|
|
Tide Basics:
At Anchor Bay, in a 24 hour period, there are 2 high tides and 2 low tides. It takes roughly 6 hours to go from a low to a high. These are diurnal tides, which means that the two highs and the two lows are unequal.
It's the positions of the sun and moon that directly affect the tides. When they are on the same side of the earth during the new moon phase, or when they are opposite each other in the full moon phase, their combined gravitational influence is greatest and causes spring tides. Spring tides occur about every two weeks throughout the year. During this time the tidal change is at its most extreme. We have our highest tide followed by our lowest tide. You will tind tide pooling the best at such times. In your tide table, look for minus tides, tides which fall below the mean low tide line. During neap tides, when the sun and moon are at right angles (quarter moon phases), the gravitational influences negate each other and the tide is less affected. The change from high to low is only a few feet, and less of the tide pools are exposed.
Tidal Zones: Top
The area between the highest possible and lowest possible tide is known as the littoral zone. This narrow band, running the length of every coastline around the world, is sometimes underwater and sometimes exposed to the ain in regular tidal cycles. The littoral zone along our coast is divided into three roughly distinct zones which are characterized by the kind of wet/dry exposures they get from the uneven ("spring" versus "neap") tides.
Zonation is most clearly seen in rocky seashores, especially vertical surfaces, where the demarcation can be quite sharpl. Hundreds of species of plants and animals, many of which can tolerate only the narrowest variations in light, temperature, salinity, etc., live in specific niches defined by exposure. A list of all the tidal species would easily fill this page. Just a few are mentioned here to help with our discussion of zones.
The Upper Littoral Top
Sometimes called the splash zone and barnacle belt, the upper littoral zone is dry most of the time except for a few days each month during spring
tide highs. Spring tides, the lowest and highest tides in the tidal cycle, occur for a few days in a row about every other week.
Look for tiny white barnacles (Balanus sp.) speckling the rocks at the higher reaches of the zone, graduating to a bit larger ones on the lower end. Barnacles, a crustacean, store water in their shells to protect them from long exposure to the air. Even though they are hermaphroditic (both male and femal), they must cross fertilize with a neighbor and, therefore, thrive in close colonies. Individuals away from the group cannot reproduce.
The Middle Littoral Top
Alternately wet and dry every day of the year, species in middle littoral zone must survive several hours of exposure twice each day (we have two low tides in a 24-hour period). Softer bodied animals and delicate plants will be seen in the more protected areas, while the tougher species, with hard shells and strong attachments, are located in the more turbulent areas.
Besides exposed rock surfaces, this is also the tide pool belt. Tide pools are special micro-habitats, not just holes to catch deep water species who forget to go home before the tide goes out. Animals here are in an extremely delicate balance but can tolerate radical temperature changes and variations in salinity as their home becomes super heated and evaporates on hot sunny days.
Aggregating Anemones (Anthopleura elegantissima) live in tightly packed colonies, filling the shallow cracks and crevices of rocks in the upper
end of the zone. During high water, they unfold like a flower, their tentacles gathering minute free-swimming prey. As the water recedes, these soft bodied creatures bloat themselves with water and fold inward. To survive the sun's burning rays, they have tiny shell fragments attached to their outer flesh. When touched, they react by folding up even tighter, causing the water to squirt out in several little streams. But this also may rob them of their protection against several more hours of exposure. Unwary beachcombers can cause harm by not watching where they step. Interestingly, these anemones multiply by dividing themselves into two. Colonies, therefore, are clones of one gene sample. The colony next door is likewise a single but different gene sample. When the two colonies grow to meet, war breaks out in the form of a stinging match until some individuals give up and fall away. Because of this, distinct little no-mans-lands can be seen between colonies.
Most noticeable among the hardier species is the California Mussel (Mytilus californianus). They form great mats covering large areas of rock. During high tide, they pump water through their systems and filter out microscopic plants and animals as their food. They close up during low tideperiods to retain moisture. In spite of their tough byssal thread attachments, stormy surf conditions rip away sections of matting. Wild mussels are considered a delicacy for many beachcombers.
Growing alongside some mussel mats are Sea Palms (Postelsia palmaeformis) competing for precious space. They need bare rock to secure themsleves and to propagate, and so will take advantage of any losses to the mussel matting. Within the passing of a single tide, spores are released onto bare rock that must grow a "holdfast" to keep from washing away at the approach of the next tide.
The starfish is sort of a "hydraulic" creature. It cannot move nor even eat without pumping water through its system.
The most common in this zone is the Ochre Sea Star (Pisastere ocraceus). Since mussels are a favorite food, these tough bodied starfish are often seen on the fringes of mussel mats. In order to eat mussels, it must move up the zone as the water comes in and eat during high tide when the mussels are submerged. They open their victim slightly using hundreds of suction cupped tubular "feet", extrude their stomachs into the shell, and digest the mussel there. When the water goes out, some starfish will cling to the exposed rock and "wait" for the next tide, while others will move down the zone. At low tide there is a noticeable band of cleared rock surface between the starfish waiting for the tide to rise and the healthy mat of mussels a bit too far for starfish to reach in the passage of a single tide.
The Lower Littoral Top
Submerged except for only a few hours per month during spring tide lows, the lower littoral zone is often shared by deep water species that overlap into the shallows, such as Red Abalone (Haliotis rufescens), Red Sea Urchins (Strongylocentrotus franciscanus), Sunflower Stars (Pycnopodia helianthoides), and several species of larger kelps.
The striking Giant Green Anemone (A. xanthogrammica), more characteristic of this zone, is easily seen open wide in the tide pools or hanging limply from the sides of exposed rocks. Purple Sea Urchins (S. purpuratus) can be seen in colonies where each individual lives in a rounded depression in the rocks which they make themselves. The depression provides them with protection against turbulent water, retains moisture during low tide exposures, and acts as a nursery for their food supply of algae.
Treat Them Kindly: Top
Most of the plants and animals living within the tidal region are distinctive inhabitants and are not recruited from an endless supply of deep water species, nor replaced with every turn of the tide. Human interference is a critical factor here. Certainly pollution has a devastating affect, but less considered is the almost daily attack by careless and thoughtless beachgoers. Heavily visited areas are soon wiped out. Plants and animals are crushed under foot. Hundreds of creatures are mishandled and needlessly die in childrens' buckets or left to rot in the bushes when tossed out before the journey home. The exact position of tidal animals is often critical. Moving them may expose them to changes they cannot tolerate. Turning over rocks can crush tiny critters, expose them to harmful ultra-violet light and fatal temperature changes, or make them easy prey as they scramble for new hiding places in an unfamiliar cranny.
Let the rocks and tide pools themselves be the laboratory and let the creatures remain undisturbed in Nature's aquarium.