The word “plankton” meaning ‘wanderers’ in Greek – living things that move mostly with the current. In the ocean they provide a lot of the energy that keeps the whole system going. They vary a lot in size, most are way less than 0.1mm, while some jellyfish can be 2 metres across. Plankton contains both animals and plants. They give the name “zooplankton” to the animals making up the plankton. “Phytoplankton” are the plants in the plankton that get energy from the sun, although some of them actually swallow other plant cells and seem a bit more like animals. Phytoplankton are usually small single-celled plants or drifting chains of small plant cells. A fashionable way to divide them up is to talk about them based on their size, “microplankton”, “nanoplankton”, or the really tiny “picoplankton”.
From 60-98% of Australian plankton is made up of the especially small picoplankton, similar to bacteria. That size is the most efficient for absorbing the sparse supplies of nutrients that are found in Australian waters. Phytoplankton are the world’s most efficient solar battery. They can also divide water into hydrogen and oxygen at room temperature, a job that many multi-celled human chemists struggle to do. As efficient in design as they are, phytoplankton are just plants, so they need light, minerals and fertiliser, to thrive. This means that you only find them in shallow depths, and only in places where there are nutrients and minerals in the seawater. The plankton isn’t everywhere and only some spots in the ocean are really suitable for planktonic blooms. Fish love phytoplankton, but because it is so patchy, 1% of the ocean produces 50% of the world’s fish catch. There is a storehouse of complex nutrients and minerals (nitrates, nitrites, phosphate and silicate) lying on the deep ocean floor. Here dead sea animals have decomposed back into basic chemical ingredients. The sea floor also has the small amounts of iron, copper, cobalt, selenium, manganese and other minerals that are needed for plankton’s more complicated body structures. Getting it up to the surface where there is sunlight, that is the problem for plankton.
Currents, upwellings and phytoplankton should always be mentioned in the same sentence as you can’t really have plankton without the nutrient enriching movement of the deep seas. Australian waters are virtually a nutrient desert compared to northern hemisphere waters and Australia’s fish productivity is low. We catch only about 0.2 of the world’s fish despite the huge size of our waters. New Zealand’s southern fisheries are 100 times more productive thanks to the West Wind Drift currents. Our major oceanic currents, like the Eastern Australian Current (EAC) and the Leeuwin Current, are very important in providing some productivity to our oceans. The Eastern Australian Current flows from the tropics down the Australian East coast and affects placed as far away as southern Tasmania. This warm-water tropical current is nutrient poor, but that is not the whole story. It causes sea to ‘pile up’ where it meets colder water. This causes a localized rise in sea levels by up to 80cms and water wants to flow downhill. That movement, along with the wind and the underwater geography of our East Coast, is enough to produce anti-clockwise eddies off the edges of this big current. The effect is a bit like the whirlpool you get if you unplug the kitchen sink. These eddy fields can be as much as 200km wide and reach all the way to the bottom of the continental shelf. They stir up the deep sea floor enough to transport minerals and nutrients to the surface. Along the Southern Ocean, currents upwellings happen after major storms in winter, but at that time of year there isn’t enough sunlight for phytoplankton to exploit this bonanza of fertiliser, so the phytoplankton bloom has to wait. Come spring, phytoplankton get enough energy from the sun to start up the engine of life. A flood of light and nutrients causes a phytoplankton cell to double its size in quick time. It then reproduces by splitting in half and massive increases in populations occur.
A phytoplankton population boom is closely followed by a jump in the numbers of predatory zooplankton who feed on them, like protozoa. These small protozoa are just the right size food for other bigger animals like little baby sea-stars and cray larvae that also swim around in the plankton stream. Major predators of smaller phytoplankton also include salps, comb jellies and stinging bluebottles. While these jelly-like animals can be a bit indigestible for fish, some animals like turtles are evolved to feast on them. Occasionally a down-welling will cause buoyant objects to form up in lines on the surface, causing a noticeable ‘slick’ of larvae, fish eggs, organic material and jellies at the surface.
When the wind is blowing onshore, it is a delight for beachcombers and a blight for swimmers. The beaches can be clogged with masses of salps and bluebottles. Schools of small juvenile fish soon get in on the act. They then attract predatory tuna, marine mammals and seabirds. The most well-known of these phenomena is the ‘sardine run’ off the east coast of Southern Africa where cold currents from the south cause a temporary but massive boom in marine life. The water will go cloudy with plankton for a while, but the surface layer will soon become depleted of nutrients because there are soon too many mouths to feed. The plankton starts to die, and only the welladapted can survive this famine.
As the numbers of plankton drop, the fish and birds will move away to a more productive area. The plankton that is a bit more mobile, like the dinoflagellates that come equipped with a pair of tiny oars, will swim down deeper where there are still some nutrients. Diatoms are small single celled plants with a hard shell. They can’t swim to greener pastures but diatoms can hang on because they are very efficient at extracting nitrogen at very low levels. Then the wind, or upwelling current eddies again stir up the surface layer and inject more nutrients. The ‘boom and bust’ cycle takes off once more.
Sometimes plankton growth is not halted by the amount of nutrients. In fact, plankton can sometimes get too much of the wrong sort of ‘junk’ food. Off Sydney it’s been shown that vitamin B12 and thiamin deficiency halts plankton blooms well before the food runs out. They have binged on ‘cheap takeaways’, so the plankton has to wait for fresh currents to bring along the marine equivalent of a dissolved ‘vegemite sandwich’ before they can start breeding again. In the open ocean the plankton usually have to handle some pretty rough food, naturally complex nitrates and nitrides that take a bit of eating, but if they travel near an estuary like Port Phillip, Port Jackson, the Derwent, the Swan, or the Port River, the plankton will sometimes find easy-to-use ammonia ‘fast food’ from sewerage works and other human pollutants. The plankton can bloom so dramatically, that it can starve the water of oxygen and outcompete other forms of life. An enclosed waterway can end up being full of plankton and algae, murky, smelly and lifeless. Despite occasionally causing us problems, plankton is the foundation of all marine life, including neing the basis of our seafoods. An area of ocean rich in plankton can be up to 5 times more productive than the same area of sea floor. Coastal reef also get a lot of their food from passing plankton. Great Barrier Reef corals have been found to get 90% of their food from oceanic plankton. There is so much of it, that it can sustain the largest animals on Earth, animals as large as a Blue Whale. We can’t see most of them, but as Yoda would say, “size not important, only life important”.