Plankton Ecology And The Effect Of Size

For plankton communities, size really does matter! Individual members of the plankton vary greatly in body size: ranging from minute viruses and bacteria, to the microscopically visible phytoplankton and small invertebrate larvae, to the large gelatinous zooplankton (jellyfish). In fact, planktonic organisms span seven orders of magnitude in length: from 0.2 micrometres to about 2 metres. A micrometre (^m), or 'micron', is a thousandth of a millimetre, that is, 1 ^m = 0.001 mm. A human hair is about 10 ^m thick (100 hairs = 1 mm); the standard pin used to package shirts is about 600 ^m (0.6 mm) thick; and a dissecting needle used in many science classes as a plankton probe is about 1 mm thick. It will be useful for you to check these dimensions using a microscope and ruler as your microscopic benchmarks -particularly for zooplankton. The resolution of the best light microscopes is about 0.5 ^m - of course, electron microscopes are much better than that (0.2 nanometres with a transmission electron microscope; Kane and Sternheim 1978).

As there are significant ecological and physiological implications of body size in plankton (Peters 1983), we use plankton size as a first step in classification.

The various size categories of plankton are as follows:

• megaplankton are those large floating organisms that exceed 20 cm in length. They are represented by very large jellyfish, salps and their relatives

• macroplankton (2-20 cm, Figure 2.1 top) include large visible organisms such as krill, arrow worms, comb jellies and jellyfish

• mesoplankton (0.2-20 mm, Figure 2.1 bottom) are very common and visible to the naked eye; they are diverse and include copepods, cladocerans, small salps, the larvae of many benthic organisms and fish, and others

• microplankton (20-200 ^m, Figure 2.2 top) include large phytoplankton (large single-celled or chain-forming diatoms, dinoflagel-lates), foraminiferans, ciliates, nauplii (early stages of crustaceans such as copepods and barnacles), and others

• nanoplankton (2-20 ^m, Figure 2.2 bottom) include small phyto-plankton (mostly single-celled diatoms), flagellates (both photosyn-thetic and heterotrophic), small ciliates, radiolarians, coccolithophorids and others

• picoplankton (0.2-2 ^m) are mostly bacteria (called bacterio-plankton). They require at least 400 x magnification for detection and counting. Marine viruses are even smaller (less than 0.2 ^m).

The size categories listed above do not reflect particular taxonomic divisions as sizes vary greatly within most taxonomic groups. In addition, size does not reflect any trophic classification. Small plankton may include photosynthetic cells (that is, autotrophs or 'self-feeders'), herbivores, carnivores or omnivores (that is, heterotrophs like us). Many phytoplankton cells maintain hundreds of other small symbiotic cells around them, sometimes for their nitrogen fixation (such as by blue-green algae). Some organisms even maintain symbiotic relationships with photosynthetically active cells known as zooxanthellae (as in many corals, sea anemones, sponges and clams of tropical coral reefs). Large plankton, such as some jellyfish, are akin to carnivorous plants - capturing copepods and small fish for their nitrogen.

Cell size has direct consequences for many physiological processes, including the assimilation of dissolved nutrients from the environment. Up until the 1970s, the importance of picoplankton (cell size: 0.2-2 ^m), relative to the larger nano- and microphytoplankton, such as diatoms and dinoflagellates,

Figure 2.1 Examples of some typical members of the macroplankton (2-20 cm, top panel, from left to right: ctenophore, krill, jellyfish, arrow worm) and mesozooplankton (0.2-20 mm, bottom panel, left to right: ostracod, salp, larval fish, cladoceran, copepod, pluteus larva of a sea urchin).

Figure 2.2 Examples of some typical microplankton types (20-200 pm, top panel, left to right: radiolarian, diatom chain, armoured dinoflagellate, centric diatom, dinoflagellate chain, nauplius (larval crustacean), ciliate) and nano-plankton types (2-20 pm, bottom panel, left to right: silicoflagellate, pennate diatom, coccolithophore, flagellate, diatom).

Figure 2.2 Examples of some typical microplankton types (20-200 pm, top panel, left to right: radiolarian, diatom chain, armoured dinoflagellate, centric diatom, dinoflagellate chain, nauplius (larval crustacean), ciliate) and nano-plankton types (2-20 pm, bottom panel, left to right: silicoflagellate, pennate diatom, coccolithophore, flagellate, diatom).

was largely unrecognised. We now know that these tiny cells, which are about the size of bacteria, can dominate the phytoplankton - contributing up to half the chlorophyll-a content in coastal waters, and up to 90% in nutrient-poor oceanic waters, and producing much of the oxygen we breathe.

Low-nutrient (oligotrophic) waters are typically dominated by small phytoplankton cells, which are much more efficient at using small amounts of available nutrients than are large cells. Small phytoplankton have a competitive advantage under low-nutrient conditions because they have a higher cell surface area: volume ratio than large phytoplankton with which to take up available nutrients across their cell membrane. For the most part, large phytoplankton cells appear in abundance primarily in response to periodic nutrient increases (for example, seasonal rain events) and/or localised inputs. Other features of plankton that are related in some non-linear way with size are growth, carbon content, sinking rates, grazing, swimming, fecundity and longevity (Peters 1983; Baird and Suthers 2007).

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