Midnight, snorkelling solo in an isolated bay, the water is frigid, I can’t see a thing, it’s amazing.

Earlier, inspired by the rare appearance of sunshine over the preternaturally overcast isles of Shetland, I walked over fences and past curious sheep to the nearest bay. Dodging jellyfish and swimming over golden kelp fronds, something looked different: the water was fuzzy, packed full of tiny particles. A closer inspection revealed the fuzz as a dense cloud of living animals, I was in the midst of a zooplankton bloom.

Plankton lies at the base of the aquatic food chain, fundamental to the more celebrated life that sits above it. It takes two forms: phytoplankton is composed of microscopic plants, whereas zooplankton is formed by tiny animals. Most of the animals we see on dives would have started life as zooplankton, living in the open sea. For fear of predation, zooplankton usually spend its days in deep water, only rising by cover of night to feed. This vertical migration takes place every evening across the ocean, and by numbers it is the greatest mass movement of wildlife on our planet.

For most, plankton is too small and unknowable to be of interest. Within the diving community, the wonders of plankton are more readily appreciated, especially by those who partake in blackwater dives. The concept sounds crazy: dive in the middle of the night, over nothing but deep expanses of open water, whilst hanging lights in the hope of attracting visitors. When Chris Newbert trialled the idea off Hawaii many decades ago, he quite reasonably used a cage for fear of how sharks and other predators might react.

The images captured on such dives have a timeless appeal and can be found in his classic book: Within A Rainbowed Sea. Blackwater diving would only garner mainstream appeal in the last few years, with dive centres in the Philippines, Indonesia, Florida, Hawaii and beyond devoting their nighttime schedule to the art of plankton exploration. This was my initiation to the blackwater club, and once you are over the first, disconcerting, drop into the inky abyss, it becomes a furiously addictive aquatic pursuit. In their early stages of life, and spending most of their time in darkness, planktonic creatures have an alien-like appearance – a space-like visual experience enhanced when visiting them at night.

With my appreciation for plankton honed in tropical seas, the dense bloom of zooplankton in Shetland ignited an irascible curiosity, convincing me to return at night for what would surely be a spectacular blackwater experience. Returning to the bay late at night, armed with a macro lens, I come face to face with plankton on a scale unlike anything I have experienced before, snorkelling amidst a bloom so thick that at times I am unable to see through it. To the naked eye, it looks like a million peach coloured spheres, as if the contents of a bean bag had spilt over the sea, but my camera reveals a mass of tiny organisms. I am in the midst of the zoo here – a rich tapestry of tiny animals pulsating all around.

Some are too microscopic to recognise, but others I can discern: larval stage crustaceans abound, some of them swimming through the darkness, others clinging to the life rafts offered by broken-off seaweed. This plankton soup has attracted an army of jellyfish, who feast upon the buffet of miniature life. Another 3 hours later, I stagger out of the water, elated but with hands so cold they can barely remove gloves and fins before trudging back up the hill to bed.

Over two years later, whilst documenting the deepwater Nekton Maldives mission, a conversation with Professor Alex David Rogers yielded the scientific name for the phenomenon I had witnessed: topographic blockage. As the great plankton migration occurs, currents push it across the sea and at times, its return to the deep is blocked by physical features. These blockages create a readily available food source which in turn attracts other, hungry, life. In the Maldives, as Professor Rogers gazed out of submersibles, a lightbulb moment occurred, he realised that the ocean shelf at 500m deep was creating a ‘Trapping Zone’, an oasis of life, brought about by plankton blockages.

Overlooked but essential, plankton is the foundation for the health of our ocean but also our planet as a whole, sustaining oceanic ecosystems and absorbing 30% of the carbon dioxide created by humans. Look closely, and you’ll also find plankton is amongst the most astonishing of underwater sights.

Henley Spiers’ pictures of plankton in Shetland have gone on to win the British Wildlife Photography Awards ‘Coast & Marine’ category.