High School Ocean Lesson Plans: The Intertidal Region

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Intertidal Region

Topics Covered in this Lecture:

The Rocky Intertidal is one of my favorite places to visit. It provides our first glimpse at the incredible diversity and complexity of marine organisms. The physical and biological forces that hammer on these organisms day in and day out offers further testimony to their remarkable adaptations. In this lecture, we take a brief look at some of the factors that impact marine organisms living on rocky shores and how these organisms make a living in such an environment.

Ecological Definitions

An ecosystem is defined as a community of organisms and its non-living environment. That means all of the organisms and all of the geological, physical, chemical and biological factors and interactions that affect them are included in an ecosystem. In truth, the only real ecosystem is our planetary ecosystem, the Earth and all its inhabitants. But in practice, we like to separate ecosystems into smaller units that helps us understand how they operate. Examples include the desert ecosystem, the tropical rainforest ecosystem, the chapparal ecosystem, even the the human ecosystem. In the ocean, we will study the rocky intertidal ecosystem, the kelp ecosystem, the upwelling ecosystem, the open ocean ecosystem, the abysso-pelagic ecosystem, the hydrothermal vent ecosystem, the coral reef ecosystem and the polar ice-edge ecosystem, to name a few.

A community of organisms is defined as a group of organisms or populations found living together. For a human community, this includes people, dogs, cats, parakeets, rats, ants, lizards, pigeons, etc, etc. You get the idea.

It is also useful to define population. It is all the individuals of a single species. That is, the human population includes only Homo sapiens. The domesticated cat population only includes domesticated cats, and so on.

Finally, we need to remember what defines a species. A species is a group of reproductively isolated organisms. Species can only reproduce with the same species. A species is an individually reproducing unit, in its simplest sense.

At the ecosystem level, species and populations and communities of organisms and their non-living environment organize themselves in ways that maximize or make efficient use of the flow of energy and matter. All energy on our planet comes from either the sun or the internal heat engine of the mantle. The sun provides continuous energy, but the flow of energy is always one way. Once energy is used, it cannot be reused or recovered. This one-way flow of energy is one of the rule of the Universe, the second law of thermodynamics to be precise.

However, matter--the minerals and chemicals that make up you and me and everything else on our planet--can be recycled. And it's a good thing. We have a limited amount of matter on our planet. A planet's worth of matter is all we've been given and we best take care of it. Of course, we could take some materials from the moon or from asteroids or from other planets, but we're not quite there yet. For the moment, the most efficient use of matter is to recycle it. And this is what nature has been doing for billions of years.

According to our best guess, some self-replicating group of molecules came into being about 3.8 billion years ago, to about 1.5 billion years ago, the only organisms living on our planet were the bacteria. This 2.3-billion year stretch of history is known as the Age of Bacteria. It was during this time that bacteria learned how to recycle matter. Photosynthetic organisms assembled raw energy and available minerals into food stuffs and heterotrophic organisms broke it down to acquire energy and the minerals they needed to live. The short of it is that all the biogeochemical cycles on our planet were formed during the Age of Bacteria. Neat, huh?

So, to recap. Energy flows, matter recycles.

These general physical laws are important to understand because they help us to understand how ecosystems are organized. The help us to better appreciate the intricate beauty of life itself. All ecosystems have general principles of organization that maximize the use of energy and make the most efficient use of available minerals. We will study the organization of ecosystems a little bit later on, but it helps to start thinking about it now.


Intertidal Zonation

Let's take the example of the Rocky Intertidal Ecosystem. The rocky part is the geological part, and it is defined as any hard substrate to which an organism (plant or animal) can attach. It may be a lava flow along the shore, a tilted and compressed ancient seafloor, or a mixture of large and small boulders and cobbles. As long as the substrate is stable enough for organisms to attach and survive, it falls into the category of rocky intertidal.

Now, where does the intertidal part come from? What does intertidal mean? Ever hear of an interstate? That's a road that goes between states. So the intertidal is the region between the tides. It includes all of the are of the shore that is alternately exposed during the lowest tides (the low low tide during spring tides) and submerged during the highest tides (the high high tide during spring tides.)

The period of exposure for an organism will depend on where it lives on the rocky substrate. If it lives right below the level where only the highest of high high tides reach, then it may spend 1-2 weeks out of water. Of it lives at the level where the lowest of low tides retreat only a couple times a month, then this organism may only experience exposure for a few hours twice a month.

Think about what this means for an organism. What kinds of physical factors impact an organism living on the shore? What are the advantages/disadvantages of living high in the intertidal region or low in the intertidal region?

Before we answer these questions, let me tell you about the most fascinating aspect of this whole intertidal thing. As a result of alternating exposure and submersion, because the rising and falling of of the tides determine the kind of physical factors that an organism living in the intertidal will experience, the rocky intertidal develops "life zones," bands of organisms that live together in distinct zones depending on their height above sea level. The phenomenon whereby organisms group themselves along the shore into distinct bands is known as intertidal zonation.

Check it out. Next time you are standing on a rocky shore at a very low tide, look down the beach, parallel to the shoreline. What do you see? If you look carefully, you will notice that there are wide bands of different colors tracking the shore. You may see a grayish or greenish band at the highest level, called the high intertidal; a brownish band at the middle level, called the middle intertidal; and a more colorful red or dark-colored band at the lowest level, called the lower intertidal. These bands of color mark the intertidal zones, and they are caused by the plants and animals that live in these regions along the shore. You might also notice a few organisms, like the sea louse, who resembles a cockroach, even living above the high water mark. These organisms live in what is called the splash zone.

Intertidal zonation occurs all over the world and it has intrigued marine biologists for centuries. What causes these bands of color, these zones where distinct communities of organisms can be found? Some scientists have spend a lifetime trying to figure that out and a few general principles have emerged.

Physical Factors

Remember those tides and the physical factors we asked about earlier? Let's take a closer look at the kinds of physical factors that might impact an organism living on the rocky shore.

Physical factors that affect organisms living on rocky shores include:

1. exposure to the air and sun causing dessication (drying out), heating up, cooling down or freezing (at night or when ice forms), abrasion by wind-borne partcles, genetic mutation by ultraviolet radiation

2. changes in salinity caused by evaporation (raising the salinity in a tidepool) and precipitation, such as rain or snow (lowering the salinity in a tidelpool); and by runoff from rivers, streams, sewage outfalls and storm drains

3. wave shock; the pressure exerted by breaking waves can be considerable. I compare it to trying to catch a small Volkswagen beetle that your friend has thrown at you. (Don't try it!). Intertidal organisms must be able to withstand tremendous forces from crashing waves, not to mention the occasional log or supertanker that slams into the shore.

4. water currents, that can change the water temperature and carry particles that cause abrasion.

5. chemical and mechanical weathering of the substrate on which the animal lives

6. chemical properties of the seawater, such as the presence of toxins or sediments that block the light

This list is not meant to be exhaustive; rather it is to give you some sense of what these poor little guys have to put up with. Despite this battery of imposing factors, the diversity of living organisms in the rocky intertidal is extremely high. Lost of different species do quite well in the rocky intertidal. I would encourage you to visit the rocky coasts of northern California, Oregon or Washington. You haven't seen rocky intertidal until you visit one of these places.

The point here is that physical factors impact marine organisms. Moreover, they determine where the organism will be placed along the shore. If a plant or animal can tolerate large fluctuations in these factors, then it will be able to live in the high intertidal zone. However, if a plant or animal has little tolerance for changes in any of these factors, then it will likely be found in the low intertidal, or even in the subtidal, which is the region below the tides.

As a result of decades of marine research on the ecology of rocky intertidal ecosystems, it has been shown that physical factors determine the upper limit of the distribution of an organism. In other words, it is physical factors that determine how high up from the water's edge that we will find a particular plant or animal.

Consider the mighty barnacle. He lives in the high intertidal zone. He can withstand weeks without air, without water, at high temperatures and at low temperatures, without food and without going to the bathroom. What a life? Yet barnacles are very successful colonizing rocky shores and the bottoms of boats and ships.

And before you get all mushy about the tough rocky life that barnacles are forced to live, consider that this hermpahoditic, sedentary arthropod, has the longest penis to body length ratios of any organism on Earth. When the barnacle wants to reproduce, it extends its penis to his neighbor and fertilizes him/her. If the neighbor is kindly, it might return the favor. It's a fascinating ritual to observe and if you look closely next time you are on a rocky shore, you might be lucky enough to witness this event.

At any rate, the barnacle is well-adapated to life in the high intertidal, especially when compared to an animal like the starfish, which dreads dessication and prefers to live in the lower intertidal. Thus, the upper dividing line between the zones is caused by physical factors.

Biological Factors

Let's move on to biological factors. These include the ecological interactions among the organisms, such as:

1) predation from land animals and subtidal animals

2) competition for space and food

3) reproduction

We could spend an entire semester talking about the incredible number of interactions among the species of organisms that live on rocky shores. One of my fondest memories of my freshman year in college, in the spring of 1975, was galloping through the rocky intertidal with Bob Payne, one of the premiere rocky intertidal ecologists of this century. Taller than most, he moved from rock to rock like a bighorn sheep, rattling off species names, food preferences, predators and habitats faster than we could write, and it was a blast just trying to keep up with him. I fell in love with the plants and animals that live on rocky shores and dedicated myself to knowing all their names. I few I have forgotten, but it was that energy and intensity that Dr. Payne fed to us that keeps me excited about this subject to this very day. Hopefully, some day, you will find a professor that has such an influence on your life.

Anyway, the point of all this is to say that as a result of Payne's research and a few others, like Joseph Connell and Paul Dayton, it came to be known that biological factors appear to set the lower limit of the distribution of an organism. That is, how low an organism lives in the rocky intertidal will depend on some biological interaction, such as being eaten or being pushed out of the way.

Your book has a great example of interspecific competition between two barnacle species, Balanus and Cthamalus, and you should read it. This example relates how a biological factor, namely, competition for space, sets the lower limit for one of the barnacles. Read it carefully and see how this works.

Another example made famous by the research of Dr. Payne involves the interaction between the ochre starfish, Pisaster ochraceous, and the blue mussel, Mytilus edulis. Mussels are the favorite food of starfish and starfish will feast on the mussels just as far up as their tolerance to physical factors will let them. The practice quiz has a great example of starfish munching on mussels. When Payne removed the starfish, he found that the mussels grew perfectly well at lower levels. What he concluded then was that predation set the lower limit for the blue mussel. Thus, we have another example of how biological factors set the lower limit of the distribution of an organism.

That concludes our brief summary of the ecology of rocky intertidal ecosystems. The World Wide Web is full of beautiful web sites covering this subject. I encourage you to explore a few of them.

from The Remarkable Ocean World