Marine Plankton
Plankton in the open ocean
Introduction
Take a second to consider the scale of our oceans. Approximately two thirds of our planet is covered in seawater. The oceans teem with life, yet humans are ill-equipped to explore this environment without specialist equipment. The fascinating world of plankton lies hidden beneath the surface of our seas.
Plankton consists of a diverse range of living organisms that spend at least a part of their life cycle suspended in water. The term ‘plankton’ is derived from the Greek word “plankton”: ‘that which is made to wander or drift’. This term is further divided into the phytoplankton and zooplankton, meaning plant- (Gk. “phyto”) and animal- (Gk. “zoön”) drifters respectively.
Planktonic organisms may have a limited ability to control their fine-scale distribution in the water column, but are otherwise at the mercy of oceanic currents and water movements. Holoplantkon refers to those organisms that spend their entire life in the plankton, as opposed to the meroplantkon, which are only planktonic for a part of their lives. Organisms that are capable of resisting the powers of currents, such as fish and squid, are referred to as neckton.
Planktonic organisms are typically classified into broad size categories according to the ‘Sieburth-scale’, originally proposed in 1978. Viruses and jelly fish sit at opposite ends of this scale, which runs from fractions of a millimetre to metres.
Plants of the ocean
Photosynthesis is the process by which inorganic building blocks such as carbon dioxide (CO2) and water (H2O) are combined using energy from the Sun to produce organic compounds. Organisms that are capable of this process can be referred to as photoautotrophs (Gk. “Photon”, “auto”, “trophe”; light-self-nutrition) and primary producers. Chlorophyll and other similar pigments are found in light-harvesting organelles called chloroplasts. These are present in all species of phytoplankton. Organic matter formed by these organisms forms the basis of almost all food chains. Approximately 50% of global primary production occurs in the oceans.
On land, plants are typically large, conspicuous organisms; trees, herbs, bushes and grasses etc. They have root structures to take up water and nutrients from the soils beneath them, and also to provide anchorage. Marine plants, the phytoplankton, are fundamentally different. They are microscopic, single-celled organisms. Smaller objects have a greater surface area relative to their volume, and hence mass. Being small in the oceans confers several advantages: Phytoplankton cells are so small that their weight is, to a large extent, offset by the frictional drag exerted on them by the water. Thus, they sink very slowly, enabling them to stay within the surface, sunlit waters. Many species of phytoplankton have ornate spines and appendages which appear to both increase drag, and serve as defence mechanisms. Phytoplankton cells do not have roots, and must take up nutrients from their surrounding environment. Having a large surface area relative to their volume ensures that they maximise their chances of attaining enough resources for growth.
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