Deep Sea 101: Introduction and What Is the Deep Sea?

This semester I am teaching a full on lecture course in Deep Sea Biology at my institution. It is a great opportunity for me and am very thankful that I can be at an institution that would enthusiastically give a PhD student this opportunity to freely develop this course how I wish. I am 2 weeks in and decided that all the work I am putting into my lectures should be shared with the world! The lectures in the Deep Sea 101 series will be rewritten into manageable blog units instead of posting my slides which I feel need a bit more context. The class is aimed at senior level undergraduates with a background in basic biology and ecology, hence this series will be written as such. I will try to have each new lecture up on Wednesday each week, but make no promises.

This series will be divided into 5 sections:

1. History of Deep-Sea Exploration and Changing Paradigms
2. Processed Operating at Ocean Scale Affecting the Deep
3. Adaptations and Patterns
4. Diversity of Deep Sea Habitats
5. The Human – Ocean Relationship

Throughout this series I hope the reader can develop a scientific appreciation for the ocean, understand the value of interdiscplinary science, demonstrate an understanding of the human – ocean interaction and get more comfortable with the scientific literature and how scientists present data. To guide your exploration of the Deep, we are using Tony Koslow’s The Silent Deep, selected chapters from Gage & Tyler’s Deep Sea Biology (1992), and papers from the popular and academic literature. You should enjoy Silent Deep, it is a very readable, nicely produced and illustrated book. It reads like a book, not a text is very affordable, a bargain for its price! (I plan on reviewing it from an instructor’s perspective at the end of the semester). We will also be using review articles from Oceanography, which provide typically well-laid out, well illustrated and readable topical issues written by scientists.

What is the deep sea? Opinions differ but the ramifications matter. Older definitions define the deep-sea as less than 200 or 400 meters, sometimes less than 1000 meters. Much of our affect on the deep-sea occurs within the first 1000 meters of the water column. But the average depth of the ocean just less than 4,000 meters so we are even barely aware of the extent of our impact.

Maybe to you the deep-sea means scary, sharp-teeth laden fish like the anglers or bioluminescent creatures and odd jellyfish. You may have heard about the real sexy habitats like hydrothermal vents or deep-sea coral reefs. But you will probably be most surprised to know that the seafloor looks like mud (see left). Occasionally there is a little stick of a sponge, lonely in the abyss. Perhaps a tripod fish moping about. Not much that appears to be going on. Just an endless expense of mud for as far as the walleye can see.

And you would be dead wrong. The muddy seafloor is alive with thousand strong armies of sea pigs on the march, millions of small worms burrowing about, or a wide variety of fish coming and going following ephemeral traces of food. The deep sea is the largest environment on the planet and is extremely variable in both time and space at any given point. The diversity and uniqueness of the deep sea fauna and the variety of deep-sea habitats rivals that of many terrestrial habitats, which we will touch on later in the series.

The ocean is characterized by distinct zones, the transitions of which are marked by thermal or pressure interfaces. In the diagram to the right you can see that the majority of the biomass in a water column occurs in the first 200 meters where the temperatures are still warmer and light penetrates through the water. But as light get filtered out, the temperatures shorten, very little biomass exists in the water column, an area referred to as the twilight zone. It is also one of the least explored and understand parts of our planet. As we approach the seafloor there is more biomass, but it appears to never accumulate so much to even begin to rival the amount of life at the surface. Part of this may be our lack of understanding of what happens in sediment communities. Traditionally we study the things we can see on top of the seafloor and have paid little attention to the infaunal community, those animals that lives among the sediment grains.

Our understanding of the deep sea is a case study in how technology allows us to peer even closer at every step. We are limited by our own imagination to conceive of devices that will carry us to the bottom of the ocean. Only once have humans gazed at the seafloor near the deepest point in the ocean – the Marianas Trench. A point known as Challenger Deep, in honor of the vessel that paved the way for large-scale, hypothesis-driven oceanographic science. Technology has allowed us to discard a paradigm of a uniform seafloor devoid of life, with few interesting topographic features or geologic history. Instead, with the onset of sonar technology and improved sampling techniques and devices, we’ve discovered an environment with a wide variety of seafloor features from submarine canyons and underwater mountain ranges to abyssal plains and deep trenches. The seafloor is littered with habitats (see left) and our pace of discovery in deep-sea shows no signs of slowing down.

NEXT WEEK: Lessons from the Census of Marine Life