One hundred marine and estuarine organisms, as well as their habitats, have been illustrated since this column started in October 2005. Many of those species are inhabitants of the intertidal zone.
Unlike the open-water areas where temperature is relatively constant and dissolved nutrients, oxygen and waste products are easily transported, the intertidal zone is harsh. The ebb and flow of the tides subjects intertidal organisms to stresses such as drying, freezing, upland predators, high temperatures, reduction in food gathering and reduced ability to obtain oxygen. Such stresses limit the number and type of species that can live in the intertidal zone. The tides also produce strong water currents that affect subtidal organisms and habitats.
Why are there tides? The solution lies in the gravitational interaction and orbits of the moon, sun and Earth. That combination results in two daily bulges in the ocean level that move around the Earth, following the moon. One bulge is opposite the moon (because of the pull of gravity) and the other is on the opposite side of Earth (because of the centrifugal force of Earth's rotation).
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The height of the bulge varies depending on the relationship of the sun to the moon. When they are aligned with the Earth, the gravitational pull is greater and the high and low tides have a much greater range (called a "spring" tide). When they are at 45-degree angles, the pull and tide range are less, the so-called neap tides. The change occurs about every seven days. The tidal range also varies with local bathymetry.
The tide can be predicted using mathematic equations that generally are reliable but can be off depending on weather conditions. For example, a storm can result in a higher tide than predicted.
What does this mean to the animals and plants of the intertidal zone? Tide information may indicate that over this 24-hour period, an organism such as a speckled limpet or a rock weed living at the 11-foot tide level would have been dry for 17.25 hours out of 24 hours, or 72 percent of the time. The same information could show that an organism such as a clam or shore crab living at the 3-foot tide level would have been dry only 14 percent of the time. Tide tables can be found at a variety of locations, such as www.tidesonline.com, www.saltwatertides.com and www.tidesonline.nos.noaa.gov.
David W. Jamison is a marine biologist and Boston Harbor resident.