Plankton - a Bit of Science

Plankton is a catchall term to describe to organisms, other than squid, fish and marine mammals (which are termed nekton), that dominate the biology of our oceans and coastal waters.  The word derived from the Greek adjective πλαγκτός - planktos, meaning "errant", and by extension "wanderer" or "drifter" (Wikipedia).  The notion of a “drifter” stems from their regional location being almost entirely at the mercy of the ocean currents, drifting with them.  Some of the zooplankton are capable of independent movement and can swim hundreds of meters vertically in a single day, nonetheless their horizontal position is primarily determined by the surrounding currents. This is in contrast to nektonic organisms that have the potential to swim against the ambient flow and control their location e.g. squid, fish, and the great travellers of the oceans – the whales.

The term “plankton” both includes plants (phytoplankton) and animals, including protozoa (zooplankton) as well as bacteria (for want of another word - bacterioplankton).  The plant forms are almost exclusively microscopic – the only large form is seaweed Sargassum – which is found in the North Atlantic Subtropical Gyre (more familiarly known as the Sargasso Sea).  The animal forms show a greater preponderance of larger forms: jelly fish – medusae – are included in the plankton and they can reach several metres in size.

Plankton and the Food Supply of the Ocean Biota: The microscopic phytoplankton are the primary source of food in the oceans.  The organic material they produce by photosynthesis is passed stepwise up the food chain by organisms of increasing size, eventually after 3 to 5 steps reaching “top carnivores” – adult fish, marine mammals.  There is something in the region of a 90% loss at each step, so in a 3 step food chain from 1 tonne of photosynthetic production just 1/1000th – 1 kg arrives at the top carnivore level; in the case of a 5 step food chain (characteristic of the open oceans) just 1/100,000th (10gm from a tonne) reaches the top carnivores. 

Diagram of Eltonian Pyramid

Biomass distribution on land

Diagram of Biomass distribution of the Ocean Food Chain

Biomass distribution in the oceans

All the Action is Down there on the Micro-scale: A curious feature of the oceanic food chain is that if the organisms are organised in logarithmical bands of size, (e.g. 1-10 mm, 10-100 mm, 100-1000 mm etc) then in a given volume of ocean water (say a cubic kilometre) there is approximately an equal amount of biomass throughout the size ranges – thus in that volume the biomass of organisms of the size range 1-10 microns (marine microorganisms – bacteria and protozoa) is similar to that of organisms 1-10 metres (the larger fish and most of the whales).  This contrasts sharply with the terrestrial system where there is a reduction of biomass as you ascend the food chain – the so-called Eltonian Pyramid (no not the piano player with funny glasses). 

Digging Deeper into the Science:  The significance of constancy in biomass with size in the plankton becomes apparent when we take into consideration a second general law in biology – that the specific metabolic rate (the rate of metabolism of an organism divided by its weight) increases as the organism gets smaller – the general relationship between the specific metabolic weight (M) and the size of the organism (as length L) is M~L-0.25.  Thus, if we do the calculation for a whale (~10m ~ 109 gm) and a bacterium (~1 micron = 0.000001m or ~10-14 gm) we get (109)-0.25 = 0.0056 for the whale and (10-12)-0.25 = 1,000 for the bacterium, thus the specific metabolic rate of the bacterium, by this calculation, would be about 1,000/0.0056 ~200,000 times greater than of the whale.  For a number of reasons a whale (or ourselves) cannot even approach the metabolic rates achieved by microorganisms.  Indeed if a whale were to have rates of metabolism comparable to bacteria it would very quickly run into a fatal problem of overheating – it would not be able to dissipate the heat generated during metabolism and it would heat up to somewhere in the region of 1,500°C, when it would be white hot – a white whale for another reason.

Have no Doubt - Micro-organisms Run the Oceans:  Historically, the biology of the oceans has been the show of the microorganisms, and although we may twiddle at the edges, thankfully our species does not have the capability to directly affect the overall biological functioning of the oceans.  That may not be the case for the physics and the chemistry, and there are concerns these effects may propagate onto the biology. 

The important piece of information we gain from the calculation above is, if the specific metabolic rate (rate of metabolism/body weight) of the micro-organisms in the plankton is many thousands to tens or more thousands greater than the fish and large mammals such as whales, then, as the spectrum biomass concentration in the oceans is sort of constant with size, the collective metabolism of the micro-organisms in the oceans will be likewise thousands to tens or more thousands greater than the fish and large mammals.  Thus, crudely, anything larger than what you can pick up with a pair of tweezers (say a millimetre or a fraction of a millimetre) is of little consequence on the general energy budget in the oceans.  Even to the specialist this is counterintuitive and, believe me, you’ll get a cold stare from many biologists when you make such a pronouncement. 

It is important to note that this generalisation does not apply in the same way or extent to their ecological importance, although it still does to a degree.  Striping out the bacteria from the oceans would have a greater immediate impact on the over ecology than removing the whales or the cod – which mankind, to its great shame, has effectively done in a number of locations – the whales in the Antarctic as an example.  The ecology will shift but will still operate.  By sharp contrast,  removing the bacteria (which thankfully we are incapable of) would probably bring the ecology to a halt, if not a close, with weeks or a few months. 

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