The Largest Ecosystem on Earth: Deep, Diverse, and Definitely Different

The Census of Marine Life brought together thousands of researchers to “Make Ocean Life Count.” (I’ve written about it here and here.) But this amazing effort was more than just an enumeration of what lives where. The goal was much greater: to draw detailed seascapes of the many diverse ecosystems that make up the world ocean. And no ecosystem yielded more surprises, raised more questions, or led to more “ah ha!” and “no way!” moments that that of the deep sea.

The largest and most mystery ecosystem on Earth

The deep sea is defined as those waters too deep for sufficient sunlight to penetrate to support photosynthesis. This means that most of the organisms in the deep rely upon the sparse rain of organic matter from surface waters (where phytoplankton can capture energy from the sun. The exceptions are hydrothermal vents and cold seeps, where chemical energy is converted to food by bacteria. (I’ve written about some of those here).

The deep-sea includes the seafloor, and the pelagic zone. The seafloor is made of up continental slopes, abyssal plains, mid-ocean ridges, canyons, and seamounts, all of which support different types of life. The open water, below 200 m but above the seafloor is called the deep-sea pelagic zone. In this zone a third dimension is added: depth. The nature of water allows creatures to attain neutral buoyancy; living, feeding, and breeding with little interaction with either the surface or bottom.

Seafloor habitats
The NE Atlantic seafloor with continental margins, canyons (arrow), abyssal plains, seamounts and the mid-ocean ridge. © A° ge Høines, MAR-ECO.

From 200m – 1000m light penetrates but does not support photosynthesis. This is the mesopelagic zone, where creatures here live in perpetual twilight. Below that, in the bathypelagic zone, no light penetrates even through clear water on the sunniest days. Forever dark, it makes up almost 75% of the total volume of the ocean.

A food-poor environment with scattered oases of life.

The biodiversity of the deep sea is the highest on Earth. Embedded within the largely nutrient-poor deep seascape are oases of incredible richness. Small, short-lived, food-rich zones such as hydrothermal vents and whale falls (whale carcasses that have fallen to the seafloor) encourage rapid diversification into an amazing range of novel shapes, sizes and survival strategies. But these oases are rare. Only 0.5- 2% of the primary productivity from the surface ever reaches the deep-sea, making it the most food-limited ecosystem on Earth.

This limitation has placed enormous selective pressures on the organisms here, favoring both gigantism and dwarfism as strategies to survive. The energy needed to live is lower for large invertebrates than for small ones, favoring gigantism. Here’s one of my favorite examples: Bathynomus giganteus (the name says it all), an isopod found at 310-2140m. Most isopods are tiny, a few centimeters at most. This monster reaches 50cm – the size of a housecat! (For more about this crazy creature, check out this post by fellow marine blogger Kevin Zelnio).

Bathynomus giganteus - the giant isopod. I found one of these creatures (a dead one) on a beach in Rhode Island when I was in grad school. I left my prize out to dry and it was ripped to pieces by a raccoon! 😦 )

However with so little food available, big critters can’t congregate in large numbers, making finding a mate can be hard. Instead, some creatures have shrunk to to survive. While dwarfism is less efficient in terms of the energy needed to live, it allows a larger number of individuals to congregate and share resources. This increases the ability to find mates. On both ends of the spectrum, the need for both food and mates have forced deep-sea creatures to strike a balance between sex and size.

Although it is far removed from the surface, the deep sea is not immune to insults from above. Research from the Monterey Bay Aquarium Research Institute (MBARI) suggests that changes in the Earth’s climate can cause unexpectedly large changes in the deep-sea. I’ve written about that here. By far, the largest human impact on the deep sea is through fishing. Because food is so limited in the deep-sea, fish grow very slowly and are very easy to overfish. (This is why I NEVER eat orange roughy, also known as deep-sea perch, which is found at 180 to 1800 m).  Management of this region is complicated by the fact that most of the deepest regions occur in international waters, and far from sight.

Seventy-one percent of the Earth is covered by oceans, and more than half of that is below 3000m. Yet, we have explored less than 5% of that area, and sampled only a few football-fields worth in any detail. The Census of Marine Life shed some light on this dark mysterious zone. I’ll highlight more of their findings in future posts.

Like what you see?  Come on back – there will be lots more where that came from in 2011. Have a suggestion for a blog post? Leave it in the comments below! Want more Brave Blue Words? You can connect to me and this blog on Facebook and Twitter.


Ramirez-Llodra, E., Brandt, A., Danovaro, R., Escobar, E., German, C. R., Levin, L. A., Martinez Arbizu, P., Menot, L., Buhl-Mortensen, P., Narayanaswamy, B. E., Smith, C. R., Tittensor, D. P., Tyler, P. A., Vanreusel, A., and Vecchione, M.: Deep, diverse and definitely different: unique attributes of the world’s largest ecosystem, Biogeosciences Discuss., 7, 2361-2485, doi:10.5194/bgd-7-2361-2010, 2010.

“From The Desk of Zelnio: Bathynomus giganteus.” Posted on: April 4, 2007 11:53 AM, by CR McClain. Accessed: January 28, 2011, 15:47.


3 thoughts on “The Largest Ecosystem on Earth: Deep, Diverse, and Definitely Different

  1. Hi Danielle,

    I’m catching up on my blogs and was delighted to see this post. Well, delighted until I saw the picture of that isopod. Oh my gosh!

    I enjoyed learning more about the deep sea, and that interesting phenomenon of dwarfism. I look forward to more such posts!


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