The Critical Tidal Factors Hypothesis

Much of the coastline of temperate latitudes displays a mixed semidiurnal pattern of tidal flux: two high tides of unequal height, separated by two low tides also of unequal height, occur over the course of approximately 23 h 50min. The amplitudes of the tidal excursions vary cyclically, so that extreme high amplitudes (termed spring tides) evolve to relatively low amplitudes (neap tides) and back to high amplitudes within a fortnight (Figure 5). A consequence of this tidal regime is that summary parameters, such as maximum duration of emergence occurring in the spring-neap cycle, do not vary gradually with respect to shore level, but rather show three- to fourfold changes within certain restricted segments of the tidal range, segments termed critical tidal factors. For example, sedentary organisms occurring above the neap level of higher high tides sometimes must endure continuous emersion for several days, provided wave action does not intervene by casting water above that level. Organisms attached at lower levels endure periods of continuous emersion of less than a day.

The critical tidal factor hypothesis maintains that the vertical limits of organisms are delimited by the abrupt change in physical conditions at the critical shore levels. The primary mechanism invoked to explain the formation of boundaries is the organism's intolerance to desiccation during emersion above the upper boundary and to submergence during immersion below the lower boundary. Secondarily, it was proposed that the critical factors constrain the incidence of suspended settlers (larvae or spores) with the substratum. In either case, it was proposed that an effect of the tidal regime directly on the adaptive limits of the species causes the boundaries of the species to cluster about the levels of the critical tidal factors.

The original statements of the hypothesis relied upon qualitative descriptions of the zonation patterns matched with imprecise estimates of the critical tidal factors. Subsequent quantitative analyses for shores of the British Isles, for which the hypothesis was first proposed,

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Figure 5 Tidal curve showing the fortnightly variation in the semidiurnal tidal cycle of the central California region for which critical tidal factors were proposed. Levels corresponding to critical tidal factors are labeled according to the diurnal and spring neap variation in the tides. For example, LLLW refers to lower low low water, which is the spring (extreme) low of the lower of the two diurnal low tides. Adapted from Doty (1949).

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Figure 5 Tidal curve showing the fortnightly variation in the semidiurnal tidal cycle of the central California region for which critical tidal factors were proposed. Levels corresponding to critical tidal factors are labeled according to the diurnal and spring neap variation in the tides. For example, LLLW refers to lower low low water, which is the spring (extreme) low of the lower of the two diurnal low tides. Adapted from Doty (1949).

showed relatively small stepwise increments of tidal emersion with respect to shore level, and the boundaries of species were not found to cluster about these modest steps in the tidal factor curves. The concept of critical tidal factors may retain some relevance for coasts of North America and other locales where tidal emersions do show marked stepwise changes at certain shore levels. However, many other physical factors correlated with zonation, the most important of which is exposure to wave action, preclude any simple relationship between predicted tidal curves and the observed levels of zones. Furthermore, the restrictive proposition about the mechanism - that all boundaries are set directly by physical factors controlled by the tidal regime - eventually was contradicted by field experiments conclusively demonstrating an effect of biotic factors (competition and predation).

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