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Word Wednesday: Microplankton

Wednesday, May 4th, 2016

Large, medium, small – and really, really small… Today’s Word Wednesday focuses on microplankton. Are they plants, animals or something else? Does the name have a specific meaning?

Much more so than land, the ocean is a 3 dimensional environment. Marine organisms are found on the ocean bottom, but also throughout what marine scientists call the water column, that is from the surface through the sunlit zone to the dark depths of the deep ocean. Since the ocean is so diverse with respect to the kinds of living organisms that live there and each ocean habitat offers different characteristics to which organisms must adapt, sometimes scientists group ocean organisms by where they live. Those organisms that live in the water column are said to be pelagic and within the pelagic realm, organisms are classified as plankton or nekton. Nektonic organisms are capable of controlling their position in the water column – they are strong swimmers and can go where they want to. However, planktonic organisms, though they may be able to swim, cannot swim strongly enough to go where they want to: they are at the mercy of large scale and small scale ocean currents. Thus, plankton are often casually referred to as the drifters of the ocean world. Although we generally think plankton are small in size, it is important to remember that even large organisms, like the human-sized jellyfish found in Japan’s waters or the long chain siphonophore, can be planktonic.

In the early days of oceanography, plankton were most commonly collected using a net. Though the size of the holes in the net varied, the net essentially strained organisms from the water: organisms larger than the holes were caught by the net; those smaller than the holes remained in the ocean. These organisms became known as net plankton. As our microscopes grew more powerful, we realized that there were a great many organisms not captured by plankton nets even when the size of the holes were very small – millimeter in size! These included organisms ranging in size from viruses and bacteria to single-celled eurkaryotic protozoans, algae, or even fish larvae. As marine scientists developed new techniques to collect and study these groups, they realized that grouping planktonic organisms by size was perhaps a better way. Thus, marine scientists refer to picoplankton, nanoplankton, microplankton, mesoplankton, macroplankton and even megaplankton. Picoplankton range in size from 0.2 to 2 micrometers (µm), a size range that includes mostly bacteria. Mesoplankton, 0.2 – 20 millimeters (mm), and macroplankton, 2 to 20 centimeters (cm) in size, are the typical net plankton that include copepods, arrow worms, krill, pteropods and many larval stages of both benthic animals and fish. Megaplankton are greater than 20 cm in size – often much greater! Microplankton are 20-200 µm in size and include most phytoplankton and many of the microscopic organisms we looked at in our school’s biology class – protozoans such as paramecium, amoebas, and foraminifera. Thus, microplankton includes both plant-like (photosynthetic) and animal-like (heterotrophic) organisms.

ACER’s Microplankton sub-group, lead by Dr. Jeffrey Krause, is studying processes and interactions within the microplankton community and how they are affected by oil and dispersant in the water.

Their current experiment is investigating microplankton structure and function in water contaminated with oil and dispersant. Specifically they have 4 treatments – seawater, seawater with oil, seawater with dispersant and seawater with both oil and dispersant. During the experiment, they are measuring a number of different parameters. These include community diversity (who is in the community), abundance (how many are present) and biomass (how big / weighty they are). They are also measuring various aspects of community function including (but not limited to) how much photosynthesis is going on, how efficient the photosynthetic process is, and how much predation (grazing) is occurring.