subtidal rocky areas discussed in Chapter 5. Most of the dominant organisms of intertidal rocks are solitary or clonal animals, whereas subtidal rocks seem dominated by colonial encrusting animals.
One of the most striking features of any rocky shore anywhere in the world at low tide is the prominent horizontal banding or zonation of the organisms. Each zone or band is set off from those adjacent by differences in color, morphology of the major organism, or some combination of color and morphology. These horizontal bands or zones succeed each other vertically as one progresses up from the level of the lowest low tides to true terrestrial conditions (Figures 6.7, 6.8). This zonation on intertidal rocky shores is similar to the zonation pattern one observes with increasing elevation on a mountain, where the different horizontal zones of trees and shrubs succeed each other vertically until, if one progresses far enough, permanent snow cover is reached. The major difference between these two areas is the scale. Mountain zones are perhaps kilometers in extent as opposed to intertidal zones extending a few meters vertically.
Rocky intertidal zones vary in vertical extent, depending on the slope of the rocky surface, the tidal range, and the exposure to wave action. Where there is a gradual slope to the rock, individual zones may be broad. Under similar tidal and exposure conditions on a vertical face, the same zones would be narrow. In the same manner, exposed areas have broader zones than protected shores, and shores with greater tidal ranges have broader vertical zones (Figures 6.4, 6.8).
Of course, these striking bands may be interrupted or altered in various places wherever the rock substrate shows changes in slope, composition, or irregularities that change its exposure or position relative to the prevailing water movement.
The fact that these prominent zones can be observed on nearly all rocky shores throughout the world under many different tidal regimes led Stephenson and Stephenson (1949) to propose, after some 30 years of study, a "universal" scheme of zonation for rocky shores (Figure 6.9). This universal scheme was really a framework using common terms that would allow comparison of diverse areas. It established zones based on the distributional limits of certain common groups of organisms and not on tides. It reflects the knowledge of the Stephensons, and other intertidal ecologists, that distribution patterns of the organisms and zones vary not only with tides, but with slopes and exposure. Therefore, under similar tidal conditions there could be different bandwidths due to different exposures or slopes of rocks. It was this universal scheme that established a standard format for describing shore zonation, replacing a bewildering host of schemes and names established by earlier biologists.
The Stephensons' scheme has three main divisions of the intertidal area. The uppermost is termed the supralittoral fringe. Its lower limit is the upper limit of barn4eles, and it extends to the upper limit of snails of the genus Littorina (periwinkles). The dominant organisms are the littorine snails and black encrusting lichens (Verrucaria type). The extreme high water of spring tides reaches part of this zone, but most of its water comes from wave splash. Above this zone is the terrestrial or supralittoral zone.
The middle part of the intertidal is termed the midlittoral zone and is the broadest in extent. Its upper limit coincides with the upper limit of barnacles, while its lower limit is the point where
zone is often subdivided and contains a host of different organisms. Perhaps the only universally present dominant group are the barnacles.
The lowest zone of this scheme is the infralittoral fringe, which extends from the lowest low tide up to the upper limit of the large kelps. This is an extremely rich zone composed of organisms that can tolerate only limited exposure to air. It is really an intertidal extension of the infralittoral zone (Stephensons' term) or what we know as the sublittoral area.
Although Stephensons' scheme did set forth a means for describing zonation on rocky shores, it does not offer an explanation of why the zonation occurs. It is this explanation of zonal patterns that intrigues many marine biologists.
Causes of Zonation
Whereas it is fairly easy to recognize and measure the extent of the zones on a rocky shore, it is more difficult to find suitable explanations for why organisms are distributed in these zones. Physical and biological factors can be considered to explain the phenomenon. We shall take up each in turn.
Equinoctial high-water spring tides
A = Supralittoral (spray) zone A' = Supralittoral fringe
B = Littoral zone
B' = Midlittoral
C = Infralittoral zone C = Infralittoral fringe
FIGURE 6.9 Stephensons' universal scheme of zonation for rocky shores. (From R. L. Smith, Ecology and field biology, 3d ed., Figure 8-4. Copyright © 1980 by Robert
Leo Smith. Reprinted by permission of Harper & Row Publishers, Inc.)
large kelps (Laminaria, etc.) reach their uppermost distribution. This
Equinoctial low-water spring tides
PHYSICAL FACTORS The most obvious explanation for the occurrence of the zones
is that they are a result of the tidal action on the shore and reflect the
different tolerances of the organisms to increasing exposure to the air and
the resultant desiccation and temperature extremes. One difficulty with this
explanation is that the rise and fall of a tide tend to follow a smooth curve
with, no obvious sharp breaks corresponding to the often sharp
boundaries observed in the intertidal zones. If, however, one observes a whole
series of such tidal curves, such a series of breaks does become apparent. For
example, Figure 6.10a gives a typical tidal curve for a mixed tide area on the
the early explanations for the zonation patterns described previously. In this explanation, it is important to remember that it is not the tide alone that causes the limit but the fact that, at these critical points, the organisms are subjected to greatly increased time in the air; hence, they experienced greater temperature fluctuations and desiccation.
The critical tide hypothesis has been tested in various places by several scientists since its original promulgation by Doty in 1946. In general, it is difficult to find good correlations, particularly at the lower tide levels. This lack of good correlation can be partly attributed to the diverse topography of the different shores and to the variation in exposure. Thus, a species may be able to exist above the critical tide level if the rocky shore is exposed to persistent violent wave action, which would throw water up higher and decrease desiccation. Similarly, caves, overhangs, and crevices remain moist when exposed areas are dried out; thus, organisms are able to persist above the critical tide level. The effects of differences in topography and exposure suggest, however, that the ultimate upper limits are set by certain physical factors: desiccation and temperature.
We have previously noted that desiccation is a serious
problem. Several natural observations, as well as field experiments, have
suggested that desiccation can set the upper limits to organisms and zones. In
rocks that have a north-facing slope often have the same organism occurring higher than do adjacent south-facing slopes. Since such rocks experience no difference in wave exposure or tides, the only reason for the height difference is that north faces dry out more slowly than south faces. Similarly, field experiments by Frank (1965) and others in which water was slowly dripped down slopes have resulted in the organisms extending higher up (Figure 6.11). In other experiments, organisms were transplanted above their normal positions. Notable in this respect are barnacles, which cannot move. When transplanted by Foster (1971) above their normal tidal height, they died. Younger barnacles died more quickly than older ones. Again, desiccation can be suspected.
Along with desiccation, and often acting in concert with it, is temperature. As noted previously, aerial temperatures have ranges that may exceed the lethal limits; hence, intertidal organisms may die from either freezing or "cooking." Upper limits of zones in part may be attributed to the tolerable temperature limits of the intertidal organisms. In addition, high temperature promotes desiccation, and the synergistic effect of these factors may be even more devastating than each acting alone. The lack of abundant organisms on rocky intertidal shores of the tropics may be at least partly the result of desiccation combined with heat and ultraviolet light stress.
Finally, sunlight may act adversely to limit organisms on the shore. Sunlight includes wavelengths in the ultraviolet (UV) region that can harm living tissue. Water absorbs these wavelengths and protects most marine animals. However, intertidal animals have direct exposure to such rays at low tide. The higher an organism is in the intertidal, the greater the exposure to these rays. At present, there is little information concerning whether UV radiation controls distribution of organisms, but the recent finding of UV-absorbing compounds in shallow-water organisms suggests its importance. Light also has been suggested as a regulator of the distribution of intertidal algae. This has to do primarily with the spectral quality of the light. As noted in Chapter 1, different wavelengths of light are absorbed differentially by water. In the case of intertidal algae, those that need light of longer wavelength (reds) that are absorbed most quickly by water would tend to be found higher in the intertidal. When submerged, they would not be too deep for the penetration of red light (about 2 m). Since the main intertidal algae belong to three different groups-reds, browns, and greens-each of which has a slightly different absorption spectrum, it might be thought that they would be arranged along a depth gradient. In such a gradient, the green algae would be expected to be the highest, because they absorb mainly red light; the brown next; and the red algae deepest, because they absorb mainly the deep, penetrating green light. This, however, is not the case. The intertidal algae are a mixture of all types at most levels. This is likely due to the interaction of other factors and the physiology of the algae. It points out again the fallacy of attempting to explain patterns of species distribution with only single factors when a multitude are acting at all times.
Although there is a large body of evidence that physical
factors are the strongest determinants of the upper limits of organisms, there
may be alternative explanations, many of which have not been investigated.
Underwood and Denley (1984) point out that other processes are at work in the
high intertidal that may be confused with physical effects. For example, some
animals in the high intertidal, particularly sessile filter feeders, may be
killed by starvation during long periods of calm weather because they are not
covered with water long enough to feed. Similarly, grazing animals may be
absent, not because they cannot tolerate the physical factors, but because
their algal food is absent, perhaps killed by severe physical factors. Finally,
in the case of larvae settling out of the plankton, they may choose not to
settle in the area, in which case the upper limit is set by processes affecting
larval settlement. In one instance reported by Underwood (1980), the upper
limit of several algal species in
BIOLOGICAL FACTORS Although the early work in the intertidal focused on the importance of physical factors in setting the zonal patterns, more recent work has begun to clearly establish the great importance of a number of biological factors in setting various observed distribution patterns. In general, these biological factors are more complex, often subtle, and closely linked to other factors. This is probably why we have only recently begun to understand how they act. The major biological factors are competition, predation, grazing (herbivory), and larval settlement. We shall discuss them in order.
Competition for a certain resource does not occur if the resource is so plentiful that adequate supplies of it are available for all species or individuals. In the rocky intertidal zone, only one resource is commonly in limited supply and that is space. This is perhaps the most restricted area in the marine environment; at the same time, it is densely populated, at least in the temperate zone. As a result, there is an intense competition for space that has resulted in observed zonal patterns.
On the intertidal shores of
Chthamalus stellatus living in the highest zone and the larger Balanus balanoides occupying the
major portion of the midintertidal. Studies done by Connell (1961) showed that Chthamalus larvae settled out throughout the zones occupied by both barnacles but survived to adulthood only in upper
zones. The reason for the disappearance in the midlittoral region was due to competition from Balanus balanoides, which either overgrew, uplifted, or crushed the young Chthamalus. Balanus was prevented from completely eliminating Chthamalus, because unlike Cbthamalus, Balanus did not have the tolerance to drying and high temperatures that prevailed at the higher tidal levels; hence, it could not survive there. Here, then, is a case where the zonation is at least partially a function of biological competition (Figure 6.12).
Intertidal algae, particularly those on temperate shores, also
often show abrupt limits to their upper and lower distribution. Earlier, these
limits were ascribed to critical tide levels, but it has also been suggested
that this could be due to competition for space or access to light. In two
experimental studies done on the Pacific coast of
Competition among the
mussel Mytilus californianus and
several species of barnacles on the Pacific coast of
ese, in turn, displace any rapidly growing algal
species. The barnacles persist
only until the mussels enter. The mussels outcompete and destroy the barnacles
by settling on top of them and smothering them. Since nothing appears to be
large enough to settle and smother the M.
californianus, they remain in control of the intertidal space. Given this
competitive edge, it would appear that eventually the rocky Pacific coast of
The role of predators in determining the distribution of organisms in the intertidal and the zonal patterns is best documented for the Pacific coast of Washington and is discussed here as an example of how complex biological interactions create the prevailing distributions (Plates 41 and 47).
The dominant abundant intertidal animals on the Pacific coast other than the space-dominating Mytilus californianus are the barnacles Balanus cariosus and Balanus glandula. These latter two species occur abundantly in the intertidal region even though they are competitively inferior to M. californianus, because a predatory starfish, Pisaster ochraceus, preferentially preys on M. californianus, preventing it from completely overgrowing the barnacles (see Plate 40). Pisaster ochraceus is a voracious predator of mussels, consuming them at a rate that prevents them from occupying all the space.
At the same time, Balanus
glandula is found primarily as a band of adults in the high intertidal,
while Balanus cariosus occurs as
scattered, large individuals or clumps in the midintertidal. This pattern, as
Connell (1970) has shown, is also due to predation. Balanus glandula, like Chthamalus
It is not known what regulates Nucella populations, but they are preyed on by large Pisaster and are also vulnerable to periodic events, such as freezing during winter low tides. Such periodic events may reduce the population by such a significant amount that the Balanus cariosus could successfully establish themselves in an area. This may explain the differences among various shores with respect to the abundance of this barnacle.
Although it prefers mussels, Pisaster ocbraceus can consume Nucella and barnacles of any size and is the primary predator of small- and mediumsized Mytilus. It is the top predator in the system. Because of its ability to influence the structure of the entire intertidal community by consuming Mytilus and preventing monopolization of the space, it has been called by Paine (1966) a keystone species as defined in Chapter 1.
The reason the upper intertidal is a refuge from perdators is that Pisaster and Nucella can feed only when the tide is in, and they require a long period to attack their prey successfully. The short period of immersion of the upper intertidal does not allow sufficient time for them to make successful forays into that area. In subtidal areas, unlimited time is available; hence, the starfish have sufficient time to attack and consume their prey. It is probably for that reason that Mytilus, for example, do not extend into the subtidal areas, even though they can live there; hence, it is the reason for the sharp lower boundaries to the zonation of this species (Figure 6.13).
The situation regarding the importance of the interaction of
mussels and starfish in structuring intertidal rocky communities of the Pacific
intertidal with a mosaic of patches-some completely dominated by mussels and others with scattered clumps, depending on the conditions and length of time the dominant predator was excluded.
Menge (1976) and Lubchenco and Menge (1978) have also
demonstrated that predation is important in setting zonal patterns in the
intertidal of the
An unexplained anomaly of the previous pattern is found in
the very sheltered bays of
The presence of the above mentioned anomalies in the keystone species concept has caused some researchers to question its usefulness, particularly how it should be defined and the generality of its occurrence. It is appropriate to briefly discuss these points.
At its inception by Paine (1969), the keystone species concept was defined as a single carnivore that preferentially preyed on and controlled the abundance of a prey species that, in turn, could competitively exclude other species and so dominate the community. It was subsequently used by
other workers to refer to such noncarnivorous species as herbivores, mutualists, prey, etc.; hence, confusion arose. Menge et al. (1994) have redefined the keystone species concept as originally stated and have clarified it by defining two other predator-prey relationships that are not keystone relationships. Diffuse predation is a condition where the total predation is strong and capable of controlling the abundance of a competitively dominant species but in which the predation is spread over several predators, not just one. If the total effect of predation on the competitive dominant prey is low such that predation does not alone control the abundance, the condition is called weak predation.
The large geographical extent of the PisasterMytilus community, extending from
In a recent test of the generality of the keystone concept,
Menge et al. (1994) investigated the high temporal and spatial constancy of the
lower limits of Mytilus distribution
at several sites in
It seems safe to conclude that the keystone species concept, at least for temperate rocky shores, remains a valid concept. At the same time, it is not a universal concept, and its occurrence is variable within the geographic range of the species or communities in question. It may be replaced under certain environmental conditions by other community regulatory mechanisms, such as diffuse or weak predation or physical factors.
central theme of marine intertidal ecology, as argued by Connell (1975), is
that wherever predaVon is reduced, competition will be increased. Indeed, the
previous examples demonstrate this. It is well, however, to remember that this
is not the inevitable result of predator removal. For example, Keough and
Data on the importance of grazing were late in being reported (not until well after World War II). In the past 20 years, however, considerable evidence has been accumulated by various methods-experimental removals of grazers, caging experiments, natural disasters-so our understanding of the role of gazing is now fairly well advanced. Grazing affects a number of parameters, including algal zonation, species diversity, patchiness, and succession. Before the implementation
of various experimental methods, it was assumed that in the intertidal the upper limits of the dominant algal species on rocky shores were a function of the species' ability to tolerate various physical factors. On the other hand, lower limits of algal species distribution seem not to be set by physical factors, because most algae grow faster when experimentally transplanted to lower levels or when they are grown constantly submerged.
Since most algae do grow better when set lower in the
intertidal or submerged, why don't we find them there naturally? The answer coming
from many researchers is that they are grazed out. For example, in various
experiments that have removed grazing animals, the result has been the appearance
lower on the shore of a number of algae otherwise restricted to higher levels,
where grazing is less intense. This has been shown by Lubchenco (1978, 1980)
to be true for Chondrus in
Grazing may also determine the community structure. For
example, Dethier and Duggins (1988) removed the chiton Katharina tunicata from the low intertidal in
scopic scopic plants into a kelp
bed dominated by the kelps Hedopbyllum
sessile, Alaria marginata, and Nereocystis
luetkeana. However, a similar removal of Katharina from a low intertidal area in
Grazing may also set the upper limits of algal distribution.
One of the more interesting pieces of evidence supporting this comes from the
mechanisms by which the removal of grazers facilitates the upward or downward
extension of algae vary. In the simplest case, the algae are directly limited
by the grazer. In other cases, the absence of the grazers may permit a rapid
establishment of ephemeral algae, in turn, trapping moisture and permitting
the newly settled sporelings of the dominant algae to survive. In still another
scenario, the upward extension of algae may be seasonal due to changes in
grazing pressure. For example, Castenholz (1961) showed that a summer decrease
in high intertidal diatom cover was due to a summer increase in Littorina grazing activity in contrast
to the less active winter condition. In a more complicated situation, grazing
and physical factors interact. Hay (1979) found that in
Surprisingly enough, some of the limits of algae are set by
the effect bf the algae themselves on potential grazers. In
Another consideration in the zonation of algae species is the ability of some algae to grow fast enough to escape grazing by becoming too large for the grazers to feed on. In these cases, the algae will mature only in areas where the grazers are low in density or restricted from grazing during some seasons; thus, the algae can reach a size at which they are immune to grazing.
So far we have considered grazing and its effects on vertical
zonation patterns. Grazing may, however, also affect
the horizontal patterns of zonation-patterns that have as the dominant physical
factor the variation in wave action. The best studied examples come from the
Atlantic coast of
We should not leave this issue of grazing without discussing a final subject, namely, the defenses of the algae to grazers. We cannot assume that algae are completely without protection from grazing. Indeed, they are not. The most obvious adaptations to grazing are morphological, and one of the most common is the laying down of calcium carbonate in the tissues. Calcium carbonate reduces the palatability of algae to grazers and makes the grazer exert more energy in feeding. Furthermore, Steneck (1982) has even shown for one coralline alga, Clathromorphum, that the meristem tissue and reproductive sites are all on the underside and protected from grazing. Growth form may also discourage grazing. Hay (1981) has shown, for example, that formation of short turfs in algae reduces grazing mortality. Still other algae, such as the Pacific coast Egregia, have considerable structural or "woody" tissue when mature; this may deter grazers.
Chemical defenses are also common. Many algae contain noxious
or toxic compounds. For example, on the Pacific coast of North
Finally, there are evolved defenses in the life histories of certain algae. Some algal species are able to exist in at least two growth forms, one a low "crust" and the other an upright frondose form. In areas with heavy grazing pressure, the slow-growing but grazer-resistant crusts predominate, but in areas of low grazing pressure or in areas from which grazers have been excluded, the frondose forms increase. Littler and Littler (1980) have suggested that having these two alternative growth forms in a single species is a strategy of bet hedging. The frondose form has fast growth and high competitive ability but low grazing resistance. The crust form, on the other hand, has slow growth rates and competitive ability but high grazing resistance. Another life history feature that may have evolved in algae to cope with grazing is reproduction. Many algae, particularly those that are favored by grazers because of their high fraction of digestible material and low fraction of structural tissue, have evolved the opportunistic habit of high reproductive output. This also makes them successful in colonizing newly available space. Quick maturity and high reproductive output mean that these species should be able to colonize an open area and reach maturity quickly before the grazers find and decimate them. Other species have evolved the opposite, or K-selected, features. They are relatively slower growing, perennial, and long-lived. To survive pressure from grazers, these algae either have evolved chemicals that make them unpalatable or grow large enough to be immune from grazers. Large size usually also means an enhanced competitive ability, at least for light.
We can conclude this section on grazing by looking briefly at
the geographical variation in grazing and its effects on various kinds of communities.
Beginning with the temperate zone, grazing is one of the main structuring
agents of the rocky intertidal on the Atlantic shores of Europe and the Pacific
We can summarize the effects of grazing in the three intertidal zones defined by the Stephensons to create a general framework for the North Atlantic Ocean as a model or example of how grazing acts. In the supralittoral fringe, grazers are either uncommon or have little time to graze. The rigorous and stressful physical environment is thus the most important agency causing changes in algal composition. The midlittoral zone is dominated by barnacles and mussels but is strongly influenced by gastropod grazers, primarily limpets, and may vary in algal cover depending on exposure. This, in turn, may limit the numbers of gastropods. Barnacles, however, may also be present due to the ability of limpets and other grazers to keep the competing algae in check. In the sublittoral fringe, the algal growth exceeds the capabilities of the grazers to control it. Therefore, the competition for space and light among the various algae is the dominant interaction that structures communities at this level. These various factors and their relative importance are summarized in Figure 6.17.
The ability of larvae of various benthic invertebrates to
select areas on which to settle has been known for more than half a century
(see Chapter 1). However, the importance of this choice to the eventual
structure of intertidal communities has remained obscure due to the difficulty
of working with larvae under natural conditions. Several studies recently
have, however, demonstrated the importance of this selection ability. In
cal factors, but by settlement of the larvae.
Furthermore, the barnacle larvae were found by Denley and
Underwood (1979) to avoid settling on substrates already occupied by other
organisms. A more comprehensive example, also from
Larval recruitment can be variable in time and in space. This variability has been documented, and its importance to the intertidal community structure can be significant. In some years, any given intertidal area may experience poor recruitment due to bad combinations of weather, reproduction by adults, larval mortality in the plankton, such water conditions as wave action, and so forth. The result is few survivors and a decrease in biological interactions, such as predation, competition, and grazing. In other years, favorable physical conditions may swamp an area with recruits, leading to increased competition or predation and probable changes in the adult population and the community. As a result, the same area may have a community that not only differs in composition over time, but may change in relative numbers of individuals. The importance of competition, predation, and grazing in determining the structure of the community will also, therefore, change.
Gaines and Roughgarden (1987) have demonstrated another
significant factor that can lead to variation in larval recruitment. They were
able to show that the juvenile rockfish that inhabit the kelp forest
immediately offshore of the intertidal zone in central
This great spatial and temporal fluctuation in the settlement of larvae of the dominant intertidal organisms means that the importance of competition, predation, and grazing are likely to be variable. It is difficult to generalize about the effects of competition, predation, and grazing if the numbers of prey or potential competitors are also unpredictable. Although it is considered by some to
be a "nuisance" in the setting up of experiments, this irregularity of appearance of various dominant intertidal invertebrates is a fact of nature and must be considered in any attempt to understand the structuring process in the intertidal zone. In other words, the irregular occurrence in a community of a numerically important, competitively dominant, or keystone predator should lead to a variety of different outcomes from experimental studies done in the area because of variations in the intensity of biological interactions resulting from the larval recruitment patterns.
In an attempt to sort out the relative importance of
competition, predation, and larval recruitment to the structure of rocky
intertidal communities, Menge (1991) undertook a multiple regression analysis
of these factors for New England and
A characteristic feature of many rocky shores is the presence of tide pools of various sizes, depths, and locations. Certain conditions affecting life of tide pools differ markedly from the surrounding intertidal and necessitate a separate discussion here. Our remarks in this section will be restricted to those pools that undergo a complete interchange with the ocean water during the tidal cycle (Figure 6.24). It should be pointed out at this juncture that the biotic communities of tide pools and the factors influencing their structure are less well understood and studied than those of the emergent rocky surfaces. It has even been suggested that tide pools do not represent an intertidal habitat because they are never exposed to the air during a tidal cycle. However, the fluctuations in physical and chemical factors in tide pools are a function of the tidal cycle; hence, it seems logical to consider them in this chapter. Furthermore, many of the organisms found in tidal pools are similar to those found on the adjacent exposed rock.
At first glance, tide pools appear more or less ideal places for aquatic organisms seeking to escape the harshness of the intertidal during its exposure to air. In reality, however, escape from such physical factors as desiccation may mean exposure to others that operate more severely in tide pools.
Tide pools vary a great deal in size and in the volume of water they contain. Since water is a great moderator of harsh physical conditions, the larger the pool and the greater the water volume, the less the fluctuations in physical factors. Other factors in addition to volume influence the physical and chemical conditions of the water held in a tide pool. These include the surface area, depth of the pool, height in the intertidal, exposure to wave action and subsequent splash, degree of shading, and drainage pattern. In addition, the physical and chemical environment of tide pools can vary vertically with depth and horizontally across the pool. Finally, all physical factors may fluctuate diurnally and seasonally. Given all these variables, Metaxas and Scheibling (1993) suggest that it is unlikely that any two tide pools will be similar in all characteristics; therefore, individual pools are unique physically. If this is true, it means ecologists cannot replicate experimental manipulations in different tide pools and may be a significant reason why there has been so little experimental ecological work done on pools. It may also be the reason why most studies have been primarily descriptive.
Three major physical factors are subject to variation in tide pools. The first is temperature. Whereas the ocean itself is a vast reservoir that heats and cools very slowly and usually within very narrow limits, the same is not true of tide pools. These relatively small bodies of water are subject to more rapid changes. Shallow tide pools exposed to the sun on warm days may quickly reach lethal or near lethal temperatures. Similarly, tide pools in cold temperate or subpolar regions may have temperatures in the freezing range in winter. An additional problem is temperature fldctuation. The pool may either heat up or cool down over a several-hour period while exposed to the air, but when the tide returns, it will be flooded with ocean water. This will suddenly change the temperature of the whole pool. Variations in daily temperature in a tide pool can be as much as 15°C, depending on its height in the intertidal, volume, degree of shading, and wave exposure. Finally, change in temperature during exposure at low tide may cause temporary thermal stratification of the pool. Organisms inhabiting such pools must still be adapted to considerable temperature fluctuation.
The second factor to vary in pools is salinity. During exposure at low tide, tide pools may heat up, and evaporation occurs, increasing salinity. Under hot tropical conditions, the salinity increase can be dramatic enough to reach the point of precipitating out salt. The opposite situation is the case when heavy rains occur at low tide and flood
pools with fresh water, dramatically lowering the salinity. Fluctuations in salinity in tidal pools vary with the position of the pool on the shore and the other factors mentioned, but values that have been measured have ranged between 5-25 psu. Salinity stratification also may develop in the pool due to fresh water runoff during heavy rains or as a result of freezing in winter and evaporation in summer. Again, tide pool animals and plants may have to be adapted to wider ranges in salinity than typical marine or intertidal organisms. As before, when the tide returns, the pool will be flooded with seawater at some point, and there will be an abrupt return to normal conditions.
The final physical factor undergoing change in pools is oxygen concentration. Since the amount of oxygen that can be held in seawater is a function of temperature, it follows that tide pools that heat up during exposure to the air will lose oxygen. Under normal conditions, this may not be serious enough to produce oxygen stress, but if the pool is crowded with organisms, it may produce a stress situation. For example, a pool filled with algae that was exposed at night would produce a situation in which the lack of photosynthesis coupled with high respiration could reduce the oxygen level significantly. Oxygen level has been recorded falling to only 18% of saturation in tropical tide pools. It is also possible to develop an oxygen stratification in the water column of certain pools.
Tide pools are areas of refuge from desiccation for intertidal organisms, but in turn, these organisms suffer from rapid changes in temperature, salinity, and occasionally oxygen; thus, the fauna and flora is restricted to those organisms able to tolerate such ranges.
organisms that inhabit tide pools are similar to those on the adjacent emergent
substrates but often with differences in abundances between the pools and
exposed surfaces. For example, several genera of algae, such as Spongomorpha and Corallina in the intertidal of
Zonation patterns have been described for tide pool organisms both within the intertidal zone and vertically within the tide pools. Along the intertidal gradient such green algae as the genera
Enteromorpha, Cladophora, and Chaetomorpha,
dominate the upper tide pools, while such brown algae as Fucus and Laminaria, and the red corallines, such as Lithothamnion and Corallina, are abundant in the lower intertidal pools. Benthic invertebrates also show a zonation. High pools are inhabited by littorine snails, whereas low tide pools have a greater diversity of snails and all invertebrates. Fish zonation in tide pools has not been quantitatively documented according to Metaxas and Scheibling (1993), but qualitatively, there is a decrease in the number of species with increasing height of the pool. In general, both in number of species and biomass, high tide pools are depauperate compared with lower level pools.
The role of biological interactions in determining the community structure of tide pools is poorly documented when compared to the studies for the exposed rocky intertidal. This is due not only to a relative lack of studies, but also to the variability among adjacent tide pools at the same intertidal level when subjected to experimental manipulation. Such variability has precluded obtaining statistically significant results in many cases, although correlative data often indicate a trend. We discuss some of these data here.
Herbivory has similar effects in tide pools to those recorded
for the emergent rock surfaces, namely, the altering of macroalgae abundance.
For example, Paine and Vadas (1969) demonstrated that the removal of sea urchin
grazers from tide pools in
Very few studies have investigated the role of predation in structuring tide pool communities. Indeed, there seem to be no well-documented manipulative studies. The few studies that have been done suggest that addition of predators to pools reduces the abundances of various prey organisms, but whether or not the predator exerts control of the community structure is uncertain.
The evidence for the importance of interspecific competition
in regulating community structure in tide pools is also sparse. Lubchenco
(1982) and Chapman (1990) have both demonstrated decreases in canopy cover in Fucus due to competition with ephemeral
algae. Kooistra et
al. (1989) have shown competitive dominance as
indicated by overgrowth for the alga Halichondria
panicea in tide pools in Brittany,
Although recruitment can be a significant factor in structuring tide pool communities, no studies have considered this factor directly. Similarly, there is little or no information about the role of disturbances, particularly large-scale episodic events, in the structuring of tide pool communities.
10 Describe each zone discussed in question 9
11 What does desiccation mean?