Water, as we all know, is a liquid, and moves freely in response to the force of the Earth’s gravity. It also moves in response to other gravitational forces, specifically those of the Moon and the Sun. But it is only in the enormous oceans that we see this movement.
These movements of the oceans are known as tides, and they follow an enormously complex pattern. The primary influence is the gravitational pull of the moon, so the tidal cycle follows very closely the movement of the moon in its orbit.
But this major effect can be compounded by the tilt of the Earth, so that there are higher tides in areas that are tilted more closely to the Moon; this results in seasonal differences in the tides. The tides are also affected by the lesser gravitational pull of the much more distant Sun, so that the highest tides occur on the New Moon, when the Moon and the Sun are on the same side of the Earth. A second smaller tide occurs because the landmasses on the side of the Earth opposite the Moon are pulled towards the Moon more than the water. The whole is further complicated by local geographic eccentricities on land and on the sea floor. The daily tidal fluctuation in British Columbia ranges from about two meters in the south to over seven on the North Coast. Despite all the complications, however, the tides follow quite predictable patterns, and a tide book is a worthwhile investment on any visit to the coast.
The tides have been having their way on the planet for eons, and thus have had their effects on the evolution of life. Just as there are plants and animals which have adapted to live beneath the waves, and in the many diverse habitats on land, so, too, there are thousands of species which live in the life zone which is affected twice daily by the tides.
As the tide recedes to its daily low point, this life zone is exposed to the elements, and the plants and animals there must survive until the next high tide. They are also exposed to the curious eyes of the intertidal explorer. Even an average tide will reveal wondrous things along the shore, or clinging to dock pilings, but it is the lowest tides of the year, in the summer, which expose the richest diversity of life.
All intertidal habitats have their species of interest, from muddy bays to wave-washed beaches, and rugged rocky outcrops. But it is these rocky shores that offer the best viewing opportunities, with relatively easy access, and many small pools left behind by the receding tide.
Marine organisms in the intertidal area fall into a few basic groups. While there are a few grasses to represent the flowering plants, most “seaweeds” are algae, and are grouped generally as either red, or green, or brown algae. The long whip-like stipes of Bull Kelp (Nereocystis luetkeana), a brown alga, with their hollow bulbous tips, can be seen in masses in the water, and often cast up on the beach. If you think it looks funny, don’t be too critical; you have probably eaten an extract from kelp that keeps your ice cream smooth and creamy.
Held fast to the rocky substrate are masses of rockweed (Fucus species), whose mucous covered fronds retain moisture, and provide cover for small marine animals. The bright green seaweed you see may be sea lettuce (Ulva species). The occasional translucence of this alga is understandable, because each frond is only two cells thick.