Intertidal zones

Intertidal zones, which are fundamentally marine, are common in coastal ecosystems worldwide and are subject to the influence of high and low tides on a daily basis, and organisms living on these shorelines are exposed to air during periods of low tides. Many factors that are either directly or indirectly related to the shoreline influence the type of habitat available for colonization of organisms. The effect of heterogeneity in topography has a subtle influence on intertidal shores, with localized irregular features such as cracks, crevices, undersides of boulders, gullies, overhangs, caves, tide pools, damp areas, and areas of freshwater drainage, all of which can serve as refuges from harsh environmental conditions and predators.

The type of substratum, sandstone or basalt, rock type, and rock texture determine how well water drains from the shore. Exposure to wave action varies greatly between intertidal shores because prevailing winds and the slope of the shore influence the type of waves and the point at which they break. Waves break far out from the shoreline on a relatively flat shoreline, then “spill” over the shore. On steep shorelines, waves come close before “surging” up the rock face.

Surging waves on steep shores may increase the effective intertidal zone to many times the height predicted purely from tidal rise and fall. Gentle sloping shores will drain more slowly than steep slopes, allowing organisms that are characteristic of lower zones to expand their ranges into the upper intertidal area. Shores with heavy wave action increase the splash area on the upper intertidal sections that would normally be covered by the tides. Waves can also exert a destructive mechanical effect on the shoreline, encourage scour by sand and shingle, circulate the water, disturb or deposit sediment, renew oxygen, and reduce dissolved carbon dioxide.

They also affect the movement of organisms, limiting feeding raids or keeping away predators. Conditions during emersion are not only affected by the position of the shore with respect to water currents, climate, and latitude, but also by the direction in which the shore faces and the amount of sunlight that it receives. The time at which the greatest low tides occur may be a critical factor, as evaporation and dessication may be intense during the middle of the day, but in places where it occurs in early morning or late afternoon, there will be less physical stress.

Tidal and wave exposure, in addition to substratum type (among other factors), have a great influence on distributions of organisms on the intertidal zones, and play a role in the myriad of adaptations of form, function, and life history that are observed among the organisms of the shore to counteract the effects of the changing physical conditions of the environment in which they live. On rocky headlands, organisms must withstand the hydrodynamic force generated by the water and tremendous impact of tons of water breaking over them, and here tough or hard bodies and strong attachment mechanisms are an adaptation. Organisms abundant in relatively protected locations are simply not equipped to take the battering and the shearing force of heavy surf. In addition to considering the physical conditions on the shore, vertical zonation is also controlled by the biological interactions that occur between species, such as larval settlement, competition between species and among individuals of the same species, predation, grazing, and behavior. Many mobile organisms have been shown to possess endogenous activity rhythms that trigger activity at each high tide, and may help to maintain the organisms in the appropriate zone, reducing the possibility of being stranded above the normal resting level on shore.

Tides

Periodic emergence in air, or emersion, is an overwhelming feature affecting life on intertidal shores, and the rise and fall of tides over the shore can be referred to as the emersion-submersion cycle. The tidal cycle is produced by the interrelation of the gravitational and centrifugal forces between the Earth, moon, and sun on the ocean waters of a rotating Earth. The Earth and moon rotate about each other in space and are held in position relative to one another by the balance of the gravitational forces that tend to attract the two masses, and centrifugal forces that keep them apart. The centrifugal force from this rotation on Earth is everywhere equal in magnitude and direction, with the direction being away from the moon. The moon's gravitational pull is directed toward the moon and is strongest on the side of the Earth closest to the moon, but is directly opposing the centrifugal force of the Earth. Depending on the location on the Earth relative to the moon position, the net effect of these two opposing forces will vary. On the side of the Earth closest to the moon, strong gravitational forces override the centrifugal forces and result in a pull away from Earth (toward the moon). On the opposite side of the Earth, centrifugal forces override the weaker gravitational forces and again result in a net force directed away from the Earth.

Tidal fluctuation is then related to the interplay of gravitational and centrifugal forces as these combined forces would create two bulges of water on opposite sides, one closest to the moon and one farthest from it. Every 24 hours, the Earth rotating on its axis would then seem to rotate under these bulges of water. A point on the Earth's surface thus would experience alternately a high tide, a low tide, another high tide, and another low tide almost every 24 hours. During the time taken for the Earth to complete one revolution on its axis, the moon has advanced somewhat on its orbit, requiring an additional 50 minutes of the Earth's rotation for a point to catch up to its previous position directly opposite the moon. This means that the tides would occur approximately 50 minutes later each day as a complete tidal cycle of two high tides and two low tides takes about 24 hours and 50 minutes, or one lunar day. Tide tables show predictions for the time and height of tides in a number of standard ports around the world.

Differences in height between tides are observed in one day, but larger changes are usually seen over periods of weeks because of changes in the relative tide-raising forces generated by the sun and the moon. The influence of the sun on producing tides on the Earth is only half that of the moon because the sun is much farther away, even though it is much larger. During a new or full moon, when the sun and the moon are nearly in a line with the Earth, their forces combine to produce spring tides, which are of much greater range than average because the tide-raising forces are at a maximum. The word spring has to do with the Anglo-Saxon words to rise, not from the season.

A few days after these occurrences, high tides are very high, while low tides are very low, thus tidal amplitude is very large. During a first or fourth quarter moon, when the sun and moon are at right angles to each other, the gravitational attraction of sun and moon act in opposition and the tidal forces of the sun partially cancel those of the moon. This produces high and low tides of minimum range in which high tides are not very high, and low tides are not very low. In these cases, tidal amplitude is very small and the tides are called neap tides. The monthly cycle contains two neap tides and two spring tides. The highest spring tides occur at the equinoxes (September 21 and March 21), whereas the spring tides are at their lowest amplitude at the solstices (June 21 and December 21).

Global Variations

Tidal patterns and amplitudes vary immensely around the world, as well as times of tidal occurrence. A common one is the semi-diurnal tidal pattern, when there are two high tides and two low tides a day. A simple pattern of two equal tides per day (with a delay of approximately 50 minutes from day to day) is also common, but not universal. These tides can be of different sizes. The overall variation of tidal patterns runs from fully semi-diurnal through semi-diurnal with unequal tides, to diurnal, where there is only one high and one low tide each day. Many factors are involved in determining the type and magnitude of the tidal changes characteristic of a particular location at a particular time, like position of land masses, and local conditions such as weather, barometric pressure, and onshore winds.

As a result, organisms on shore in different parts of the world experience quite different patterns in the times when they are forced to tolerate life in air. Intertidal zones are characterized by many different organisms, which in one way or another have adapted to life in this zone. On shores with similar exposure to wave action, the distribution patterns of organisms are remarkably similar around the world. Almost everywhere, organisms are found in distinct vertical bands or zones at particular heights, and are not distributed randomly; this is known as vertical or tidal zonation. Organisms occur in certain vertical zones, but do not always occur uniformly in those zones. Patches of organisms, or even bare space, frequently occur. One boulder in a boulder field may be covered with mussels, and next to it, another may be covered with algae. The mosaic in the horizontal dimension is a vital contributor to the richness of intertidal life.

Shores that are neither extremely sheltered nor pounded by extreme wave action usually have three distinct vertical zones and are appropriately called the littoral fringe, eulittoral zone, and the sublittoral zone. The high shore zone, or littoral fringe, often extends well above the levels reached by tidal cover, and is dominated by small snails and black lichen with blue-green algae. The midshore, or eulittoral zone, is characterized by barnacles and limpets, mussels, and algae. The sublittoral zone at the bottom of the shore extends gradually downward to regions well below those that are ever uncovered by the tides, and contains red algae and kelps (e.g., laminarian algae) or large tunicates (as in parts of the Southern Hemisphere). Even though the zones have been defined entirely in biological terms as they are related to tidal levels, zonation patterns on the shore are caused by the relative influence of multiple biological and physical factors.

In many cases, intertidal species are controlled by some physical factor at their upper limit of distribution, while they tend to be controlled by sets of biological factors at their lower limit. A good example is the distribution of the barnacle Chthamalus stellatus because its upper limit is set by tolerance to dessication, while the lower limit is set by competition with the barnacle Semibalanus balanoides and predation by the dog whelk Nucella lapillus. Second, the zones are defined using conspicuous organisms as indicators, but these organisms are far from evenly distributed across the zone. Some eulittoral zones may have clumps of algae forming an irregular and changing mosaic among a background of barnacles. Third, the boundaries between zones are not necessarily sudden, but can occupy a transition belt. For example, barnacles and black lichens might overlap considerably.

Inhabitants of Intertidal Zones

Organisms on intertidal zones can be divided into four large subgroups: algae, grazers, suspension feeders, and predators. Algal growth is a major factor influencing shore ecology because they provide a primary energy source and thus form much of the basis for intertidal food webs. Great biomass of brown, green, and red algae, collectively known as macroalgae or macrophytes, can be abundantly present in some shorelines. Shores that do not have abundant macroalgae usually have a film of microalgae, which is a collective term that includes the blue-green algae, diatoms, and other protists such as the euglenoids, and the spores and sporelings of macro-algae. The microalgae are often more important than the macro­phytes in providing a food supply for grazers because, in spite of their low biomass, they have very high rates of production. On most exposed shores, there is a rich assemblage of algae at the bottom of the littoral and sublittoral zones, with brown kelps and often a belt of short red algal turf above them. On sheltered shores, the diversity of algae increases and the whole shore may show extensive algal cover, and in many cases this may be because of growth of brown algae. On all shores, the variety of species increases toward the sublittoral, and red algae tend to take over the browns at the bottom of the shore. Green algae do not usually occur in large masses, but some of their genera are found worldwide.

The grazers are herbivorous gastropods that are specialized for intertidal zones, such as limpets, winkles, topshells, neritids, chitons, and urchins. Some of these organisms can be found worldwide. Herbivores rasp microfilms off rock surfaces, taking in microalgae, sporelings, fungi, and probably algae detritus. Urchins that feed sublittorally may take both live and dead macroalgae. Suspension feeders, which are sessile, such as barnacles and mussels, usually outnumber mobile organisms on rocky shores. On top of rocks, barnacles and mussels may entirely cover rocks, while the underside of rocks and macroalgae are covered with bryozoan colonies, tunicates, sponges, hydrozoans, and polychaetes. Predators such as sea stars, crabs, whelks, and nemertine worms are common on all vertical zones of the intertidal.

The slope of the shore will affect the foraging ranges of the many predators that move up in the intertidal to capture prey during high tide, and then retreat to lower zones at low tide. On steep slopes, such predators can cover greater vertical distances in a given amount of time, and as result, the vertical distribution of both predator and prey may be affected. Despite their mobility, gastropods such as winkles and crustaceans such as crabs usually maintain a well-defined zonation pattern. When covered by high tide, many species roam widely. Some crabs move up-shore as the tide rises, but retreat again as the tide falls. Some species of snails begin to move down-shore as soon as the tide covers them, but return to their usual zone when the tide rises again. Some species, especially grazing gastropods, have been shown to exhibit movements that are random in direction and extent, at least in the short term. But since these animals usually remain within a relatively restricted zone, there must in the long term be some limits to movements in the vertical plane. Very mobile species, such as crabs, show directional movements. Grazing gastropods displaced from their normal position are often capable of returning to it.

Wave exposure may influence the distribution of organisms indirectly through the effect it can have on the foraging ability of predators and distribution of sessile organisms. On exposed areas to full wave impact, predators such as crabs or predatory snails may be largely restricted to protective crevices from which they can only make quick foraging raids on prey populations. On the other hand, for other organisms such as barnacles, the vertical distribution may become broader along a vertical gradient of increasing exposure to waves. This is often observed with sessile, upper intertidal organisms as the more frequent wetting of the upper zones by splash and surf in exposed areas lessens the restrictive influence of desiccation and allows these organisms to live higher on the shore than they would under conditions of quieter water.

One of the structural features of intertidal zones are tide pools, which are dynamic and often constantly submerged, undergoing changes in temperature of the water, carbon dioxide concentrations, and pH, depending on the time for which they are isolated from the sea. The amazing diversity of fauna and flora in tide pool systems vary greatly in relation to height on the shore, shelter, shading, size and depth of the tide pool, and also from random disturbance events. Pools high on the shore undergo the consequences of prolonged separation from the main body of the sea, and conditions within them fluctuate dramatically. For example, in tide pools on the Atlantic coast of Cape Peninsula, South Africa, the maximum temperature during the day increases from about 16 degrees C in pools near the mean low water in spring tide, to near 86 degrees F (30 degrees C) near mean high water in spring tides.

The situation with oxygen concentration, carbon dioxide concentrations, and pH is less simple because here organisms directly affect the variables, as well as being affected by them. Salinity may increase from evaporation, or decrease from rainfall and water seepage, and these changes can be overwhelming in high shore pools. Algae in tide pools produce oxygen during the day by photosynthesis, and oxygen concentrations may rise above saturation value, so that bubbles rise from dense beds of algae. Carbon dioxide, absorbed by algae, is followed by a rise in pH, which can be extreme. Tide pools can exhibit equally drastic changes in the opposite direction during the night, when photosynthesis stops.