The ocean is a vastly mysterious thing to humanity.  It's a widely lauded factoid that we know more about space than we do about our marine ecosystems, the deep sea in particular.  This by no means shows humans don't want to know more about it.  Quite the contrary really.  The ocean just so happens to be one of the most hostile environments for us to explore and we struggle to create tools that can withstand the harshness of our planet's deepest regions.

What little we do know is quite interesting, so we're going to spend a bit of time delving into this beautifully treacherous marvel that is the marine biome.  We'll be taking a look at some of the more accessible ecosystems before going for the deep dive, so let's get started!

What is a Marine Ecosystem

First things first, we should define what a makes a marine ecosystem a marine ecosystem.  The key characteristic that sets them apart from other water-dependent regions is their salinity.  It's not a big secret that the Earth's oceans are salty.  Anyone who has accidentally caught a mouthful of seawater tends not to forget!  This factor is how we categorize marine ecosystems as different from freshwater counterparts.  You may even find a bit of overlap here with our wetlands page from a while back!

Beyond that key identifier, the rest of the biome's ecosystems are left up to more nuanced categorization.  Likely with the exception of deep sea ecosystems.  Though they make up a majority of the Earth's habitable space, we don't know much about them beyond categorization by ocean depth.  Still, there are a good number of ecosystems to explore before we jump off the deep end!
Marine ecosystems make up a vast majority of the Earth's habitable regions, but due to their harshness are some of the most mysterious as well

Types of Marine Ecosystems

Let's start at sea level, where humans are able to get a decent idea for how marine ecosystems thrive.  As discussed in our wetlands topic, water and land don't tend to stay completely apart from one another.  Where they intersect (and how they intersect) introduces us to an array of saline wetland ecosystems.  These areas are highly subject to tides in addition to how the continental shelf forms around them.  They may not exist all the time, or may at least vary in size throughout the year depending on tidal activity.  Regions that have this dependency are known an intertidal zones.
Not all marine ecosystems are in the ocean!

​Salt marshes and mangroves are areas where salty ocean water has creeped in and created ecotones with the land!

Intertidal Zones

Ecosystems that are highly intertidal include rocky shorelines, beaches, and tide pools.  Marine life capable of dealing with some time out of water can be found here along with terrestrial and seafaring bird species.  While not as volatile, marine flooding influences another ecosystem known as a salt marsh.  Recalling our wetlands knowledge, we know that to be a wetland where a grassland is inundated with water enough to build communities resilient to, and at times reliant on, that factor.  What makes this different from a typical marsh is...you guessed it, salt.

Mangroves

On the swampier side of wetlands, we have our mangroves which are forested saline wetlands.  The trees and other plants that grow in these regions can dip their roots into the saltwater and have adapted to thrive under that condition.  If we stay at the intersection of the open ocean and continental shelf, we can also find lagoons.  Lagoons are wetland ecosystems where its communities are separated from the larger ocean by sandbars, reefs, or other terrestrial structures.

Kelp Forests

As we move a little further out from shoreline, but not so far out the seafloor isn't exposed to sunlight, we can find some pretty diverse ecosystems.  The kelp forest is essentially an underwater forest where kelp is a keystone species.  Being a keystone species, the communities that inhabit kelp forests depend highly on kelp for their energy and shelter needs.

Coral Reefs

Similarly, we have coral reef ecosystems where the keystone species is coral.  Coral is, despite its relative sedentary lifestyle, a part of the animal kingdom.  What we see as coral reefs are the result of many corals excreting calcium carbonate via polyps.  These stony skeletons form ecosystems for some of the most diverse communities across the world.  Coral reefs are extremely fragile systems, however, and are subject to even minor changes in water quality and behavior.
There is a good amount of research material on these ecosystems closer to the ocean's surface.  There are even more nuanced categories of these systems based on where they are located in the world.  It's probably fair to say that each of these systems could be explored more on their own, but let's take a look at something a bit more mysterious now...

The Deep Sea

Just to preface, as with the other ecosystems, the open ocean probably deserves its own page in kind, but we'll take a look at how we currently categorize its sections.  We split the open ocean into 5 major parts: epipalegic, mesopalegic, bathypalegic, abyssopalegic, and hadopalegic.

The epipalegic zone is something we're already familiar with!  This is the surface region or sunlight part of the ocean.  Only in this region can plant life perform photosynthesis.  This region can go as deep as 200 meters, but generally stops around 150 meters.  This is the most biodiverse depth of the ocean and the other depths, in some way, depend on the organic matter and systems that drive this region.
Kelp forests and coral reefs are not only some of the most biodiverse areas in the ocean, but the entire world.

Coral reefs are even affectionately referred to as the rainforests of the sea​!
Up next is the mesopalegic zone, also known as the twilight zone.  Very little sunlight reaches the top of this depth, and creatures have adapted to this by having very wide eyes to take in as much of the minimal available sunlight as possible.  This region spans depths of around 200 to 1000 meters and is also home to what is known as the thermocline, or the range of depth where the ocean's temperature drastically changes, dropping around 20 C at the bottom of the region.  Any creatures trekking across this zone need to be capable of surviving both the drastic temperature shift as well as the pressure shift.

Below that is the bathypalegic zone, also deemed the midnight zone.  From this point downward, we are in complete and utter darkness.  Creatures here likely have no form of traditional light-based sight.  Most have also shed any skeleton or fragile organs.  That's because here, every 10 meters you descend, you introduce yourself to another atmosphere worth of pressure!  While the twilight zone housed predators that may swim up to the sunlit zone for meals, this level is almost completely dependent on detritus from the higher zones.  This falls in the form of what is called marine snow, since the organic remains that fall through the ocean tend to look like snowfall to those who see it.
Beyond the sunlit zones of the deep sea, every 10 meters of depth adds another atmosphere of pressure
Finally, we have the abyssopalegic and hadopalegic zones.  As their names insinuate, these regions are the harshest the ocean has on offer.  Ridiculously cold temperatures accompanied by hundreds of atmospheres of pressure make this a region of sheer survival or immediate death for most that would be exposed to it.  The hadopalegic zone is named after Hades for reason, a region so deep, it is reserved for the deepest depth of the Earth's oceanic trenches, reaching over a whopping 1000 atmospheres of pressure! 

​Interestingly, these areas are not lifeless.  Especially around the oceanic phenomena of hydrothermal vents, some creatures have adapted with what is known as chemosynthesis, or the ability to transform hydrocarbon minerals into energy, as sort of replacement for the photosynthesis of the ocean's surface.  Granted these are mostly tube worms, but it's crazy to think that anything could thrive in these conditions!

Humans and the Sea

Keeping on the topic of the oceanic zones for a bit longer, humans long thought the deepest regions of the ocean to be barren.  Even in our modern era, we've rarely visited the depths of the ocean.  The only human-piloted excursion to the abyssopalegic zone was in 1960, where we had first-hand observations of life this deep within the Earth's waters.  We've had several unmanned vessels make the trek, but it is still a challenge to make it that far without succumbing to the unrelenting pressure.

Back up toward the surface, humans have a lucrative relationship with marine ecosystems.  Prior to air travel being a proven and eventually affordable mode of transportation, marine ecosystems provided the primary means of human movement across the globe.  That holds less true today, but we still depend on our waterways to transport goods across long distances.  For humanity's entire time on this planet, we've also used marine ecosystems as a source of food and materials, which does still hold true today.
Despite our clear gains from marine ecosystem services and our general lack of knowledge about how they function, humanity has done quite a number on these biodiverse regions.  Overexploitation of resources, offshore drilling, and agricultural runoff push a ton of stress onto these environments, causing quick and drastic shifts that can leave entire areas devoid of life.  And, of course, our most amorphous contribution to the planet, anthropogenic climate change throws the ocean's equilibrium with the atmosphere off.  To summarize there, the ocean constantly trades oxygen for carbon from the atmosphere.  With the rise of carbon in the atmosphere, the ocean has been pulling in more carbon than it normally does, leading to ocean acidification.
The ocean surface constantly pulls carbon out of the atmosphere, serving as one of the planet's greatest natural carbon sinks
In short, humanity's behavior on land, in addition to our surface-level interactions with marine ecosystems, has fairly substantial ripple effects on the entirety of the global marine footprint.  What we can learn from this is that where we were able to negatively impact oceans from land, we can have the reverse positive effect as well.  Helping our marine ecosystems can be done on multiple levels, by directly interfacing with the marine biome itself and by doing our best to enact greener policy and behavior on land.

Marine ecosystems are one of the Earth's remaining mysteries and humans should be doing everything we can to ensure they survive alongside us so we can, in turn, witness their beauty in the future.  And there is a bit of hope knowing that in doing our part treating the Earth better on land, the oceans will thank us too.
~ And, as always, don't forget to keep wondering ~
Sources
Ecosystem Types
https://www.thoughtco.com/types-of-marine-ecosystems-2291779​
Ocean Zones
 https://oceanexplorer.noaa.gov/edu/curriculum/section5.pdf
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