Arrival at the mid-Atlantic ridge

Yesterday we transited to our ‘ridge’ site – almost time to test our theory of increased productivity on the mid-Atlantic ridge! We used the afternoon to host another mini-conference of science talks – and today was my turn to present. It was a really supportive and curious atmosphere and, though it was only a 10/15 minute talk, sparked much discussion afterwards. While we’re here, I’ll take the opportunity to tell you a little about my PhD.

My focus is marine phytoplankton and their role in the carbon cycle. Phytoplankton are key players in the biological pump, that is, the flow of carbon from the air, to our oceans, to the deep ocean. This is a key process as it locks carbon out of our atmosphere, reducing the effect of CO2 in warming our planet. However, as our planet is warming and the ocean is absorbing that heat, its surface is increasing in temperature. This causes an increase in the temperature gradient between the surface, low nutrient, high plankton layers, and the lower, high nutrient, low plankton layer. The plankton in the surface rely on mixing between these layers to get the nutrients they need to grow. As the stratification of the ocean increases, there is less mixing, and less nutrients are delivered to the surface. We think this will alter the community structure of phytoplankton in the surface ocean, as a result altering the capacity of the ocean to absorb carbon dioxide.

One theory is that this will shift phytoplankton communities towards smaller phytoplankton, which is where my PhD comes in. With my fieldwork and lab-based study, I am seeking to understand a) how is community structure of phytoplankton in the surface ocean likely to change in response to changing nutrient supply?, b) how and why are picoeukaryotes (particularly a few groups) so well adapted to these conditions (physiological and biochemical mechanisms)? and c) what does photosynthesis and carbon cycling really look like in these organisms - and can we optimise this? As part of this, I’m also looking at specifically-adapted communities, such as those in the deep chlorophyll maxima, and the effect of different nutrient and light regimes, such as those on and off the mid-Atlantic ridge.

Left: my peristaltic pump filtering rig. Middle: fitting 0.2 micron Sterivex filters to the tubing ends. Right: running chlorophyll samples on the spectrophotometer.

Having transited overnight, we arrived around 0400 and immediately set about getting the CTD in the water. Unfortunately we didn’t manage to get it out before dawn but it was a good day to have a bit of a play with methods - I was able to try out my new filtering rig, with the consensus being that you cannot filter 5L of seawater through a 0.2um filter under vacuum (at least, not in less than 5 hours!). So, lesson learned (the long way).

Filtering in hand, it was time to measure some chlorophylls - we were measuring total and size-fractionated chlorophyll, to enable us to understand the structure of the phytoplankton community in the water. We measure this with a spectrophotometer, once the filters have been in acetone for around a day after sampling. This measures the absorbance of light through the sample, from which we can understand how much of the chlorophyll pigment was present in the original filter.

Today we also dropped our second mooring – this time on the ridge. This works much the same as the first (see 25th Feb entry) except now we’re trying to drop the anchor on a specific location on this underwater mountain range. To quote our chief scientist, Jonathan, ‘it’s a bit like drifting over the Lake District in an airship at a height of 3000 metres trying to drop a rock onto a barn roof. At night.’ These two sets of moorings, together, will enable us to observe differences between the two sites – on and off the ridge.

Sunset over the aft deck during mooring deployments.

In the evening (or, our version of evening, which is essentially 4-5pm) we were lucky enough to witness a part of the planetary parade - seeing Venus, Jupiter and Mars while the wirewalkers were deployed over the side.