Life in the Epipelagic

The Epipelagic Zone

The epipelagic zone is the part of the ocean where there is enough light for photosynthesis to take place. In the clearest parts of the ocean this is the top 200 meters. All most all of the marine life that you can think of lives in the epipelagic, most of it along the continental shelf.

What is a Continental Shelf?

The Continental Shelf along South Central Alaska

The Continental Shelf along South Central Alaska, as seen on google earth.

Continental shelves are parts of continents that are submerged under water. Continental shelves extend from the shore line to a steep drop off known as the shelf break. Water depth along the continental shelf averages about 200 feet (60 meters.) At the shelf break, at an average depth of 460 ft or 140 meters there is a steep drop off known as the continental slope. In just a few kilometers water depth may drop to depth of 12-15,000 feet or 4-5,000 meters. Widths of continental shelves vary widely. Passive (non-geologically active) margins tend to have broad shelves and gently sloping continental slopes. Broad continental shelves can be found spanning the Bering Sea, the South China Sea, the Gulf Coast of Florida, and New England. Active margins tend to have narrow shelves and steep continental slopes. Active margins make up nearly all of the coastlines of the Pacific Ocean, including the West Coast of the United States, and the South Coast of Alaska.

Life on the Shelf:

Image from: Fisheries and Oceans Canada

Image from Fisheries and Oceans Canada

Continental shelves only make up about 6% of the ocean’s surface area, but they are biologically one of the richest parts of the ocean. Why so much life on the shelf? First the water is shallow enough that light can penetrate to the bottom. Second, wind and wave action (especially in the winter) continuously replenishes the supply of nutrients that the smallest marine organisms need to grow.? Third, many organisms are going to require a substrate or surface to attach themselves to.? On the continental shelf, the bottom surface is highly accessible.

For our purposes here we will group organisms that live on the shelf as plankton, nekton, and benethos. Plankton are drifters. Nekton are swimmers. Benthos are bottom dwellers.


While oceanographers are much more concerned about the physical characteristics of the ocean than the biological. The study of plankton and the factors that make them grow are where oceanographers and marine biologists meet.

Plankton are organisms that drift with currents close to the surface. Many are small algae, but there are large examples as well. The Portuguese Man-o-war can grow to over 100 feet long. What makes a plankton is not an organisms size, but rather its lack of capacity to move against the current. Plankton are drifters. They spend their lives caught in the currents. When we talk about plankton its helpful to divide the group into phytoplankton (plant like algae) and zooplankton (animal or animal like.)

Algae bloom near Seattle, WA

An algae bloom near Seattle as viewed from a passing airplane, July 2015, credit Brian Mason


Phytoplankton, (which include the diatoms, dinoflagellates, blue-green algae, and coccolithophores) make up the bottom of most ocean food webs. Phytoplankton are able to manufacture sugars and other biological molecules by combining carbon dioxide and water in the process of photosynthesis. Oceanographers use to term primary productivity to describe the rate at which phytoplankton and other producers convert carbon dioxide into organic compounds. The rates of primary productivity vary greatly based on location and time of year.

Large growths of plankton are often called plankton blooms. The physical factors needed to get a plankton bloom are highly variable but generally involved the combination of light and key nutrients such as nitrogen, phosphorus, silica, and iron. In Alaskan waters, sunlight is highly abundant in the spring and summer, and nearly absent in the winter. Most suspended nutrients get used up early in the spring by blooming phytoplankton. As those plankton are eaten or die, nutrients work their way back to the bottom.  Intense winter storms will mix shallow waters along the continental shelf and bring nutrients back to the surface. When sunlight returns in April, all of the conditions necessary for phytoplankton growth will be present, and the North Gulf of Alaska will experience a phytoplankton bloom. You can read my NOAA Teacher at Sea Blog or Katie Gavenus’s (NOAA teacher at sea from Homer, AK) blog to see how oceanographers study primary productivity in the Gulf of Alaska.

February average nitrate level at the ocean surface.

In the winter nutrients (such as nitrate) are abundant.

August average nitrate levels at the ocean surface.

By August plankton have absorbed surface nitrates and either died or become part of the food web.


Zooplankton get their names from zoo meaning animal and planktos meaning wanderer. The term zooplankton, loosely describes an incredible variety of animals and protozoans ranging in size from microscopic to over one hundred feet long. What they all share in common is that they are heterotrophic eaters of phytoplankton and they float at the mercy of the oceans currents. Zooplankton are generally found drifting in the upper reaches of the water column but many will make daily vertical migrations to several hundred meters.

Daily Commute

In what might be the largest migration on the planet, zooplankton make a daily commute between the lower reaches of the epipelagic during the day and the surface at night. Zooplankton need to spend much of their time near the surface feeding on the phytoplankton that float on or near the surface. However, they prefer to do so between dusk and dawn while they are less visible to the predators hunting them. As day light approaches they descend into the depths trying to stay just below the level of visible light.

daily zooplankton migration

You can see the daily migration of zooplankton up to the surface at night and down at dawn. Image from Ocean Networks Canada

Marine biologists had known for years that trawl samples taken at night produced more animals than those taken during the day. However, the extent of the migration wasn’t realized until World War II when, ships sonars looking for enemy submarines detected a puzzling layer on their sonar screens. This deep scattering layer, or false bottom as it was called, rose toward the surface each evening, and sank again the next morning.


Nekton are organisms that are capable of swimming against currents. They can be found at a variety of depths, from shallow to deep. They move through the water column with ease and can be highly migratory. The nekton includes a variety of organisms but is dominated by marine mammals and fish.


Benthos (or benthic) are organisms that live on the bottom. Benthic communities vary greatly based on what is on the bottom. Roughly speaking, the bottom is either hard or soft. Hard bottoms are made up of exposed rock with very little sediment. Depending on location, rocky bottoms may be dominated by algae, kelps, or corals. These hard surfaces provide places for organisms to attach themselves. Soft bottoms may be sandy, silty, or muddy. In a few places of the world they may be inhabited by sea grasses, but in other places they may be completely un-vegetated. Muddy bottoms are often dominated by organisms that can burrow themselves into the sediment.

Rocky Bottoms

Rocky bottoms are rich and productive places, often colonized by kelps and seaweed’s. Coral reefs also require hard bottoms to grow, but we will look at them in their own chapter. Rocky bottoms are home to many types of animals including; suspension feeding organisms (such as sponges and barnacles), grazers (such as urchins, chitons, limpets, and abalone), and carnivores (such as crabs, octopus, and fishes). In Alaska, rocky bottoms are dominated by a variety of attached filter-feeding organisms that include corals, sea pens, sponges, sea anemones, barnacles, and mussels. Like all suspension feeders, these animals take water into their bodies and filter out detritus and dissolved organic matter. They make up the bottom layer of a food web that includes numerous species of shrimp, fish, and marine mammals. Any place with a hard surface to cling to, and provides a seasonally abundant food source, is considered prime habitat for kelps and seaweeds.

bullwhip kelp

Bullwhip Kelp

Kelp Forests and Kelp Beds

Kelp is a generic term for large, fast growing, brown algae. Kelp generally grows in cold, nutrient-rich, sun-light waters. Under ideal conditions, kelp grows rapidly and will accumulate into dense stands called kelp forests. Kelp forests cover as much as 25% of the worlds coastlines and maintain high levels of biodiversity. The kelp provides protective places to hide for some species and abundant food for others. Kelp is not a plant, but rather a brown algae. Kelp doesn’t have roots, shoots, or leaves. Rather kelp attaches to the substrate (or hard bottom) via a holdfast, have a stemlike stipe, broad flat blades, and floats called pneumatocysts.  Instead of leaves kelps have leaf-like blades called fronds.

Soft Bottoms and Seagrass Communities

Un-vegetated soft bottoms are often unstable, shifting constantly in response to waves, tides, and currents. They can include sandy, muddy or silty bottoms. These are difficult places for plants or algae to grow. However, in a few places around the world soft bottoms may be carpeted by seagrasses. Seagrasses, are true plants with roots anchored into the soft bottom. Seagrasses help to stabilize soft bottoms; leaves cut down wave action and currents. Roots collect finer sediments and water in sea grass beds is generally clearer because of sea grasses capacity to trap sediments in the water. Seagrass beds are also highly productive.  They feed herbivores and detritivores directly, and other organisms indirectly, by providing sheltered habitat.

By Colin.faulkingham at en.wikipedia (Create by Colin Faulkingham in Summer 2007.) [Public domain], from Wikimedia Commons

Sea grasses, image from from Wikimedia Commons

Benthic Sediment Types

There are three main types of sediments found on the ocean bottom. They are classified by their source of origin. Lithogenous or Terrigenous Sediments are inorganic, formed by the breakdown of rocks on land. Biogenous Sediments are organic, composed of the skeletons, shells, and decomposed remains of marine organisms. Biogenous sediments tend to be found close to the continental shelf. Click to read more about these different sediment types.

Types of Sediments

Sediments are inorganic debris that settles on the ocean floor. Sediment can be classified by by origin, i.e. where it comes from. There are four types: lithogenous, hydrogenous, biogenous and cosmogenous. Lithogenous sediments come from land via rivers, ice, wind and other processes. Biogenous sediments come from the mineralized skeletons of plankton and other organisms. Hydrogenous sediments are precipitated from chemical reactions in the water. Cosmogenous sediments come from space, filtering in through the atmosphere or carried to Earth on meteorites. Sediments can also be classified by size.

  • Gravel is any material greater than 2 millimeters in its largest dimensions.
  • Sand is any material between 2mm and 0.06 mm in size.
  • Silt is material which is finer than sand, but still feels gritty when rubbed on your teeth.
  • Clay is the finest material of all, and pure clay will feel smooth on your teeth, and will form a sticky ball when wet.

Sediment is not distributed uniformly throughout the ocean. Not surprisingly, lithogenous sediments dominate the near shore areas of the continental shelf. Biogenous dominate the deeper areas of the ocean, because there is so little of other kinds of sediments but they are also common in near shore areas, particularly in the tropics. Ooze refers to deep-ocean sediment that is comprised of more than 30% biogenic material. Calcareous ooze is comprised mostly of plankton with calcium carbonate skeletons. Siliceous ooze is comprised mostly of plankton with silicon-based skeletons. Hydrogenous sediments make up only a very small portion of ocean sediments and are primarily found near geologically active areas.

Questions to Research:

As always, your textbook may be used as a resource. For questions about life on the continental shelf, see chapter 13.

  1. Describe three physical features of the epipelagic.
  2. Use the Nauplius data explorer  to identify what months nitrogen, phosphorus, and silicate are most available in the Gulf of Alaska, and what months they are least available.? Do your best to explain why the levels of these nutrients go up and down.
  3. Why is most life in the ocean concentrated on the continental shelf?
  4. Read my NOAA Teacher at Sea Blog or Katie Gavenus’s (NOAA teacher at sea from Homer, AK) blog and describe one way oceanographers study primary productivity in the Gulf of Alaska.
  5. Ready my NOAA Teacher at Sea Blog about collecting zooplankton or Cara Nelson’s (NOAA Teacher at Sea from Bartlett High) Blog and describe why oceanographers who collect zooplankton primarily work at night.
  6. Mixotrophs are organisms that can both photosynthesize and feed on other organisms.? Read NOAA Teacher at Sea Katie Gavenus’s Blog about mixotrophs.? Describe the two different strategies that lead to mixotrophy.
  7. Describe at least two ways in which a kelp is not a plant.
  8. Explain one key ecological role that sea grass plays in the continental shelf.? In your explanation, be sure to talk about photosynthesis, nutrients, or habitat.
  9. Check out this map of ocean sediment thickness. ?Based on your observations, is ocean sediment thicker along the continental shelf or in the deep ocean? ?Explain why you answered the way you did.
  10. Compare and contrast biogenous and lithogenous sediments.? What is the source for each and where are you most likely to find each kind of sediment on the ocean bottom?