The intertidal zone is a narrow fringe area a few meters in extent between high and low water. It forms the junction between land and sea. Despite being restricted, the intertidal zone has the greatest variations in environmental factors of any marine area, and theses can occur within centimeters of each other. Coupled with this is a tremendous diversity of life, which may be as great as or greater than that found in the more extensive subtidal habitats (Nybakken, 1997; Round, 1973).

In the rocky intertidal zone, space is the principal limiting resource, since to live there, organisms have to be firmly attached to the substrate. If dense strands of seaweed develop, however, light becomes the critical factor as the canopy of algal fronds shades the rocky surface (Littler and Littler, 1985). In the intertidal zone, organisms have to be able to withstand high mechanical stress caused by wave action, extreme temperature and salinity fluctuations, and the absence or overabundance of light and of nutrients. Hence, physiological tolerance is very important (Littler and Littler, 1985). The physiological tolerance of algae to temperature, light, and salinity fluctuations, or the combination of these factors, has been investigated by several authors (Davidson, 1991; Karsten et al., 1990; Thomas et al., 1990 and 1988; Russel, 1987; Lubchenco, 1980; Schonbeck and Norton, 1980 and 1978; Quadir, 1979; Ramus et al., 1976; Lewin, 1962). Nevertheless, physiological tolerance is not the only factor leading to the typical vertical zonation found along the rocky intertidal zone. Competiton amongst the organisms, as well as predation, also have an affect on the zonation pattern (Darley, 1982; Lubchenco, 1980 and 1978).

Zonation of algae in the intertidal zone has been investigated by several authors (Nybakken, 1995; Dring, 1992; Barnes and Hughes, 1988; Littler and Littler, 1985; Darley, 1982; Lubchenco, 1980 and 1978; Schonbeck and Norton, 1980 and 1978; Evans, 1949; Stephenson and Stephenson, 1949; Coleman, 1933). The authors agree that the littoral zone can be divided into three zones, namely the Littorina zone, the Balanoid zone, and the Laminaria zone (Evans, 1949; Stephenson and Stephenson, 1949; Coleman, 1933).

The rocky intertidal zone of Menai Bridge, Anglesey, is a region of high algae diversity. The tides and the water currents of the Menai Strait are very complex and they have an immense effect on the intertidal organisms (The National Trust, 1996; Troëng, 1994). The aim of this experiment is to investigate the diversity and the zonation of macroalgae in the intertidal zone of the rocky shore near Menai Bridge, Anglesey, and to relate the observed zonation patterns to the morphology, cytology and the physiology of the algae.

  Approximately 200 meters of coastline were included in the survey, approximately four hundred meters west of the Menai Suspension Bridge in Menai Bridge, Anglesey (Wales), at the level of the swellies (see map, Fig. 1). The Menai Strait has two high tides and two low tides a day. On the flood tide, seawater funnels into the Strait through the very narrow channel at its southwestern mouth. Although it flows very quickly, the constricted entrance nevertheless holds back incoming water. At the same time, water is also flowing around Anglesey to enter the Strait through its northeastern end. The two streams eventually meet, usually at some point between Bangor and Menai Bridge, and for about half an hour there will be no horizontal flow of water although the water level continues to rise. Only when the stream flowing from the northeast overcomes its opposite number does all the water begin to flow to the southwest. The water continues to flow to the southwest as the tide ebbs, but close to low water the last of the tide in the northeast changes direction and flows back out into the sea past Beaumaris. The tidal currents are very strong, up to 8 knots in the area of the swellies. Material entering the Strait from the northeast may take two or three days or more than a week to pass through to the southwestern end. Severe weather and strong wind from south west strongly affect the flow through the Strait. They can even reverse the residual flow of the Strait. (The National Trust, 1996; Troëng, 1994).

Tidal data for Menai Bridge was obtained from the Menai Strait tide tables 1999 by Sherwin (1999). On October the 6^th 1999, which is the day on which we performed our experiment, hight tides were at 08:39 a.m. GMT (6.24 m) and at 08:59 p.m. GMT (6.69 m). Low tides were at 02:42 a.m. GMT (1.61 m) and at 03:06 p.m. GMT (1.68 m). In our experiment, the lower tide-mark referes to the level at low tide at 03:06 pm GMT, at 1.68 m. At Menai Bridge, the tidal range at extreme spring tides is approximatively 7.8 m and at extreme neaps it is approximatively 0.5 m. The intertidal zone included in the survey had a width (distance between the upper and the lower tide-marks) of approximatively 35 meters, and the difference in height between the two watermarks was approximatively 6.5 meters.

In the Strait, light penetration is limited due to poor water clarity caused by large amounts of sediment held in suspension.

The Strait's water temperature varies with the seasons. It is about 17°C in summer to about 4°C in winter. On sunny days, the water in shallow rock pools can be considerably warmer than 17°C (The National Trust, 1996). According to Harvey (1972), there are semi-diurnal temperature oscillations of 0.07-0.15°C caused by advection by tidal currents, diurnal temperature oscillations of 0.04-0.20°C due to the day/night heating cycle, and forthnightly oscillations due to the transport of heat awy or towards the coast through mixing by tidal currents varying with the tidal range.

The monthly average salinty of the Menai Strait varies from 31.93‰ in January to 33.82‰ in May (Troëng, 1994).

  The survey of the zonation of macroalgae in the intertidal zone took place on October 6^th 1999 at Menai Bridge, approximatively 400 meters west of the Menai Suspension Bridge, at the level of the swellies (see figure 1). The measurements were taken between 2:00 and 4:30 pm local time (i.e. 1:00 to 3:30 pm GMT), which corresponds to the period of low tide. The actual height of the lower tide-mark to which we refer in this experiment is 1.68 m. On the days before October 6^th and on October 6^th itself, it had been cloudy and rainy and the temperature was normal for the time of the year. Along 8 different transacts spread along the 200 meters long survey area, the distribution of algae was evaluated using quadrats. For each of the eigth series of measurements, the first measurement was taken at the upper limit of the intertidal zone. A cross-staff (originally designed by J. Moyse and A. Nelson Smith) was used to determine the position of the next measurement site down the shore. The distance from the top of the spirit level to the base of the upright was 50 cm, which corresponds, approximately, to 1/10 of the spring tidal range at Menai Bridge. The cross staff was positioned such that, when the top edge of the cross staff was at the same level as the middle of the first quadrat, the bubble in the spirit level was central. The cross staff, in its new position, determined the centre of the second quadrat, and so on, down the shore. The distance between two measurement sites was measured with a measuring tape. 25 x 25 cm² quadrats were used to evaluate the percentage of surface covered by each algal species along the shore. In addition to the measurements taken at 50 cm height intervals, measurements were taken along the same line as the other measurements, in between the latter, in order to obtain further data from the flatter regions (at which the frequency of measurements had been lower, using the spirit method, due to the lack of slope).

The algae which were common in the surveyed intertidal zone were identified and analysed at naked eye, as well as under light microscopes, in order to investigate their histology and their cytology. Iodine was used to stain the cells. Slides of transverse sections of Laminaria digitata stipes, which had been provided, were also analysed under light microscopes.

As we can see in figure 2, within the 200 m long survey area, the shore profiles were rather diverse. The slopes were unequally steep, and the substrate varied along the shore. The distribution of the algae varied from one transact to another. The reasons for this variation are the differences in the slope and the rock configuration between the transacts and the linked differences in the degree of exposure to physical and biological stress. The measurement techniques varied slightly between the different surveyers groups, and the measurements of the percentage of coverage was very objective. However, along all of the transacts, there was a similar global pattern of distribution. Pelvetia canaliculata was only found at the upper limit of the intertidal zone, at the same level as Verrucaria maura lichens, and Laminaria digitata was only found at or below the lower limit of the intertidal zone. Ascophyllum nodosum and Fucus serratus were very abundant and ubiquitous in the intertidal zone. They were only absent at the upper limit of the littoral zone and in very steep zones with little bulky substrate to attach onto. Fucus serratus was less abundant than Fucus vesiculosus, and it was only present lower down the shore. Fucus spiralis was also less abundant than Fucus vesiculosus, and it was only found in the upper part of the intertidal zone. Red algae (Ceramium rubrum, Mastocarpus stellatus, Chondrus crispus, Cryptopleura ramosa, and Corallina officinalis) were only found in the lower part of the intertidal zone. Because they are very small and thin, their percentage of coverage was small. Ulva lactuca, Enteromorpha linza, and Ectocarpus siliculosis are also only present in the lower part of the intertidal zone. Ulva lactuca is very thin but it has a big surface. Hence, its percentage of coverage can reach up to 50%. In the rockpools, thin algae such as Ulva lactuca, Enteromorpha linza, Mastocarpus stellatus, Chondrus crispus, and Cryptopleura ramosa were very abundant.

  In order to understand the zonation pattern of the algae along the intertidal zone, we analysed the structure of the algae in the laboratory. The common green algae (Chlorophyceae) found in the intertidal zone at Menai Bridge are Ulva lactuca, Enteromorpha sp., Codium tomentosum, and Cladophora rupestris. The common brown algae (Phaeophyceae) found in the intertidal zone at Menai Bridge are Ascophyllum nodosum, Fucus serratus, Fucus vesiculosus, Fucus spiralis, Pelvetia canaliculata, Ectocarpus siliculosus, and Laminaria digitata (lower limit of the intertidal zone only, more abundant in the sublittoral zone). The common red algae (Rhodophyceae) found in the intertidal zone at Menai Bridge are Mastocarpus stellatus, Chondrus crispus, Cryptopleura ramosa, Corallina officinalis, and Ceramium rubrum.

In most of the cells of fresh algae, we could observe under the microscope, at a magnification of x400, an intracellular flow of the cytoplasm and of the chloroplasts.

 Enteromorpha linza is very similar to Ulva lactuca, except that the thalli form thin, sometimes spiralling, elongated grass-like stripes. This reduces the surface/length ratio. Hence, Enteromorpha linza is more resistant to the drag force than Ulva lactuca. Another species of Ectocarpus commonly found in the intertidal zone at Menai Bridge, although not on our transact, is Enteromorpha intestinalis. Enteromorpha intestinalis has an inflated tube-like structure.

 Codium tomentosum has an 8 cm long dichotomously branched dark green frond. The frond has a double wall. In October, yellow-orange fruiting-bodies can be found at the tips of the frond.

Cladophora rupestris forms 6 cm long tufts of very thin dark green threads. The observation of threads under the microscope, at a x400 magnification, shows that the cells are multinucleated and that they contain many angular chloroplasts and many pyrenoids.

 Ascophyllum nodosum has a dark olive green midrib-less strap-like dichotomously branched frond. Along most of its length, there are short stalks to which reproducing bodies can be attached. Every 5-10 cm along the frond, there is an egg-shaped gas bladder. The leathery tissue of the frond is approximatively 1 mm thick, and rather resistant. Ascophyllum nodosum is attached to the rocky surface by its holdfast. Ascophyllum nodosum can be up to 30 cm long. The red alga Polysiphonia lanosa is frequently found attached at the junctions of the short stalks with the frond, Polysiphonia lanosa is a very thin branched dark red-black alga.

 Fucus serratus has a cylindric wooden stipe attached to the rocky surface by a discoid holdfast. The edges of the olive-green dichotomously branched frond are serrated, and a midrib runs through the middle of the flattened leathery frond. The reprodutive bodies are located at the end of the frond. Gas bladders are absent on Fucus serratus. Small barnacles and bryozoans are often found firmly attached onto the frond of Fucus serratus. Fucus serratus can reach a height of 100 cm.

 Fucus vesiculosus has a stipe similar to the one of Fucus serratus. The frond is yellow to olive-green, and it is dichotomously branched. Its margins are smooth. Along the midribs, there are many small air-bladders, which have a diameter of 3-15 mm. The reproductive bodies are at the end of the frond. Fucus vesiculosus can reach a height of 100 cm.

 Fucus spiralis is similar to Fucus vesiculosus, except that it does not have any air-bladder. The reproductive bodies are located at the heart-shaped ends of the dichotomously branched frond. The frond is often twisted. The frond has a double wall. Hence, it can be partially filled up with water.

 Pelvetia canaliculata has a curled dichotomously branched frond which forms a channel in which the alga can trap water before it gets exposed to air and to sunlight. This prevents Pelvetia canaliculata from drying out. The frond of Pelvetia canaliculata has a length of approximatively 8 cm.

 Ectocarpus siliculosus has 10-30 cm long irregularly divided fronds made up of thin yellow-brown filaments. Ectocarpus siliculosus is attached to the surface by its holdfast.

 Laminaria digitata has a strong leathery oval cylindric stipe firmly attached to the substrate by a small root-like holdfast. The smooth, leathery, and very flexible golden-brown frond can be very large. Growth occurs at the base of the frond. At its end, the frond is cut into approximatively 5-10 cm wide lamina. The split between the lamina can run almost down to the junction with the stipe. When uncovered with water, Laminaria digitata is not able to maintain itself upright. The transverse section of the stipe of an old and a young plant of Laminaria digitata are shown in figure 4. In figure 4, we can clearly recognize the cell differentiation characteristic for most Phaeophyceae. The stipe of an old Laminaria digitata plant is composed of a cuticle and a layer of epidermal cells which protect the plant, followed by a large layer of cells which are rich in chloroplasts. In the middle of the stipe, there are cells responsible for the transport of water, nutrients, and waste material through the plant. This cell differenciation allows the plant to optimise its photosynthesis and its protection mechanism against dessication and other stresses. Not every single cell depends on photosynthesis, as nutrients can be transported throughout the plant. In younger plants of Laminaria digitata, the stipe is less oval and the cuticle is thinner, and the central cells have not maturely specialised yet into cells responsible for nutrient and water transport through the plant.

 Mastocarpus stellatus has an 8 cm long dark red-brownish biramously branched frond. The frond is channeled and it's thickened along its margins. The ends of the frond are sharp and snake-tongue-like. Mastocarpus does not have any midrib. It is rather easy to confuse Mastocarpus stellatus with Chondrus crispus. However, in October, it is easy to make the difference between the two species of red algae, because the fruiting bodies of Mastocarpus stellatus are very characteristic. They form sticky papilla and they are located all along the frond of the alga.

 Chondrus crispus is similar to Mastocarpus stellatus, except that it is not curled and that its fruiting bodies don't have the characteristic shape of the ones found on Mastocarpus stellatus. The ends of the frond of Chondrus crispus are curved, the y are not sharp as in Mastocarpus stellatus.

 Cryptopleura ramosa has a very thin, very fragile, flat, red transparent frond. The frond is biramously branched, and a midrib runs through the middle of each branch, although it's difficult to be recognized. The frond is approximatively 6 cm long.

 Corallina officinalis has a very characteristic calcerious structure. It has a pinnate branching structure and the branches are always opposite to eachother. The 3 mm long coarse calcified segments of the frond are bleached red in colour. The frond has an average length of 6 cm.

 Ceramium rubrum is very thin and filamentuous. It is often found in tufts. It is dichotomously branched. The ends of the branches bend inwards to form a claw-like structure. Ceramium rubrum is 5-6 cm long. It is dark brown in colour when the plant is wet and it becomes black as the plant dries out.

Along the intertidal zone at Menai Bridge, there is a clear zonation pattern both for the fauna and for the macroalgae inhabiting the area. The upper part of the intertidal zone forms a habitat for Littorina littorea, for some barnacles, and for some whelks, as well as for macroalgae such as Pelvetia canaliculata and Fucus spiralis. The middle of the intertidal zone is characterized by the presence of barnacles, some little crabs and some dogwhelks, and macroalgae such as Fucus spiralis, Fucus serratus, Fucus vesiculosus, Ascophyllum nodosum, and Cladophora rupestris, and lower down in the middle part of the zone, we find periwinkles, barnacles, limpets, as well as red algae such as Mastocarpus stellatus, Chondrus crispus, Cryptopleura ramosa, and Corallina officinalis, and small or thin green algae such as Ulva lactuca, Enteromorpha linza, Enteromorpha intestinalis, and Ectocarpus silicious. At the lower limit of the intertidal zone, we find periwinkles, a few crabs, and tube-worms, and a few barnacles, as well as Fucus vesiculosus, Ulva lactuca, Chondrus crispus, and Laminaria digitata. At the location of our survey in Menai Bridge, Laminaria digitata is only found at the very end of the intertidal zone and further down in the sublittoral zone. This agrees with the zonation pattern described in litterature. The intertidal zone is usually divided into three zones. The "Littorina zone" corresponds to the upper part of the intertidal zone, the "Balanoid" zone corresponds to the middle part of the zone, and the "Laminarian" zone corresponds to the lower part of the zone (Barnes and Hughes, 1988; Schonbeck and Norton, 1980 and 1978; Evans, 1949; Stephenson and Stephenson, 1949).

The Balanoid zone is the widest zone on the shore. It is the zone most subject to modification in response to local factors of the rock configuration, surf,... The distribution of the algae within the Balanoid zone is significantly affected by the steepness of the slope and by the structure of the slope. The algae have to attach to heavy stones in order to maintain their position and not to be washed away by the waves, the tides, and the currents. If the slope is steep and the substrate is sandy, no macroalgae can permanently attach to the surface. The Balanoid zone can be divided into the "Balanus perforatus" and the "Fucus serratus" subzones. The Balanus perforatus subzone is an area in which the slope is steep and the conditions are too rough for Fucus serratus to settle down. Lower down the Balanoid zone, before the Laminarian zone, sheltering rock masses facilitate the settlement of Fucus serratus, and membranous and calcerous Rhodophyceae and Chlorophyceae are found in that area as characteristic species of under-flora (Evans, 1949). This explains why, on our profile, there was, in the middle of the shore, a gap where no algae could be found.

In the Balanoides zone, two very definite types of association can be found, namely an association characteristic of the seaward faces of loose rock slabs, exposed both to waves and to direct illumination, and a cryptofaunal association developed on the landward, sheltered and shady overhangs of the same loose rocks. The distribution of algae can vary significantly around a single rock, because of the water currents passing non-uniformely around the rock. The water currents affect the attachment of the algae onto the substrate. They also affect the presence of nutrients, of predators, and of filter-feeders in the algae's environment (Evans, 1949). Ulva lactuca is usually free-floating in the seawater. As the tide goes out, Ulva lactuca is left behind on the shore, wherever the substrate is rough, or where small obstacles are present to catch the membranous alga. Free-floating Ulva lactuca does not attach on smooth rock-surfaces. Hence, the distribution of Ulva lactuca in the intretidal zone at low tide varies every time the tide goes out. It depends on the structure of the substrate and on the speed of the tidal current (Evans, 1949; Stephenson and Stephenson, 1949).

Competition and predation play an important role in the zonation of organisms along the intertidal zone.(Darley, 1982; Lubchenco, 1980 and 1978). Another very important factor, however, that determines the zonation pattern along the intertidal zone, is the tolerance of the algae to the physical stresses such as temperature, light, and salinty fluctuations and the exposure to atmospheric conditions. The tolerance of macroalgae to these stresses and to the combination of the stresses has been investigated in depth by various authors (Davidson, 1991; Karsten et al., 1990; Thomas et al., 1990 and 1988; Russel, 1987; Lubchenco, 1980; Schonbeck and Norton, 1980 and 1978; Quadir, 1979; Ramus et al., 1976; Lewin, 1962).

According to Schonbeck and Norton (1978), all the littoral macroalgae reach their physiological limit within the intertidal zone, and in general they do not survive a transplantation into another zone of the intertidal region. At the upper limit of the intertidal zone, the major limiting factor is desiccation. Pelvetia sp. is the alga found the highest up the intertidal zone. It spends up to 90% of the time in atmospheric conditions, and it can survive up to 65% of desiccation and resume its original rate of photosynthesis afterwards. Fucoids and Ascophyllum nodosum are only found at levels below Pelvetia sp., unless they are protected by wave splash, by overlying canopy, or by inhabiting shady north-facing slopes (Barnes and Hughes, 1988, Schonbeck and Norton, 1978).

Ramus et al. (1976) have investigated the chromatic adaptations of macroalgae in the intertidal zone. The zonation of the algae is related to their pigmentation. Rhodophyceae are found very low in the intertidal zone. Olive-brown Phaeophyceae are found mainly in the Balanoid zone, and brown Phaeophyceae such as Laminaria digitata are found at or below the lower limit of the intertidal zone. Fucus spiralis is the fucoid found highest up on the shore, and it does not have any air-bladders. Ascophyllum nodosum and Fucus vesiculosus are found attached to substrate lower down along the intertidal zone, and they have air-bladders along the frond to suspend the photosynthetic tissue (Darley, 1982; Ramus, 1976). In sheltered areas, Fucus vesiculosus is found to have more air-bladders than in areas of important exposure (Stephenson and Stephenson, 1949). The bright green Chlorophycea Ulva lactuca is generally found free-floating in the upper part of the water column or, uncovered by water masses on the shore and in rockpools at low tide. Membranous algae such as Porphira sp. and Ulva lactuca can show differential photosynthetic response to the light quality at different depths (Darley, 1982; Ramus, 1976). According to Quadir et al. (1979), the different algae present in the intertidal zone also vary in their ability to use CO₂ and HCO₃^- as a substrate for photosynthesis. Lewin (1962) mentions that in most algae, as well as in higher plants, the chloroplasts can move through the cell and orient themselves towards light. This mechanism is called phototaxis.

Quadir et al. (1979) have investigated the relationship between the blade morphology and the water currents to which the algae are exposed. Laminaria digitata has a very large and very flexible frond. Growth occurs only at the base of the frond. At its end, the frond is cut into approximatively 5-10 cm wide lamina. The split between the lamina can run almost down to the junction with the stipe. This allows Laminaria digitata to cope with strong water currents and to survive damage at the extremities of the blade. Ulva lactuca has a very large surface. Hence, it is less resistant to damage by strong water currents or by obstacles than Enteromorpha linza, which grows in thin grass-like straps. The branches of the frond of Pelvetia sp. form channels in which water can be trapped. This explains why Pelvetia sp. is found very high up in the intertidal zone.

Davidson (1991) has studied the effect of temperature on algal photosynthesis. According to Davidson, temperature acclimatisation of the light-harvesting photosynthetic apparatus is a wide-spread phenomenon in algae. A short-term increase in temperature causes an increase in the rate of photosynthesis up to a certain point, at which the rate of photosynthesis rapidely declines. In most macroalgae, maximum photosynthetic rates occur over a range of several degrees, and this range varies from one species to another.

The interactive effect of temperature and salinity on Fucus vesiculosus has been investigated by Russell (1987). Russell (1987) mentions that at a salinity of 34‰, Fucus vesiculosus is indifferent to temperature and that only at extreme temperatures, there is evidence of serious damage to photosynthetic systems. At extreme salinities, Fucus vesiculosus has difficulties to survive for 48 hours temperatures outside those of the limiting isotherms. Freezing is lethal to tissues pretreated with the lowest but not the highest salinities. The tolerance to the combination of temperature and salinity varies from one algal species to another, and this affects the zonation of the algae along the intertidal zone, as the fluctuations of both temperature and salinity are highest at the upper limit of the intertidal zone and lowest at the lower limit of the shore (Schonbeck and Norton, 1980 and 1978).

 It is important to understand the zonation pattern of algae within the intertidal zone, as algae are becoming of major economical importance, and as the intertidal zone is the marine region which is the most accessible to humans. Macroalgae are becoming widely used in the pharmaceutical industry, in the food industry, and in biotechnology (Dring, 1992). A clear understanding of the response of the algae to salinity fluctuations is also important, as DMSP, which is produced by the algae in hypersaline conditions, seems to play an important role in the regulation of atmospheric processes (Karsten et al., 1990; Dickson and Kirst, 1986).

Hence, further investigations have to be done about the mechanism regulating the zonation pattern of algae in the intertidal zone. It is important to repeat our experiment at different seasons, and at different phases of the tidal cycle in order to be able to generalise our results.

  Along the intertidal zone of Menai Bridge, we can recognize a clear zonation patter of the macroalgae. The upper part of the zone, usually called the Littorina zone, is characterized by the presence of Pelvetia sp. and Fucus spiralis. The intermediate part of the zone, or the Balanoid zone is characterized by the presence of a wide range of brown, red, and green algae. It can be divided into a steep zone called the Balanus perforatus zone and a more sheltered an flat zone called the Fucus vesiculosus zone. At the lower limit of the intertidal zone, we find the Laminaria zone, which is characterized by the presence of Laminaria digitata and of Fucus vesiculosus. This zonation pattern is determined by biological factors such as competition and predation, and by the tolerance of the algae to physical stresses. The tolerance of the algae to the different stresses is related to their morphology and to their physiology.

  I would like to thank Andy, Gareth, Sam, and Stephany for the excellent collaboration within our group, as well as all the people from the other groups which profided us with the data from their shore profile.