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Ocean Habitats

Did you know that the ocean occupies 99% of all the living space on our planet and houses, perhaps, the greatest variety of living organisms? Yet when most people think of the ocean, they think of a place that doesn't have a lot of variety. I call this the "bucket of water" mentality. Scientists used to think this way too, that the ocean was a fairly homogeneous place with not a lot of places for organisms to exploit.

But the ocean (and even a bucket of water) displays incredible variations across its depths and surfaces, across its seasons and years, in virtually all of its physical, chemical, geological and biological variables. It's a highly dynamic place where the environment changes in space and time over all possible scales.

In this section, we explore further the nature of the ocean environment, the types of habitats we may find and the multiple forces that each endures. We also examine the variability of these habitats as a primary factor in determining the kinds of species we may observe at any given time and location. The combination of these variable most certainly has given rise to the incredible diversity of organisms that have evolved in the ocean, a diversity still largely unknown and perhaps unappreciated.

Topics covered here include:

A Few Helpful Ecological Definitions
Before we launch on a whirlwind, worldwide tour of the oceans, let's make sure we are all talking the same language and define a few useful terms.

What do we mean by the ocean environment?

The term environment is defined in the ecological sense as the complex of physical, chemical, geological and biological factors that act upon an organism and determine its form and survival. So the term ocean environment refers to all those factors in the ocean that affect an organism's ability to live, survive and reproduce.

The term habitat is similar to environment but more general. Most simply, a habitat is the place where an organism lives or where it is most commonly found. We usually designate habitats by location or physical setting.

Any given species occupies space on our globe, in what called its area. A species' area is simply where it occurs. We represent species areas by shaded regions on a map. Check out an Audubon bird book sometime (or any other kind of field identification manual). More times than not, they include maps with shaded regions where a particular plant or animal can be found.

A species' area may occur over a range of habitats. Thus, an organism may live in a number of habitats. Think about migrating birds or whales. Within the span of a year, they may travel across and utilize resources from many very different habitats. The point here is that a single habitat does not always define where a particular species live. However, just to confuse things, some species are confined to a single habitat. Got that?

For this reason, ecologists (scientists who study the relationships and interactions among organisms came up with what is known as an ecological niche. A famous ecologist named G.E. Hutchinson in 1957 defined a niche as a region (n-dimensional hypervolume) in a multi-dimensional space of environmental factors that affect the welfare of a species. That's a little complicated (but useful!) so we'll use an earlier definition, that provided by Joseph Grinnell in 1917, who defined niche as all the sites where organisms of a species can live (where conditions are suitable for life).

Let's explore this concept of a niche just for a moment because I think it will help you better understand one of the most fundamental topics of life: how an organism reacts to and within its environment.

Hutchinson imagined the niche like a multidimensional graph. Since most of us think in three dimensions only, let's stick to that. Imagine, if you will, three yard sticks arranged in three dimensions parallel to each other. Use Popsicle sticks or pencils if it helps you but basically what you are constructing are the axes of an X-Y-Z graph.

Let's start with just one axis, the X axis, and consider how we might represent the range of salinities (the saltiness of water) within which a particular fish can live. Paul Maslin at Chico State offers on his web site at http://www.csuchico.edu/~pmaslin/limno/niche.space.html this simple illustration for a Largemouth Bass and a Chinook Salmon.

Note that the range of salinities tolerated by a Chinook Salmon is much greater than the range of salinities tolerated by a Largemouth Bass (you knew that, right?). Thus we might conclude for this one-dimensional depiction of niche that the salinity niche for a salmon is larger than the salinity niche for a bass (and we would be correct if we concluded that!)

Now start thinking in two dimensions. How would we plot the range of tolerance of a species to two variables? Maslin does it like this:


What conclusions can you draw from this graph? Which species is more tolerant to temperature? Does it make sense that an organism living in a lake tolerates higher temperatures?

For the grand finale of this little niche discussion we're having, try to imagine three dimensions. This is where your yardsticks (or Popsicle sticks or pencils) might come in handy. In this example, we'll look at a single plant, affected by water, fertilizer and sunlight. Here we borrow from a Virginia Tech web page:
http://www.sv.vt.edu/classes/surp/surp96/laughlin/stat/3D_tutor/color_cube.html.

Spend a few moments and trace each axis. Can you pick out the ideal conditions for this plant?


If it helps, consider this drawing, which depicts plant growth versus water for varying amounts of sunlight.

 


If you're starting to think about niches as "space" in the sense that an organism occupies a "space" that might have multiple dimensions where each dimension is some variable that affects how it lives and survives, then you are starting to think like an ecologist!

Don't feel bad if these concepts totally baffle you. Come back to them from time to time. They are very heady topics and require simmering and meditation to fully understand them. The point here is to get you to think about the environment as a multi-faceted, multi-dimensional set of factors that determine where and when an organism lives, including humans.
You may also want to consider what Dr. Seuss has to say about the subject. (You do know Dr. Seuss, don't you?) In On Beyond Zebra (1955), he writes:

And NUH is the letter I use to spell Nutches,
Who live in small caves, known as Niches, for hutches.
These Nutches have troubles, the biggest of which is
The fact there are many more Nutches than Niches.
Each Nutch in a Nich knows that some other Nutch
Would like to move into his Nich very much.
So each Nutch in a Nich has to watch that small Nich
Or Nutches who haven't got Niches will snitch.

Read that aloud a couple times. It definitely will get your head spinning!

Back in two-dimensional space (this text!), we have another couple terms that help us better understand the relationship of organisms to their environment. Biogeography is the science that deals with the geographical distribution of animals and plants. Biogeographers seek an understanding of the relationship between an organism and its environment, with an aim towards describing the evolution of species in various locales and habitats across the globe.

Here's a good place to mention the word ecosystem. Formally defined, an ecosystem is a community of organisms and the non-living environment in which they interact. For our purposes here, it's useful to describe our ocean habitats as ecosystems because it implies something dynamic and variable, as most ocean habitats are.

The role of an organism in an ecosystem is one of the most important aspects of its ecology. Ecologists ask "what function does an organism play in an ecosystem?" And you might answer "plants provide food, herbivores transfer material and energy from plants to carnivores, bacteria decompose organic matter and make it available to plants, as so on." Some ecologists divide organisms into guilds, a category of organisms based on their ecological role. For example, fishes that eat zooplankton (animals that drift in the sea), whether in the open sea or a coral reef, are said to belong to the zooplanktivore guild. Birds that eat insects might belong to the insectivore guild. Actors who act might belong to the actor's guild. You get the idea.

All these ecological definitions serve a purpose: they not only help us to speak the language of ecologists and to better understand what we read (not only in textbooks but in newspapers and magazines), but they get us thinking about the interaction between physical, chemical, geological and biological processes, which is the major theme of this textbook and the field of oceanography. No organism is an island, to paraphrase John Donne, and for that reason, understanding the relationship of organisms to their environment is one of the most fundamental ideas we can study.
Major ocean habitats and ecosystems
Armed with these succinct definitions, let's see how we can divide the ocean into different "life zones" or geographic provinces. These zones define the major habitats in the ocean in which we find the major ocean ecosystems discussed below.

The most basic distinction between habitats in the ocean is the one that separates oceanic or open ocean waters from coastal or neritic waters. Oceanic waters (also known as the open ocean) are truly oceanic; they are not influenced by terrigenous or land-derived factors. Terrigenous factors include runoff of inorganic sediments (clay, silt, sand), dissolved substances (salts, nutrients, chemicals), non-point source pollution (oil, fertilizers, heavy metals), organic matter (fecal matter, sewage, plant remains, food) and debris (straws, plastic bottles, cigarette butts, etc.), which are largely confined to coastal waters (also known as neritic waters). Oceanic waters may receive terrigenous materials through the atmosphere, such as dust, aerosols and even balloons, but the influence of these factors is negligible. Nonetheless, there are no sharp lines in the water, so to speak, to separate these zones, so there is bound to be some overlap.

Oceanographers also use the sea bottom to separate oceanic from coastal waters. Waters above the continental shelf (the margin of the continents) are defined as coastal waters, while waters beyond the continental shelf (i.e., the deepest parts of the ocean) are defined as oceanic waters.

These distinctions help to define not only the physical and chemical makeup of the water, but the organisms that live there as well. Coastal waters are among the most productive on Earth; it's where the major commercial and recreational fisheries make their haul and it's where you and I go to the beach. Oceanic waters have been compared to deserts but don't let that fool you. The open ocean hosts many diverse and fascinating life forms, like blue whales, viperfish and giant squid.

In addition to proximity to land, depth also helps us to define major ocean habitats. But depth in and of itself is not a factor. When we talk about depth, we are really talking about light penetration and water pressure, physical factors that we'll discuss in more detail later. Just keep in mind that when you read the word "depth" that what we are really talking about are the factors that vary as a function of depth. (Remember your niche dimensions?)

To be continued...




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