by Steph Allen
The Extended Ellett Line (EEL) has traditionally been sampled by oceanographers to examine changing hydrological conditions year after year. However, whenever there are free berths available, biologist and ecologists alike jump onboard to get a glimpse of the region’s biota, and this year this included me.
My purpose on the cruise is to examine the net growth rate of marine phytoplankton and to examine their community composition, essentially finding out how much they are growing over a 48-hour period and what types of phytoplankton are living here.
What is marine phytoplankton?
Marine phytoplankton are tiny plant-like organisms that drift around in the ocean currents and are responsible for producing half the world’s oxygen (they are so small you can get as many as 1 million in a teaspoon of seawater!).
But this isn’t all that they get up to! They help regulate atmospheric carbon dioxide levels, transporting carbon to the seafloor (allowing the ocean to act as a carbon sink) and because they are the primary producers of the ocean they are the base of the marine food chain, responsible for feeding a whole array of other organisms.
Net Growth Rate Experiments:
To examine the growth rate of phytoplankton we measure the abundance of a pigment called chlorophyll-a, which phytoplankton contain in order to photosynthesize. The water samples collected from the CTD are dispensed into clear bottles and are placed an incubation box that I have set up on the back deck of the discovery. These incubation boxes will house the water samples and allow the community to grow over a fixed period of time. The blue lid on the incubator that you can see below, helps mimic the light conditions that would be experienced by phytoplankton when they are under the ocean’s surface.
Seawater from our current location is pumped into the incubators, around the samples and back into the ocean, this keeps the samples at the same temperature as the environment they were collected from, which here is around 8˚C.
The bottled samples from the incubator are measured at 0, 24 and 48 hours and we use the change in the amount of chlorophyll between time periods to calculate how much the community has grown, in this case, over a 48-hour period. This technique is repeated at different points along the EEL.
As well as measuring how much phytoplankton there is in the sample between 0 and 48-hours, we also want to know what types of phytoplankton are present. Does this change over a 48-hour period? Do certain types begin to outcompete others?
In order to find this out we filter the water, again concentrating the sample, at 0, 24 and 48 hours onto specific membrane filters using a filtration rig. A filtration rig essentially, a pumps the seawater sample through the membrane filter, but phytoplankton are too large to pass through so they get caught on the filter paper. Once filtered the samples are stored on petri slides so they can be transported back to the laboratories in the National Oceanography Centre and examined using highly specialized microscopes, so that we can count what types of phytoplankton are present.
Above I mentioned that phytoplankton contain a pigment called chlorophyll-a which they use to photosynthesize. One defining aspect of this pigment is that it appears green in colour, and it’s one of the reasons why the sea has that blue/green appearance.
Yesterday afternoon I finished filtering the samples that had been incubated from the first CTD station near Iceland’s coast over a 48-hour period. Although I cannot put quantities to the sample for a little while longer, I wanted to share with you what I’m seeing.
The second photo shows three filtered samples from the sampling location off the coast of Iceland. The left hand filter was filtered straight away at 0-hours, the middle filter was filtered after an incubation period of 24-hours and the right hand filter was filtered after an incubation period of 48-hours. As you can see after only 48-hours the filter is considerably greener, indicating a growth in the phytoplankton population!
Photos by Steph Allen