You are on a raft, drifting in the middle of the Pacific Ocean. The rippling surface surrounds you, stretching in every direction to the horizon. There is nothing but salt water and an empty sky.
This desolation also extends below you. That is because once you move away from the coasts of the continents, there is little life in the open ocean except for a scattering of miniscule nanoplankton.
The food chain in the sea starts with phytoplankton, microscopic plants. Where there is an abundance of phytoplankton, you will find a rich diversity of marine animals feeding on it.
If you remember your biology, chlorophyll is what plants use to capture the energy of sunlight and make it available; most life on earth is dependent on this process.
The map to the left is satellite imagery of chlorophyll levels – i.e. phytoplankton levels – of the oceans of the world. Green to red is an abundance of chlorophyll, blue to purple is a scarcity. Most of the open oceans of the world are blue in this map, indicating that there is little life in these ‘biological deserts’.
Why is this? There are a number of factors, but the main one is lack of nutrients that phytoplankton need to grow. Most organic matter in the sea – mucus, faeces, dead fish and whales – falls from the surface waters to the abyssal plains below. This is known as marine snow. On those dark plains microorganisms break down the organic matter of the ‘snow’ into nutrients. However, it is beyond the reach of phytoplankton – sealed in the depths.
What happens next? From Pelagia: “In the southern hemisphere, the cold Humboldt current slid quietly up the coast of Chile and Peru. The winds blowing up the coast, along with the earth’s rotation, contrived to drive the surface waters away from shore and out into the Pacific Ocean.
As the surface waters moved offshore into the open seas, they were replaced by cold, nutrient-rich waters upwelling from the dark depths. Phytoplankton bloomed abundantly in this swell of nourishment. This in turn attracted swarms of anchovies with gaping mouths to gorge themselves on this bounty.
Schools of tuna waited, biding their time in the depths. These cheetahs of the sea had crossed an ocean seeking opportunities like this. Now the predators darted in, slashing through the school of fattening baitfish. Terror sent the anchovy shoal to draw tightly together, then flash and wheel as one, in their instinctive but futile defence.”
Undersea mountain ranges and seamounts also form a physical barrier to bring up these deep nutrients to produce productive seas. Seamounts are oases in the ocean desert.
This is why many of my Pelagic pioneers build communities above these undersea peaks. Read about this and more in the settling of the South Pacific Gyre.