the pigment algae use to harvest light and turn it into food (photosynthesis) and is green in colour, because this is the only colour in the light spectrum it doesn’t absorb and so reflects back into our eyes (why we see plants as green)!

After filtering 150mL of water we can see the colour from the algae on a filter

We have also been measuring oxygen production and uptake over 24 hours. Photosynthesis produces oxygen, whilst the process of respiration (by bacteria or grazers) consumes it. By incubating samples in the light (the total oxygen production and consumption over 24 hours) and other samples in the dark (just consumption) we can work out the balance between photosynthesis and respiration! As the diatom bloom has been developing we find our system is producing more oxygen than is being consumed, but this could change now that the M. leidyi is in the tank, or as bacteria begin to break down the dying algae.

Oxygen bottles kept in a water bath, the probe is to measure oxygen and temperature from each bottle. Half of these were kept in the dark and half in the light. 

We will keep you updated with more information next week!


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