Microscopic Ocean Forests and Underwater Gardeners

by Emily Gregory

Between you, me and the garden post, baleen whales have a green thumb… or bum! We’re talking big animals with big impacts but in order to understand, we need to think small, really small.

You could think of the upper surface waters of our ocean as a tiny free-floating forest, the kind of tiny forest that can only be seen with the aid of a microscope. This forest is made up of microscopic organisms like plankton, in particular, phytoplankton, or more fittingly known as plant plankton. The word plankton is derived from the Greek word ‘planktos’, meaning to drift or wander. A founding organism of marine food webs, these drifting single-celled photosynthetic organisms use energy from the sunlight, and regulate carbon dioxide in the atmosphere and ocean, inevitably giving the by-product of oxygen!

Sounding pretty important right? Well, these little microscopic plankton are actually responsible for the big job of producing over 50% of the earth’s oxygen (scientists say there is oxygen present from these tiny ocean plants in every breath we take!). To achieve all this carbon-sequestering fun, they require nutrients like nitrogen and iron, often scarce in our Southern Ocean surface waters.

So, what do whales, and their bums, have to do with really important microscopic, free-floating ocean forests responsible for regulating carbon dioxide in our atmosphere…that we can’t even see?!

Plant plankton make up the base of the marine food chain. They play a huge role in supporting our marine life, from the small guys (like krill) to the largest fellas (like whales).

It’s strange to think that the world’s largest mammals can quench their appetite when surviving almost entirely on tiny crustaceans, that we call krill. Krill are nutrient-rich, multi-cellular animals and these friendly little fellas can reproduce quickly. Scientists estimate that they amass to a whopping 500 million tonnes, just in our Southern Oceans.

A big enough mouthful of these guys is sure to satisfy even our largest animal on earth, the blue whale! They’re also full to the brim with important nutrients like iron and so it’s no surprise that a diet of krill would result in a baleen whales’ faecal matter being chock full of these vital nutrients. That’s right, we’re talking what goes in, must come out. These hugely abundant crustaceans feed directly on plant plankton and there’s no shortage of plant plankton…thanks to whale poo!

So, does it all come down to whale poo?! Actually...yes!

Turns out these gentle giants of the oceans could be just as busy fertilising under water as bees are, pollenating above. Whale poo just so happens to float (due to being characteristically flocculent; a fun word for saying loosely aggregated particles). This, along with their inability to release their bowls under pressure means all their poo inevitably ends up in our energy-rich ocean surface waters!

This upward movement of nutrients like iron and dissolved organic carbon in whale poo is quite literally fertilizer when absorbed by our tiny free-floating ocean surface forests (or plant plankton), stimulating their growth. There’s a fittingly big word for a hugely important process like this, and now you can nod along when scientists refer to the ‘biogeochemical cycling’ of our oceans!

So, let’s break this down; baleen whales eat krill, krill eat plant plankton, whale poo fertilises plant plankton, plant plankton regulate carbon dioxide. Not forgetting that they produce oxygen as a by-product, and we breathe that very oxygen on a daily basis. How cool is that?! We call these kinds of linkages positive feedback loops.

Whales gardening in tiny free-floating ocean forests are not only recycling nutrients through their poo, sustaining ocean ecosystems productivity, some researchers suggest this stimulation of carbon export could have a big impact in counteracting global warming. This is what scientists refer to as a geoengineering technique, as whales poo out all these nutrient goodies into our ocean surface waters, these poo-loving plant plankton absorb all those nutrients and carbon in the atmosphere, photosynthesising carbon to biomass and inevitably creating ocean carbon sinks in our sea floor sediments. Therefore, essential nutrients like iron in whale poo can restore balance to ocean ecosystems and their food chains but could also play a role in offsetting climate change! So, next time you take a deep breath, stop to appreciate the beauty of nutrient-rich whale poo and microscopic ocean forests!

You can now see why scientists would be so interested in understanding more about these feedback loops and the crucial role whales play when it comes to the balance and health of our marine ecosystems. Conservation of our great charismatic ocean giants is so important when recovery of these guys could mean a huge increase in productivity and life in our Southern Oceans!

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