Karen Boswarva is floating in a most peculiar way... in the epipelagic zone with some of our planet’ s most important organisms
OCEANSCIENCE
The soft parade
Karen Boswarva is floating in a most peculiar way... in the epipelagic zone with some of our planet’ s most important organisms
At the end of a dive, we take a couple of fin kicks to aid us as we begin our ascent back to the surface. As we rise, we adjust to the changing pressure, releasing the expanded gases from our suits or BCDs. The light increases and if we are lucky, the sun’ s shimmering columns of light bathe down on us. We may spend a good portion of time hanging out in the water column performing a series of safety stops. During this time our bodies are neutrally buoyant and float effortlessly, drifting along at the mercy of the current.
As we wait for the time to tick down, we monitor our depth and make minor adjustments when needed. Something effects our buoyancy as we join currents of warmer water, but our adjustments prevent us from rising to the surface. Finally, we surface exclaiming excitedly“ did you see that …!”
Now what if I told you that this part of the dive( the end) is the bit where we most resemble the marine organisms around us? Crazy thought? Bear with me, for the ocean is a three-dimensional environment. We( usually) go there to find its bottom – the biodiverse rocky reefs or aweinspiring wrecks. But to get there( and back) we pass through one of the richest ecosystems of all – the water column, also known as the pelagic zone and this is where the magic happens!
The pelagic zone can be so deep that it is split into a series of subzones. The zone that we experience as divers and snorkellers is the epipelagic zone( 0-200 m deep); it’ s also known as the sunlight zone because there is enough light for photosynthesis.
We learn quite early on in our diving career that the oceans are not static. There are tides governed by the lunar cycle, and currents forming underwater conveyor belts circulating and mixing colder denser seawater with warmer, more buoyant, seawater.
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Seawater is a viscous liquid that naturally impacts movement. Whilst larger marine animals( and some birds) can propel themselves through it, most of the smaller species must either attach to a surface or remain on the seafloor, the smallest are left drifting with the current.
These microscopic marine organisms are collectively referred to as plankton. Plankton comprises of phytoplankton( plant-like) and zooplankton( animals). Some we can see with the naked eye; phytoplankton, for instance, is what makes the sea turn to pea soup. Jellyfish and luminous comb jellies make up the larger zooplankton that drifts in. However for every litre of seawater there are between 10 to 100 billion( mostly invisible) lifeforms.
Shells of planktonic radiolarians, drawn by Ernst Haeckel in 1904
Phytoplankton are mostly microscopic plant-like microalgae( coccolithophores, dinoflagellates and diatoms) that contain chlorophyll- they photosynthesise just like land plants. A powerful microscope is required to view their intricate body shapes comprised of shell plating, silica, or cellulose. Dinoflagellates and some coccolithophore species move through water using a whip like tail called a flagellum, while diatoms and other coccolithophore species drift on ocean currents. Phytoplankton are the powerhouse of the oceans; the key driver of productivity and they produce almost 50 % of the oxygen we breathe!
Zooplankton includes microscopic animals such as krill( whale food), sea snails, the eggs and larval stages of invertebrates and fish, plus larger weak swimmers( jellyfish). They often have fine hairs or appendages to make small movements with great resistance( imagine trying to swim through treacle). Most zooplankton eat phytoplankton and in turn are prey for larger species. During daylight hours they migrate to deeper depths to avoid predation, rising towards the surface during periods of darkness to feed.
The composition of plankton changes throughout the year. Phytoplankton has two key blooms – spring and autumn – which coincide respectively with increased sunlight and increased nutrient upwelling from storms. Following environmental cues, marine animals will broadcast eggs and sperm or release larvae into the water column. These zooplankton will live a nomadic life, eating and developing as they drift across the vast oceans.
And so, for a brief window in time we too are floating in the epipelagic zone, among the dense soup of microscopic plankton – how marvellous. �