Clockwise from left: The zooplankton team deploying the MOCNESS (look closely to see the doors that open at different depths); a riot of plankton collected with the MOCNESS; an image of an arrow worm, or chaetognath, taken at Station ALOHA by the Scripps Plankton Camera; the author assembling the SPC aboard the Kilo Moana.
Ocean scientists have all sorts of tools and toys they use to sample the ocean. There is a huge variety: from satellites to remotely operated vehicles to good ol’ fashioned binoculars. We have had the opportunity to play with an assortment of them on our cruise to Station ALOHA.
Broadly speaking, there are two ways of getting data from the ocean: bringing water into the lab or putting an instrument in the drink. Oceanographers spend a lot time thinking about which is most appropriate to get at their scientific question. We have had the chance to do a bit of both to accommodate the diverse interests of our science party.
We’ve already heard about a few ways to look at sediments and particles. The sediments traps, McLane pumps, and bottles are all great examples of pulling samples out of the ocean. The traps accumulate sinking particles over several days, the pumps filter living organisms, and the bottles capture whatever happens to be suspended in their volume. All three methods bring material back on board for further analysis on the vessel or in the lab.
The team that I am working with is looking at slightly larger objects: the zooplankton. These microorganisms are an important part of the marine ecosystem. At Station ALOHA, they are responsible for much of the carbon export from the surface to the deep ocean. Without them, life would not be possible much below the surface.
Observing these organisms is a challenge. They are sparsely distributed relative to their phytoplanktonic cousins and can move great distances over the course of a day. To study their behavior, we had to think about ways to look at them at different depths and times.
The humble net is the workhorse of zooplankton research. Different types of nets have been in continuous use since the early days of biological oceanography. Over the years, scientists have refined net-based sampling strategies, adding bells and whistles to better answer their questions. Since we are interested in observing the Diel Vertical Migration, we decided to use a Multiple Opening/Closing Net and Environmental Sensing System (MOCNESS).
This specialized tool is much more than just a fine-mesh that is dragged through the water: it has multiple nets stacked on top of each other coupled with a suite of environmental sensors. As the MOCNESS is towed behind the Kilo Moana, an operator triggers it to open and close nets based on where it is in the water column. When the system comes back on deck, we end up with 9 discrete depth bins that give us a snap shot of the community structure.
To get a different perspective on the zooplankton assemblage, we have also been deploying the Scripps Plankton Camera (SPC) to look at plankton without pulling them out the water. The SPC is an imaging microscope that goes right in the water; essentially a light and a camera sealed inside a pressure housing. The system can go down to 500 meters to image the tiny denizens of the ocean. Besides taking pretty pictures, the SPC can spot fragile organisms that might get damaged in a net and resolve community differences at very fine depth intervals.
Together, the MOCNESS and the SPC let us ask interesting questions about how these microscopic creatures influence the larger environment and ecosystem. But they are just a few tools of the trade; the ocean is too dynamic a place to be observed with a just a few instruments and it takes lots of effort to come up effective methods. That to me is part of the fun of being an oceanographer. We get to spend our time thinking of new, creative ways to understand what is happening on two thirds of our planet!