By Epmatsw - Own work, CC BY-SA 3.0, Link
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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. |
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? |
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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 & HighwaysBeyond 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.
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The freshwater biome only exists thanks to the water cycle and the Earth's wind system. |
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 & FreshwaterOf 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.
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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.
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Over 100,000 species across the world depend on freshwater ecosystems |
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