Tidal characteristics

Wave exposure

Water movement and the hydrographic regime have a number of effects on both individual kelp plants and on the kelp bed as a whole. In areas where the kelp bed is exposed to heavy wave action (e.g. on an open coast, a headland or at the mouth of a loch), the plants and animals found differ markedly from areas where there is little wave action (within a bay or cove or within a loch). In addition, within sheltered areas there may be kelp beds in regions of high current flow (e.g. at the narrow entrance to a semi-enclosed embayment) or low water movement (within the body of a semi-enclosed embayment). In these different hydrographic regions, a number of major differences may become apparent:

• the species of kelp will vary
• the depth distributions of kelp species may extend or contract
• the structure of the kelp forest may change
• the plants of individual kelp species may exhibit different morphologies
• the understorey algal population and species composition may alter
• the fauna covering the bedrock may be different

As a general rule, tidal heights and ranges have very little effect on kelp beds, since they are mostly subtidal. Although individual plants which grow at the upper edge of the habitat may on occasion be exposed to desiccation and die as a result, this has little effect on the kelp bed as a whole. In locations where LWST (low water, spring tides) occurs around midday, a period of calm weather combined with high barometric pressure and sunshine can result in a catastrophically low tide exposing the upper band of kelps to severe desiccation stress. The results of exposure can be seen in the form of damaged or dead, bleached tissue in the kelp blade. In contrast, where LWS occur in the morning and evening (such as on the Isle of Man) L. digitata may be found as much as 1.5 m above LWS (T. Hill, pers. comm.). Low tide coinciding with severe wave action as the result of storms also has a profound effect, removing larger plants and less flexible species in the surge and swash zones (see also section II.D.3. below). As a result of the episodic exposure and storm events, the population of kelp plants (of whatever species) living to about 1 m depth below MLWS usually has a reduced age range in comparison to deeper parts of the kelp bed. There is some evidence that plants which are only shallowly submerged at midday may be subject to cellular damage as a result of increased UV radiation. However, under field conditions such episodes are of limited duration and plants can repair the damage during the subsequent periods of submergence and darkness (R. Forster, pers.comm.)

In areas that are sheltered from wave action, the tidal current may have a critical effect on the appearance and productivity of the kelp plants. Particularly within coastal embayments, strong tidal water movement can result in the growth of luxuriantly large, long lived plants, which lend a degree of stability to the more usual dynamic turnover of the kelp population. This, in turn, can result in a more than usually diverse population of associated plants and animals within the kelp bed. The tidal flow prevents thick depositional layers of silt building up on the blades of the kelp plants (and effectively reducing productivity through shading) and the water movement over the blades maintains the maximum concentration gradients of external dissolved nutrients (particularly of nitrate and phosphate), which are actively taken up and stored within the kelp tissue. The reduced wave action results in much longer, broader laminae than are found in kelp beds on open coasts.

Kelp species found in the upper sublittoral (A. esculenta, L. digitata, L. saccharina) may extend their habitat into the lower eulittoral in exposed locations subject to a great deal of wave action. However, a period of calm weather in conjunction with sunshine and a low tide will generally result in the loss of these marginal plants. Consequently, the upper band of kelp plants in areas where there is such an upward extension of the habitat, often have a reduced age range in comparison to plants in deeper parts of the kelp bed. The severity of wave action on a kelp bed in an exposed location will result in a more rapid turnover of kelp plants within the kelp bed. As plants get older, they become larger and more bulky - this is particularly the case for L. hyperborea (Kain, 1973). The holdfasts of many of the older plants are also progressively weakened by the browsing and internal tissue excavations of Helcion pellucidum. Both these factors render the older kelp plants more susceptible to removal by wave action.

Laminaria hyperborea is unable to survive where wave action is extreme (such as in coastal surge gullies), since its stiff stipe, topped with a large lamina, is prone to being snapped. In some areas, wave action depresses the upper limit of the L. hyperborea habitat to several meters below MLWS (several 10’s of meters on St. Kilda, T. Hill, pers. comm.) and, under very severe wave conditions, the species may be absent (as reported from Rockall: S. Hiscock in Lüning, 1990). Those kelps found in the eulittoral and upper sublittoral zone (A. esculenta, L. digitata, L. saccharina) have very much more flexible stipes, and narrow (or at least streamlined) blades. Alaria esculenta (and, to a lesser extent, L. saccharina) has a thickened midrib or central part of the blade which acts as reinforcement. In very sheltered areas, the upper sublittoral is dominated by L. saccharina which combines with, and then gives way to, L. digitata as the degree of exposure increases. With ever-increasing wave exposure, L. digitata in turn combines with and is then replaced by A. esculenta in the lower eulittoral and upper sublittoral zones.

The morphology of kelp plants has been shown to be plastic. If kelp plants from a sheltered habitat are transferred to a more exposed environment, their growth form changes to that typical of plants in the new environment (Svendsen & Kain, 1971).

The differences in appearance of the same species growing in different exposure regimes can be striking (see table below). A brief review of the hydromechanical adaptations of kelp species in response to water movement is given by Lüning (1990, pp 344-346).

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