Concerns about declining life in our oceans have scientists looking to microscopic creatures at the bottom of the food chain for ways to help populations recover. Now, as this ScienCentral News video reports, studying the food of the sea has gotten a whole lot easier.
Sea Food
Typically smaller than a grain of sand, and yet hugely important to the health of the world, they are microscopic ocean dwellers that not only help control the world's climate but also form the base of the food chain in the ocean. Called plankton, these tiny creatures freely drift in the water and come in two basic types: phytoplankton and zooplankton.
Phytoplankton are at the very bottom of the food chain. They photosynthesize, producing sugar from light and carbon dioxide, one of the greenhouse gases. By removing this gas from the atmosphere, phytoplankton help to prevent global warming.
Zooplankton eat phytoplankton, then other organisms eat the zooplankton, and so on, up the food chain. "Most organisms in the ocean at some time during their life cycle feed directly on zooplankton or phytoplankton," explains Scott Gallager, an associate scientist and plankton ecologist at Woods Hole Oceanographic Institution. "Larval fish, the fish that we consume from the market, all commercially important fish ultimately— particularly during their larval stages— feed directly on zooplankton. So the fish that we depend on are directly dependent upon zooplankton in the ocean. That's one of the reasons why it's so important for scientists to understand how phytoplankton and zooplankton interact and form the basis of the food chain in the ocean."
In the past, scientists studied plankton virtually one creature at a time. They'd drag a fine net with a bottle in the end through the water, gathering large numbers of plankton from a large volume of water. They'd take a tiny sample from the bottle and use a microscope to identify and count each creature in the sample. Tedious and slow, this method damaged some types of plankton so badly that scientists couldn't identify them.
Gallager and his team have changed all of that. They have created a series of machines that identify and count plankton in real time— without even touching the creatures. Collectively called video plankton recorders, or VPRs, these new, "smart" machines take sixty pictures each second. A computer analyzes each picture and identifies and counts the plankton in the photos. "We've taken the microscope off the bench top and put it into the ocean," says Gallager. "It allows us to rapidly make surveys of the plankton and understand how the plankton are interacting with their physical environment, so we literally get a real-time plot of the distribution of the plankton."
The newest, smallest, towable VPR image: Scott Gallager, Woods Hole Oceanographic Institution
Five different versions of the VPR have been developed, each for a specific use. One, called the profiler, rides up and down a mooring line, measuring the types and quantities of plankton that are carried past it in the current. It allows scientists to monitor plankton continuously in one area, typically where currents, freshwater run-off from land, or storms affect plankton populations. A second travels up and down in the water as it is towed behind a boat, and can be towed at a very fast speed of 10 to 12 knots. This towed VPR gives scientists the ability to do high speed surveys of fairly large areas. It also allows them to do "adaptive sampling," where scientists monitor the data and if needed, go back and get more. "Once we see what the distribution looks like in real time," Gallager explains, "we can actually turn the ship around and go back and re-sample an area if we feel as though we haven't sampled it well enough," a feat that was virtually impossible for scientists using a net. The third and fourth versions of the VPR are attached to either a remotely-operated vehicle (ROV), which is "driven" by a person on the surface, or an autonomous underwater vehicle (AUV), which follows a path laid out by its programming and doesn't require a person to steer it. These vehicles can monitor plankton in areas where sampling from the surface would be difficult, such as under ice. The fifth, called an underway shipboard sampler, does not require that a scientist be present. The sampler monitors water that passes through an inlet and then an outlet that are built into the bottom of a boat. It automatically sends plankton data to shore. Wherever the boat travels, maps of plankton are made. When used for example, on fishing boats, scientists repeatedly receive information about plankton populations in the area where the fishing is done.
One type of plankton, an animal called a copepod
Compared to nets, VPRs not only make it easier and faster to keep track of plankton, they also allow scientists to get a better idea of where the plankton are in the water. "Nets are designed to collect a huge volume of water and concentrate that into a small bucket," says Gallager. "Because it's sampling over a much larger volume, it's smearing the spatial scale such that you don't understand what the individual interaction is between plankton." In fact, different kinds of plankton live in different regions of the ocean. "Each water mass has a very characteristic distribution of a specific kind of plankton, and that's one of the very fascinating features of using instrumentation such as the VPR. We're beginning to understand the boundary conditions between these water masses and how it actually captures the organisms into a water mass and doesn't allow them to mix between water masses."
VPRs also allow scientists to study creatures so fragile that they are impossible to study with a net. "So we can capture images of this very fragile Ctenophore, or jellyfish," says Gallager, "whereas a net, as you tow it through the water would damage this to the point where a human would not be able to identify it."
Gallager and his team have taken their research using the VPR even further by studying predator-prey interactions of plankton. He says they can "obtain pictures of plankton to study, let's say, the interaction of krill and larval krill behavior with their predators, such as penguins and other organisms that feed directly on the krill."
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