More confidence in the makeshift solution

Whether water soldier is hampered by crayfish in its role combating methane emissions remains unclear, but master’s student Thomas Celie has gained valuable methodological insights.

‘A large part of the Polderlab, where researchers investigate new methods for sustainable agriculture, consists of ditches. One problem is that a great deal of methane escapes from them, and water quality is poor. Water soldier could help address this – it is a true ecosystem engineer. The only issue was that water soldier was not doing equally well everywhere in the Polderlab. I investigated whether that might be due to the crayfish present, which can damage the plants’, introduces the now-graduated master’s student Thomas Celie, setting out the subject of his thesis. He conducted his research at Polderlab Vrouwe Venne, a project involving, among others, Leiden University.

Recalcitrant
Celie placed exclosures in Polderlab ditches with and without water soldier. Half of them also had small holes, allowing crayfish to enter. ‘Making those exclosures was a very interesting part of the thesis – one I hadn’t anticipated beforehand. I really had to think carefully about the experimental design. In the end, I sawed the bottom out of a mortar tub to get a watertight ring onto which we could place the measurement chamber for methane emissions. Inside that sat a circular ring of wire mesh, with bamboo sticks attached so I could push it into the sediment.’ In practice, things proved stubborn: ‘I’d have walked the whole site with a wheelbarrow, only to find the bank of the ditch was a bit steeper or deeper than expected, and then I’d have to adjust the cages again. There were days when I stood in the mud in my waders all day long and everything went wrong – the mesh would be bent out of shape, for instance. At those moments you do think: this is never going to work. But fortunately I had great supervisors. It was an experiment in itself whether the experiment would succeed: I couldn’t follow someone else’s methodology. Being the architect of your own research like that was also really enjoyable.’

Instruments
In the end, Celie did manage to place enough exclosures and carry out the measurements. ‘For methane emissions I used two instruments: a portable gas chromatograph to measure diffuse methane flux, and inverted plastic bottles to capture bubbles. One is highly technical and the other more makeshift, but funnily enough I ended up having more confidence in the makeshift solution. With that method, you at least know for certain that you’re measuring a bubble. Whereas with the other method, the question is whether you’re measuring only diffusion, or secretly capturing bubbles as well. That data is much harder to interpret, and I think this often goes wrong’, says Celie.

Cages
That conclusion is also the most important finding of his research, because the experiment itself yields no clear results. ‘The water soldier grew very well under all conditions. And in terms of methane emissions, we saw no difference either. Perhaps water soldier retains the methane bubbles, which you then disturb when you start measuring. Of course, I was secretly hoping for a more interesting result, but my cages are still in place and the measurements are continuing. Perhaps a warmer summer will create more of a difference between the groups; methane emissions are higher then. It also remains to be seen whether there really were crayfish inside some of the cages – we won’t know until the cages are removed.’

Is your master’s thesis, bachelor’s thesis or extended essay interesting reading for your fellow biologists? Send an e-mail to redactie@bionieuws.nl