When looking out at the sea ice that is now piled up in jagged forms along our rocky coast, it is sometimes hard to believe that there is water underneath it all. It seems like that water has temporarily turned into part of our terrestrial landscape. You can venture out on it in places on foot or on skis, animals use it to find new habitat, and people have even transported houses across it on sleds pulled by oxen. But, it isn’t part of the terrestrial world at all. Underneath, there is a lot of activity – fish swimming in the depths and lobsters peeking out from rocky caves. This is all part of the world’s biggest habitat – the aquatic one.

Biome is just a neat word. It comes from the Greek for life and is a community of plants and animals that live in the same habitat. There are seven biomes and the aquatic one is just one of them. But, it is bigger than all of the others put together – by buckets. Water covers more than 70% of our planet in some way or another. Not all that water is the same, though, and the biggest difference between “waters” is salt.

At the most basic, there is freshwater, which has less than 1% salt and saltwater, which is everything else. To get more refined, though, there is a whole mess of water in between that is a mixture known as brackish water. The varying salt content is empirical evidence of just how much mixing occurs.

While salt content varies widely, there are some characteristics that hold true across all aquatic biomes. The biggest one is insulation. This is something top of mind right now as we are deep in winter and often working hard to find ways to stay warm. Water is an amazing insulator and ice is even better, trapping air in much the same way that a down jacket does and keeping the water underneath it from getting too cold for life.

Rather than keeping warm, one of the biggest challenges for all things aquatic is getting enough sunlight. Sunlight only gets so far underneath the surface of the water. There is obviously not much sunlight getting through when there is ice on top. But, normally, it shines down into what is called the photic (“light”) zone.

Just how far down it gets depends on how clear that water is and how bright the sun is, all of which shift throughout the year and the planet. All the photosynthetic stuff lives in the photic zone and provides lots of nutrients for other creatures, including those living deeper in the aphotic (“not light”) zone.

There’s some really bizarre stuff living in the depths. Chimaeras, which get their name from the fire-breathing creatures or Greek mythology, are shark-like fish that are specially adapted to live up to 8,500 feet below the surface. Sometimes called ghost fish, they have large translucent eyes to see in the dark and, like sharks, use electroreception to find their prey when they can’t see them.

In addition to being very dark down there in what’s known as the twilight zone, there isn’t much oxygen either. Water absorbs oxygen from the air as well as gets it from photosynthetic organisms that need to live where there is light, which means there is plenty of oxygen at the surface, but little down deep. What there are lots of, however, are nutrients. Everything that once lived closer to the surface eventually dies and sinks to the bottom where it is broken down into nutrients by decomposers – a good recycling scheme.

Water is, perhaps, the best recycler of all. It turns from liquid to clouds to ice and salty to fresh, mixing and remixing along the way. At the end of the day, though, of all the water in the aquatic biome, about 95% of it is salty. We, in Maine, have a lot of that salty water around us – even when it seems like the two biomes have coalesced into one during the frosty winter months.

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