Plankton

The word “Plankton” refers to a whole living community. It takes its name from the Greek for “drifting” and it comprises all those animals, plants and bacteria capable of swimming or floating in the so-called “pelagic environment” of the sea, of lakes or large rivers, independent of the surface, the shore and the bottom, but not capable of maintaining their distribution and movements against the force of currents. As such these organisms are distinguished from those that are found floating at the surface (neuston), living on the bottom (benthos), and swimming strongly enough to maintain their distribution or migrate from place to place (nekton).

Plankton is fundamental to the ecology of all watery environments, particularly in the sea by virtue of its vastness. Almost all life on earth is dependent on energy from the sun, but little of that penetrates more than some tens of metres below the surface. Most of the sea bed receives too little for any plant growth to occur there through photosynthesis, and harsh conditions of weather and sunlight right at the surface prevent any substantial “meadows” to develop, analogous to fields and forests on land. The shore is a narrow strip around the edge, where conditions – waves and exposure – can also be harsh.

So, virtually all life in the sea is ultimately dependent on the photosynthesising algae and bacteria that occur below the surface and down to a depth of a hundred metres or so. This “phytoplankton”, the primary producers comprising, for the most part, microscopic algae and bacteria, grows as individual cells or into long chains and is capable of developing intense “blooms” which can colour the water various shades of green, milky blue, brown or even red, form the basis of the pelagic food chain, and for the benthos as well. Light and nutrients are both essential for this primary production, conditions that are met better in some places and at certain seasons than at others. Many coastal areas where deep water rich in nutrients is driven near the surface can be highly productive; other areas can be relative deserts. Scottish coastal waters are generally noted for their high productivity.

As on land, a variety of plants can be fed upon by an even greater variety of herbivorous animals [illustration]. Prominent among these are small crustaceans that are very effective grazers on microscopic plant life. Opportunities are many and varied, so the wide range of these “copepods” (photo above), as they are called, has been likened in the sea to the role played by insects on land. Just as there are many insectivores on land, so there are many forms of carnivorous zooplankton, belonging to many different groups of animals; in fact there are not many animal “phyla” (i.e the major groups of animals, such as arthropods, molluscs and the chordates which include all animals with backbones) that are not represented at some time or place in the zooplankton.

Such a varied community demands some order in people's minds. Like pretty well everyone, scientists love to classify objects so that they and others know what they are talking about. Because there have been many people studying plankton, there are many different ways to classify it. First there are the plants, the phytoplankton, and the animals, the zooplankton, and one also hears about other groupings such as bacterioplankton and ichthyoplankton.

How to sample plankton, and what can be done with the samples, is very dependent on the size of the organisms, so there are divisions into picoplankton (the most minute of all), nanoplankton, microplankton (which includes a large proportion of the microscopic algae that can make the water quite opaque, green, brown or red according to the plants in the “bloom”), macroplankton (visible to the naked eye) and megaplankton (centimetres to metres in length and including such giants as the largest jellyfish – despite their size, they are still drifters, not swimmers.

All the photosynthetically active plants have to live quite near the surface where there is enough light, and all herbivores have to live there too or at least visit surface layers in order to feed. Carnivorous plankton and those that eat detritus can live in deep, dark layers, so one finds quite distinct communities at different depths. Groupings such as epiplankton, mesoplankton, bathyplankton identify these communities, in relatively well-lit water near the surface, at moderate depths with some residual light, and in the deep, dark waters respectively. There is much movement between these layers, particularly between day and night: many herbivores move upwards in the evening and down in the morning, a pattern called diurnal vertical migration (DVM) which has been associated with the trade-off between the need for food and the danger of predation (and has been the springboard for much research in marine ecology). That trade-off may also change as animals grow, require different diets, and develop stronger swimming ability, so seasonal changes in vertical distribution also occur.

The most extreme cases of such seasonal changes occur when larvae or juveniles which start off in the plankton metamorphose or grow and cease to live as plankton, either because they change into the adult form and settle on the sea bed, like barnacles and crabs, sea-urchins and starfish, octopus and flatfish, or simply because they grow into strong swimmers, like most fish and squid, and so join the nekton, the community that is still pelagic but capable of maintaining its distribution in spite of ocean currents – or perhaps even by exploiting them. These temporary members of the plankton are termed meroplankton, in contrast to the holoplankton, which are adapted to a permanent life in the plankton, unbounded by solid boundaries, shore and sea-bed.

The growth of organisms that are capable of photosynthesis is limited around the shores of the world's seas and oceans relative to their vast surfaces areas.  Plants that live on or near the sea-shore have to withstand harsh conditions (storms, extreme temperature and salinity changes, and desiccation) and therefore are tough and make poor fodder for marine herbivores. Therefore, virtually all animals living in the sea are dependent on phytoplankton. These plants are mostly unicellular algae (though they may form quite long chains), so most obligate herbivores are similarly microscopic and all are planktonic.

Hence plankton provides the fundamental basis for all life in the sea. Without a rich, healthy plankton community, there would be little else living in the sea. Yet geological history has shown us that life started there and has flourished ever since. Just how much is there is dependent on many factors – there are deserts and green pastures in the sea just as on land – and how much we see, exploit or enjoy depends to a great extent on the form of the food-chain that develops. In one place or at a particular time the phytoplankton may be minute, the food chain may be long and complex, and culminate in forms of little use (or, let's face it, of little attraction!) to man. Under other conditions the food chain may be short, say phytoplankton to copepods to herring to cod, and the sustainable yield may be great. The prime example of a short food-chain with a spectacular end is that of krill in the Antarctic. This is a small shrimp-like creature that feeds on rich blooms of phytoplankton and is in turn eaten in enormous quantities by the largest creatures that have ever evolved on Earth – the blue whale and the other great rorquals.

 


Humpback off the West Coast of Scotland feeding on sprat

But we don't need to travel to the Antarctic to see this. The waters off the west coast of Scotland – and elsewhere around the North Atlantic and in the wider world ocean – are highly productive. That is what the cetaceans around our coasts are ultimately thriving on; this gives us cause to cherish “our” plankton, to seek to know more about it, and to ensure that it maintains itself in a healthy environment.

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