Measuring the unexplored: How do we sample the ocean?

The ocean covers around 71 percent of the Earth’s surface and holds up to 97 percent of the world’s water. Much of the technology revolving around the research and discover, such as mapping, have only been developed within the last few decades. Even so, it still leaves us with a staggering 80 percent of the ocean unmapped, unexplored, and unseen by humans due to its sheer size and immense depth of the deep ocean. The far distances from human populations make it very resource intensive to research far out locations. Not only do we want to discover the unseen, as is the nature of humans, but we also want to understand the ocean as a body and how it interacts with the lithosphere, atmosphere, and biosphere (among others). The ocean absorbs a large amount of radiation from the sun, especially around the equator, and the distribution of this heat towards the poles like a conveyer belt around our planet is what drives our climate systems.  Over the last century, many different methods of ocean data collection have been used. 

An example of this is the Argo programme, which uses floats to record ocean temperature, currents, and salinity. The so-called “Argo-fleet” consists of up to 4000 floats producing 100,000 profiles of temperatures and salinity every year. Each float weighs approximately twenty to thirty kilos, diving up to depths of 2 kilometers every few days, and resurfacing again by adjusting their buoyancy levels. While temperature or salinity might be slightly easier to measure, much of ocean research is centered around the sampling of various organisms in the ocean. While in terrestrial ecology it can be easy to place a quadrat frame to count a plant certain species in, the organisms in the ocean move through a three-dimensional space, driven by various seasons and currents. Organisms such as plankton will mix into deeper depths during the winter and stratify during warmer summer periods. This makes it difficult to determine exactly the distributions of these organisms. Even if the capturing of the samples is the easy part, unlike the atmosphere, it is difficult to have a reliable power source because of the issues that batteries and electronics pose in the water, especially at deeper depths. Depending on the machinery and the desired resolution of the sampling data, regular visits and maintenance requires the commitment of travel and of a (research) vessel, something that might need to be shared amongst other scientists conducting their own research and might thus not always be available. So, what are some methods that can be used?

Sediment traps are a useful tool for collecting samples falling from the photic zone down to the bottom of the ocean. These traps are deployed for a year or longer and are especially useful for more isolated areas further off the coast and dangerous locations, such as Somalia. The falling debris is caught in a large cone, specially designed to trap the debris at correct amounts. Under the bucket, there is a collecting carousel which will turn at different intervals, from a few days to a month. The shape of the funnel was widely researched and discussed throughout the 70’s and 80’s, as the ocean being a liquid medium made it a challenge to capture floating substances without the material swirling in and leaving again, or over trapping too much of the material. The bottom of the carousel is lined with mercury chloride as well as denser water, trapping the debris and killing the bacteria that could degrade the sample. Several sediment traps may be lined up at different depths as well, to determine the settling speed of the measured organic matter. These sample materials can then be analysed to see how the ocean environment and composition is changing. The carousel nature of the sediment trap has potential for high temporal resolution, which can provide for even more insights into organism fluxes and temporal habits.

(Source: Monterey Bay Aquarium Research Institute (MBARI))

There are various types of traps that exist. Once the moored trap is full, an electric signal allows for the trap to release itself from the metal planted on the sea floor and drift to the top. Another issue when it comes to moored or freely drifting traps can be retrieval. Through the waves, some researchers find themselves having lost the trap completely.

(Source: Woods Hole Oceanographic Institution)

Like each method, sediment traps need to sit vertically in the water column to correctly catch falling sediment. Strong currents can tilt traps and influence results, and so much research has also been done separately in that area. While nets block the top of the traps, many fish and organisms look to find their next meal sitting inside the traps, which can cause disruptions or bias in data, especially in traps deeper in the ocean where food is scarce.

Other methods such as plankton tows and pumps exist, which are self-explanatory. Plankton tows allow us to have a time snapshot at a certain depth but are a one-time job and would require consistent revisiting of the location, as the plankton could have sunk the next day right after the sampling. Plankton pumps are also temporally one-dimensional, but require less resources and expertise to execute. Sometimes, they can be conducted by other non-research vessels who have some extra time on their hands.

By researching ocean productivity and dynamics, we can better understand the world’s largest carbon sink and biggest climate influence. While there are many different methods to do so, each has its own different strengths, and we need to select the method which is best catered to the research we are aiming to conduct. As technology advances, we can hope for robust, more longer-lasting batteries and better signalling which can help us navigate and research the unexplored more consistently, effectively, and safely.