Recognizing the ocean zones
Oceanographers have divided the ocean into zones to better understand and describe the physical characteristics of the ocean, the ecosystems (biological communities) in each zone, and the inhabitants of those ecosystems. Zoning can be simple, such as dividing the ocean into two zones — photic and aphotic:
Photic (light): The top 200-meter (650-foot) layer of the ocean through which enough light penetrates enabling photosynthesis to occur. (Photosynthesis is the process of using the sun’s energy to produce food from carbon dioxide and water.)
Aphotic (dark): The part of the ocean from 200 meters down to the bottom, where it’s totally dark.
Another simple zoning system involves dividing the ocean into pelagic and benthic layers:
Pelagic (top): The water above the ocean floor.
Benthic (bottom): The seafloor and the thick layer of sediments below the seafloor.
In Chapter 4, we cover two more-detailed approaches to zoning the ocean — one that divides it into five horizontal layers (like layers of a cake) based on depth, and another that divides it into three vertical zones from coast to open ocean.
Dropping in on the different ecosystems
An ecosystem is a biological community of organisms interacting with their physical environment as a whole. Think of it as a mostly self-contained, self-reliant neighborhood with a diverse population. Land-based ecosystems include grasslands, deserts, rainforests, and wetlands. Common marine ecosystems include coral reefs, estuaries (where fresh water and salt water mix), kelp (seaweed) beds and forests, mudflats, rocky shores, sandy shores, seagrass meadows, and more. Lesser known ecosystems develop near the bottom of the deep sea and include communities that form around hydrothermal vents (which spew hot, mineral-rich water that some bacteria feed on), whale falls (literally, dead whales that sink to the bottom), and cold seeps (where methane gas is released that some bacteria and archaea feed on).
What’s so fascinating about ecosystems is that the community of residents that live within them evolved together, adapting to the unique conditions of a particular place as well as each other. In Chapter 5, we explore numerous marine ecosystems and introduce you to the plants, animals, and other organisms typically found in each.
Going with the Flow: The Physical Properties of the Ocean
Even without all the wonderful living organisms that call it home, the ocean is amazing. In fact, it is very much like a living thing itself; it breathes, it moves, it’s constantly changing, and it interacts with everything around it — land, water, and air. It plays a huge role in controlling Earth’s climate and making the weather, and it distributes heat and nutrients around the globe, making the entire world more habitable for every living thing.
In this section, we get physical by focusing on the salt in seawater, the various processes that maintain a steady flow of water around the world, and the interactions among land, sea, and air.
Getting up to speed on the water cycle
While plants and animals have life cycles, water has a life cycle of its own. Following the rule of “what goes up must come down,” the life cycle of water, commonly referred to as the hydrologic cycle, describes the way water travels around the globe from ocean to air to land and back again to the ocean (see Figure 1-2).
©John Wiley & Sons, Inc.
FIGURE 1-2: The hydrologic (water) cycle.
Water is a wonderous element which can take on three different states of being. It can exist in a solid state (ice), a liquid form (water), or a gaseous state (steam), in which it evaporates and becomes vapor (humidity). When humid air is cooled, the water forms droplets and falls back to the earth as precipitation — in either liquid form (as rain) or in solid form (as snow or hail).
Knowing what makes seawater salty
Why are most lakes, rivers, ponds, and streams all freshwater, whereas the ocean is salty? Mostly because of the water cycle. Most of the salt in the ocean comes from freshwater rivers and runoff from land. As the water moves over the land and rocks and through the earth, it picks up minute traces of salt and other minerals, which it then carries to and deposits in the ocean.
When water evaporates from the ocean, the salt remains, while the water vapor either precipitates over the ocean or is carried over and deposited as precipitation on land, where it washes more minerals into the ocean. This process “quickly” (that is, on a geological time scale) increased the concentration of salt in seawater to the level at which it is today — on average about 3.5 percent.
Investigating variations in salt concentrations
Because water evaporates faster from some parts of the ocean than others and rainfall and river discharge to the ocean isn’t uniform around the globe, certain parts of the ocean are saltier than others. For example, salinity in the Red Sea ranges from 36 ppt (parts per thousand) to 41 ppt (about 3.6 to 4.1 percent). In the Mediterranean Sea, surface waters average 38 ppt but can approach 40 ppt in the extreme western parts. Saltwater taffy, anyone?
Salinity also fluctuates in certain areas of the ocean. For example, in polar regions, the salinity of water near the surface increases as ice forms, leaving much of the salt behind. As the ice melts, salinity decreases due to the influx of fresh water. Likewise, in coastal areas, salinity is lower in areas where freshwater from rivers and streams enters the ocean. When the salinity dips below about 30 ppt, the water is often called brackish.
Realizing that sea water is more than just salty water
The recipe for cookin’ up a batch of seawater might seem quite simple: Start with 1 gallon (about 4 liters) of water, add about 4 tablespoons (60 grams) salt, and shake till dissolved. However, seawater is more complex than mere salt and water — it contains a very large number of chemical elements and compounds, including dissolved organic materials, particulates, and dissolved gasses, including oxygen. (Marine mammals can’t breathe the gasses dissolved in seawater, but apparently the crew of the movie The Abyss can.)
Don’t worry, we’re not going to deliver a lecture on the chemical composition of seawater, but knowing that seawater is more than just salty water is important for when we get into discussions about certain issues related to preserving the ocean’s health, such as climate change and ocean acidification (see Chapter 21).
Checking out what’s at the bottom of the ocean (and below)
At the bottom of the ocean is a landscape that’s as varied as what you see above the ocean, complete with plains, ridges (mountain ranges), seamounts (mountains), hills, canyons, trenches (valleys), cliffs, volcanoes, hydrothermal vents (underwater geysers), and even rivers and massive “waterfalls” of sand and sediment that you do indeed want to go chasing.
The shape of the ocean floor (as well as all land) is largely determined by a process called plate tectonics — the bumping, grinding, and separating of tectonic plates (massive sections of rock that form