…the soup of life

We find plankton near the surface of the water, and at the bottom of the food chain.  “Plankton” is an umbrella name that takes in more than 5000 kinds of drifting life forms living in the upper layer of the world’s seas and oceans, rivers and lakes.

Some members of this community – viruses and bacteria – are so small they’re only visible with a high-powered microscope.  Others – jellyfish, for example – are much larger.

Some – called “phytoplankton” – act like plants.  These include the algae that produce mass seasonal blooms.  Others – the “zooplankton” – are animals.  These include the earliest stages of some familiar marine life.  Some don’t fit neatly into either class.

Some spend their whole lives as plankton.  Others, including squid and sea urchins, some fish and corals, crabs and lobsters, spend their early childhood drifting and feeding with the plankton.  After a few weeks – or a few months – they mature into the forms we know, and settle into bottom-dwelling life as juveniles.

The one thing these billion of little specks have in common is their lack of control over where they go.  Some can move up and down the water column, but other movement – forward, back or sideways – is almost totally controlled by waves, tides and currents.  Even the name “plankton” comes from a Greek word meaning “drifting” or “wandering”.

The only time plankton gets any public attention is when a “bloom” happens.  With just the right combination of sunlight, water temperature and minerals in the water, drifting algae go into a frenzy of birth and growth.  They get so dense over a wide area that the blue, green or reddish blooms can be seen from space.

Blooms can happen anywhere but they are more common in colder climates, where there are strong seasonal differences in light and water temperature.  They also need to be fed.  Land-based runoff from agriculture and industry supplies a lot of the nitrates and phosphates they need, and Sahara dust is a major source of iron.

Some of these blooms – commonly called “red tides” --are dangerous.  The algae involved produce poisons that can kill other marine life, or get stored in the bodies of fish and shellfish and kill the people that eat them.

However, plankton is important and useful to all of us.  First, it is a vital element in the marine food chain.  At some stage in its life, almost every marine animal eats plankton, alive or dead.

Second, plankton is a vital link in the world’s carbon cycle.  The total mass of marine plant life (including phytoplankton) is tiny compared to the mass on land, but a UNEP report issued last October estimates that it traps just as much carbon dioxide every year.  It’s even been suggested that “seeding” plankton blooms might be helpful in controlling greenhouse gases.  The effect is short-lived – a few weeks at most – but when the blooms die, most of the trapped carbon settles into long-term storage on the sea floor.  Marine carbon sinks are almost permanent.  Carbon that finds its way to the sea floor will stay there for thousands of years, compared to a few decades for rainforests.