Islands in the Intertidal

Note: This program first aired December 15, 2012.

If the intertidal zone can be considered one of the most rigorous habitats on the planet, due to its consistent inconsistency, part time ocean, part time land, then tide pools are the oasises in the intertidal zone’s deserts.  A tide pools is like an island. An island is a little piece of land, terrestrial environment, surrounded by water, a physiologically hostile environment (at least to the land based organism on the island). A tide pool is the same thing in reverse—an oasis of habitat stranded in a larger hostile environment.

As a general concept, tide pools allow organisms to live further away from the contiguous coastal ocean than they “normally” would; further away meaning higher up the intertidal zone. The pools provide a little piece of sea like habitat in the middle of a dry beachfront or rocky shore. Of course, all tide pools are not created the same, and it would be a misconception to think that they perfectly recreate the conditions found subtidally. No, tide pools, though they do offer marine organisms a more desirable situation than simply being stranded on a rock in the sun at low tide, do present their own set of physiologic challenges and inconsistencies to the plants and animals that call them home.

The further away a tide pool is from the low tide line, the less like the ocean it tends to be. And the smaller or lower volume a tide pool is, the more easily its environmental parameters can stray from those found in the mother ocean as well. The shape of the tide pool matters too—a big wide but shallow tide pool may have a high volume of water but that water will have a wide surface area in contact with both the atmosphere and the underlying substrate, both vectors for change of the physical parameters of the pool. Finally, the tidal cycle itself varies. Sometimes of the month the tide is low during the hottest sunniest part of the day, at other parts of the monthly tidal cycle, the intertidal zone is covered during this time. The organisms that live in this environment must be endlessly and admirably flexible, dealing with dramatically different parameters on a daily basis.

All of this is to illustrate the main point, that though tidepools are typically characterized by their height in the intertidal zone, no two tidepools are the same, and the high and low characterizations I’m about to talk about need to be taken with a grain of sea salt.

The higher the tide pool is in the intertidal zone, the further it is from the contiguous coastal ocean and the longer the organisms that live in it are castaways “on the island”. The temperature of these pools can vary greatly, as air temperatures vary much more than ocean temperatures due to the differences in the physical properties of air and water. In winter these pools get colder than the ocean, in summer they get much warmer. The salinity can vary much more than the ocean as well; when it rains or snows, the salinity in a high tidepool can drop dramatically as the sea water is diluted by fresh water precipitation. When the sun is intense and the water is easily heated, salinities can increase as evaporation takes place. Even the pH of a high tidepool varies. At  night the pool can experience low pHs (acid conditions) because the organisms in the pool are respiring and producing carbon dioxide, which when dissolved in water, creates carbonic acid. During the day, the photosynthetic organisms in the tide pool will photosynthesize and take up carbon dioxide, which causes the pH to increase and the water to become less acidic. High tide pools tend to have lower biological diversity because of the wide range in physical parameters.

Tide pools that are closer to the ocean, have less variation in all those parameters, because they are replenished by the contiguous coastal ocean sooner and more often than the higher tidepools. As a result, the lower tidepools exhibit higher biodiversity and host communities that look more like neighboring subtidal zones. The closer the island is to shore, the more like shore it will look.

Tide Pool Fanatic

Note: This program first aired December 1, 2012.

Looking through an old resume recently, I noticed that I listed “tidepool fanatic” on my list of other skills and interests. Seriously, and it got me the job too. And I am not the only one; there are millions of us*, you can find us pawing through seaweed, looking under rocks, and staring into tide pools every time we go to the edge of the sea. The organisms we find in the intertidal zone are our windows to the mysteries of life under the ocean surface. Twice daily the veil is pulled back and the tribe of the curious gathers to explore the unknown.

The intertidal zone is the area at the edge of the ocean that is regularly covered and uncovered as the tide rises and falls each day. Places like Maine, with high tidal ranges have a large and diverse intertidal zone. Places like Florida, with very small tidal ranges have minimal intertidal diversity. The intertidal zone is populated with marine organisms, meaning organisms adapted to live in the salty aquatic environment we call the ocean. These are some of the toughest organisms in the world, and this is why: imagine yourself as a land based, air breathing being, having to survive, day after day, totally submerged in sea water half of the time. This would certainly challenge your physiology, to put in nicely. This is the kind of challenge (albeit in the opposite direction) that organisms in the intertidal zone face twice daily, every day. As organisms dependent on the ocean’s water, they are stranded out of it for some, if not most of their lives.

Consider the benefits of the ocean, if you are a marine organism that is. Being submerged in water means that you can breathe; water brings with it food for many organisms, water provides a medium for motility (and breeding and hunting), water provides environmental stability in terms of temperature, salinity and pH, in short, the intertidal zone comes alive at high tide. Consider the difficulties of living in the terrestrial environment, if you are a marine organism. You can’t breathe air (with a few exceptions), the temperature and salinity can fluctuate dramatically with season and weather events, you can’t eat, and you could easily dry out. If you are algae, you just have to lay there as you fell. Its rather undignified when you think about it. Clearly, these organisms would be much better off if the tide just came in and stayed in.

Or so we think. The reality is that the organisms we see in the intertidal zone live in this marginal environment because they get outcompeted (which is just the scientific word for bullied) in the sub tidal environment. The intertidal organisms do better in that incredibly stressful intertidal zone, because they evolved physiologic and structural adaptations that protect them from drying out, suffocating, cooking and freezing. Subtidal organisms just can’t do that. Take them out of water and they die.

I have huge respect for the resilience of the intertidal organisms. They eke out a living in a consistently inconsistent habitat, organisms of the ocean that can exist on land, at least temporarily. This sounds pretty significant, but how different is it from our own situation? We humans are predominantly visual animals, yet, we evolved in and exist in an environment that grows dark for half of the time we are in it, effectively negating our primary sense. So perhaps the achievement of the intertidal organisms isn’t as amazing I led you to believe. I don’t care, you’ll still be able to find me staring into the nearest tide pool, imagining the scene when the tide comes in and all of the organisms live the fullest expressions of their lives. Perhaps that is why so many of us love the edge of the sea. Its rhythms of activity and rest, and abundance and famine so readily mirror our own.

*Full disclosure: this is a complete guess.

References:

Oh there are so many books on the intertidal environment! Though it is old, I still love the Sierra Club Naturalist’s Guide to the North Atlantic Coast (Cape Cod to Newfoundland) by Michael and Deborah Berrill.