INTRODUCTION TO MARINE BIOLOGY
The Deep Sea (2012)
It is odd that a planet with 65% of its surface covered by ocean and 57% by 1000+m of deep-sea is called the ‘Earth’. Some 49% of the globe’s surface is 3-6 km below the ocean waves and the average ocean depth is 3.8 km. The deep sea is dark, devoid of plants but home to many animals which have been collected on hooks and lines, in nets, from the stomachs of other deep-sea beasts, viewed and photographed from tethered cameras, ROVs and deep-sea submersibles. Yet we still know very little about them. The total area of seen/ photographed deep sea bed is less than central Cambridge – and we have better pictures from the dark side of the moon or even Mars!
Environmental Conditions – are extreme:
Temperature: at the ocean surface is from 0 to 30ºC yet the deep sea is a constant 2 to 3ºC. Cold but stable, allowing extremely stenothermal animals to develop biochemical mechanisms to cope with other environmental extremes.
Sunight: only penetrates to c.100 m in nutrient rich/ turbid temperate regions to as much as 600 m in nutrient poor but transparent tropical waters. Photosynthesis is limited to the upper 200 m of the epipelagic or euphotic zone though some fishes and cephalopods can see surface or downwelling light at 1000 m.
Pressure: increases by one atmosphere or, 14 psi, for every 10 m depth:
15838 (¾ tonne)
Nutrient: Deep-sea animals that cannot migrate up into the euphotic zone are reliant on either prey, the rain of dead material that sinks from the surface to the ocean floor, or (non-solar) geochemical energy.
The deep-sea is divided into two principal environments – open ocean ‘pelagic’ and on/ near-ocean floor ‘benthic’, both of which are further classified according to depth:
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