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Flowing with

The Freshwater Biome

Water.  It's truly a necessity for life on Earth.  And some of that life needs a particular variety of that water to boot.  A vast majority of our planet's water exists as ocean, which isn't particularly helpful to these lifeforms, humans included.  The salt content in the ocean is simply too much for our bodies to process, counter-intuitively leading to dehydration.  Which is rather unfortunate, since oceans make up 97% of Earth's reasonably available water.

Today's topic is about a portion of that remaining ~3%.  It's this remaining water, freshwater, that humans and many other plants and animals rely on for their source of hydration.  How this type of ecosystem even exists is a wonderful example of the balance that the Earth's ecological systems are in.  So, without further ado, let's dive in!

Crater Lake Panorama, Aug 2013.jpg
By Epmatsw - Own work, CC BY-SA 3.0, Link

What are Freshwater Ecosystems

We've already mentioned that freshwater ecosystems are different than the vast majority of aquatic ecosystems (at least by the space they take up) on Earth.  There are other differences like temperature, nutrient availability, and how much light penetrates their water, but the primary factor setting them apart from the marine ecosystem is salt content.  Freshwater, by definition, contains 500-1,000 ppm (parts per million) of dissolved salt content.  By comparison, ocean water contains 35,000 ppm of dissolved salt!

Beyond the composition of freshwater ecosystems, they can be classified into 2 general types: lentic and lotic.  Lentic translates to "still" which would include freshwater systems the likes of lakes and ponds.  Lotic, on the other hand, translates to "flowing" which would encapsulate our moving water systems such as rivers and streams.  Each of these ecosystems can blend into one another (termed an ecotone), so it's good to recognize that these categories aren't always hard-and-fast rules.  Another ecosystem that we've also discussed, wetlands, have fresh- and saltwater variations as well, further breaking down types of ecosystems based on the aquatic ecosystem's relationship to a terrestrial area.
The marine biome makes up more than 97% of the Earth's water, leaving less than 3% freshwater on the planet.

​2% of that is glaciers, so freshwater ecosystems make up less than 1% of the Earth's water!
All of this variety may make it sound like these ecosystems are plentiful, but you would be quite surprised.  At least, I was when realizing that the "remaining ~3%" mentioned earlier isn't the whole story.  Of that percentage, 2% is locked in glaciers, leaving less than 1% for available groundwater and surface-level freshwater ecosystems.  That's right, this ecosystem responsible for the life of tens of thousands of species on Earth only makes up less than a percent of available water on the planet!  They truly are a precious resource, and the fact they exist at all is something of a matter of luck.

How Do Freshwater Ecosystems Form

We mentioned earlier that oceans are very saline.  They get this property from the minerals and nutrients that dissolve into the water from the land as the water interacts with it.  This does happen on the fringes of the marine ecosystem, where water touches the land, but that's not the whole story.  The bigger picture is that most of these added minerals are from water finding its way back to the ocean.  But how does water "travel back" to the ocean?
Freshwater always flows from a high point to a low point
Thanks to the hydrologic cycle, water does have a way of leaving the ocean.  By way of evaporation from the ocean's surface, water can exit the ocean and rise into the atmosphere as water vapor.  When this happens, the mineral content (including salt) is left behind in the ocean.  This water vapor eventually reaches an altitude where it will form clouds with other water vapor molecules.  Changes in atmospheric pressure will eventually cause this cloud to precipitate, or convert the water vapor into liquid water which rains down from the sky.

But there's an interesting problem there.  Did you notice it?
When water vapor rises into the sky, it's not going to move anywhere but up.  With only this evaporation at play, clouds will form and the water will come back as rain, but it's just going to rain back down on the ocean.  The cycle only takes water out to immediately put it back in.  That's not how the Earth works though.  It has a little help from the wind!  Wind pushes these clouds of water vapor over land (a process termed advection) so when the cloud does precipitate, it does so over terrestrial area rather than the ocean.  It's this precipitation that allows that saltless rainwater to collect and form freshwater ecosystems.  Without these systems existing and in balance, freshwater on Earth would likely be entirely made up of glaciers!

With freshwater precipitation as a catalyst, ecosystems follow by falling into basins to form lakes and ponds, or places of high elevation where the water can flow "downward" forming streams.  Water that lands in areas where these criteria aren't met are generally absorbed by plants or pulled into groundwater aquifers, essentially underground lakes where they may sit for eras waiting to be tapped or slowly drain to the ocean.
On the note of streams, these usually start from mountainous regions, but that isn't a hard requirement.  As long as the water can flow downward from a higher elevation, it can form a stream.  These streams may eventually flow into other streams, acting as a tributary for a larger lotic ecosystem called a river.  These rivers eventually make their way to the ocean, forming a delta, the meeting point with a lentic ecosystem.  It's here that the river deposits not only water, but the dissolved (and undissolved) contents of all the minerals that water collected on its epic journey from the sky to the sea.  The cycle repeats, and persists the existence of the ecosystems that formed around the systems that make life possible on this planet!

Veins & Highways

Beyond sustaining life and providing places for plants and animals to call home, freshwater ecosystems uniquely affect the planet.  These habitats depend on the movement of water, and with the movement of water comes the deconstruction of the land around it.  We call this effect erosion, where continued stress from land meeting a force eventually gives way to that force.  Water erosion is particularly interesting since it doesn't just pull minerals and nutrients from the land, it takes them somewhere else.
The freshwater biome only exists thanks to the water cycle and the Earth's wind system.

If not for these massive and balanced systems, clouds would not be able to rain over land!
In other words, how the land is shaped is largely dictated by how these ecosystems flow.  And further, how nutrients this planet's life depends on move do so largely with the help of these ecosystems.  After all, rocks don't move much, yet they hold a lot of minerals.  How do they move without the help of complex life?  Freshwater ecosystems.

In a sense, rivers and streams (and the lakes and ponds that may feed and direct them) form the veins of the Earth.  So much life would not be possible without their existence.  Even the way they meander influences how much surface area a flowing water source covers.  If rivers traveled straight, how life took hold on Earth might be totally different.  While these shapes are a side effect of how streams and rivers extend themselves and how force pushes land over time, these effects allow for life to exist in more than just around a straight line.
Rivers and streams are effectively mineral highways, unlocking nutrients from the Earth and shuttling them to the ocean to cycle back into the land
Even as humans, we eventually recognized a desire to make travel easier for ourselves and the things we need to transport.  Our "invention" was highways.  Dedicated routes for us to move ourselves and things quickly over long distances.  In many ways, we can see this as an inspiration from our rivers, which before they carried us, they carried (and still carry!) the minerals this entire planet needs to cycle to continue allowing life to exist at all.

It's crazy to consider how much is accomplished with so little of the Earth's stores of water.  And how much comes of an extremely tuned and intricately balanced system.

Humans & Freshwater

Of course, humanity has done more than take inspiration from our freshwater ecosystems.  We're pretty good, as a species, at using resources at a rate that isn't healthy for the environment at large, and freshwater systems aren't much different.  But because they are uniquely important for our survival, the ways we exploit them have a much greater ripple effect.  Similar to rainforests, which we exploit for lumber, extinction rates for fauna of freshwater regions are about 5 times higher than other types of habitat.
Though, one interesting thing to note is that humans did recognize early that monitoring freshwater was an important thing to do, albeit for selfish reasons.  Early monitoring of freshwater ecosystems wasn't so much for the health of the environment, but for the health of humans around that ecosystem.  After all, drinking unhealthy water wouldn't be great for people.  As a result, these monitoring efforts were mostly chemical observations, and extremely focused on whether levels were good for humans, not balanced and healthy for the communities in the habitat.
This rampant over-exploitation leads to other rippling effects such as habitat degradation, water pollution, and flow modification.  The last one is particularly interesting, since freshwater streams and rivers are intended to reach the ocean.  They're a part of a system intended to deposit minerals into the ocean for other cycles to be maintained.  When we interrupt this system for our own gain, we risk interrupting systems we benefit from without even realizing it.  The worst part is that these system work on geologic time, so we won't feel the effects of today's interruptions for eras to come.
Over 100,000 species across the world depend on freshwater ecosystems
Conversely to acts of modifying flow, we also saw the "mineral highway" effect when agriculture began industrializing.  This is in the form of runoff from chemicals used in farming, taking those minerals away from the source, but depositing them in the ocean.  The largest offender here has been (and still is) nitrogen-loaded fertilizers.  These not only impact the freshwater ecosystem with an over-saturation of nitrogen, but when this dump of nitrogen hits the ocean, it supercharges algae at the delta, causing a massive bloom followed by death that drains the area of oxygen, forming what have been deemed "dead zones."  Using these systems in unintended ways has effects not only on the system we exploit, but has downstream (literally in this case), effects make life as we know it literally impossible.

Unfortunately for freshwater ecosystems, the damage we've done is quite extensive.  This isn't to say that helping clean our lakes and rivers isn't good, it very much is and we should help to the extent we are able, but the larger problem is bigger than picking up after ourselves.  Policy and systemic changes are necessary to behaviorally change how humanity interacts with our freshwater resources.  Doing the bare minimum of keeping our environment clean and well-maintained is still necessary, but it's short-sighted to think that's all that needs doing.  As a species, we need to change how we approach water as a resource and foster mindful environmental stewardship for key factor of our very existence.

It's well within our ability to care for our environment and ensure our coexistence with freshwater ecosystems is one that fosters a healthy habitat and lets our Earth keep working in a way that supports us for eons to come.​
~ And, as always, don't forget to keep wondering ~
Prismatic Planet
Sources
https://en.wikipedia.org/wiki/Freshwater_ecosystem​
https://www.annualreviews.org/doi/10.1146/annurev-environ-021810-094524​
https://coreybradshaw.files.wordpress.com/2015/09/dudgeon-et-al_2006_freshwater-biodiversity-importance-threats-status-and-conservation.pdf
​https://www.thoughtco.com/rivers-from-source-to-sea-1435349​
​https://www.britannica.com/place/Mississippi-River/Physical-features​
​
https://www.usgs.gov/special-topic/water-science-school/science/saline-water-and-salinity?qt-science_center_objects=0#qt-science_center_objects​
https://phys.org/news/2020-10-fourteen-freshwater-biodiversity.html

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