The Significance of the Substratum for Intertidal Algal Growth on the Artificial Rocky Shore of the Netherlands

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    Hydrobiological Institute, division Delta-Research, Yerseke. Communication No. 75

    The Significance of the Substratum for Intertidal Algal Growth on the Artificial Rocky Shore of the Netherlands

    C o n t e n t s

    1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207 2. Material and methods. . . . . . . . . . . . . . . . . . . . . . . . . . . 208 3. Resul t s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208 4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213 5. S u m m a r y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214

    1. I n t r o d u c t i o n

    Centuries ago benthic algal growth on the coast of the Netherlands was only possible on mud flats and salt marshes, while the only solid substrata consisted of shells of molluscs, peat-banks and - as regards epiphytes - the leaves of sea grasses. The epilithic algal vegetation made its entrance not before man begam to protect the mainland against the sea by sea-walls, reinforced with palisades and bouldcrs. In this way il great number of algae could settle on the artificially constructedrocky shore. DEN HARTOG (1959) gave an ample survey of the ultimately developed algal communities.

    The slopes of the dikes, as we know them nowadays, are of a rather recent date. Not before 1827 the first revetment of Vilvorclian limestone was constructed in the south-western part of the Netherlands. Basalt was used for the first time in 1858, and in the beginning of the 20th centjury the first concrete slope was built. After 1953 dike-revetment of asphalt came into use in Zealand, which method became the usual practise in recent years ( W ~ D E R O M , 1964). Owing to these variegated methods of constructing dikes, it is possible to find on one slope different kinds of substratum, connected to one another, and influenced by exactly the same ecological conditions. In such a case the substratum is the only variable factor.

    The purpose of this article is to elucidate the significance of different types of substrata for algal growth in the intertidal region, illustrated by the position of certain algal belts in the zonation pattern.

  • 208 P. H. NIENHUIS

    greyish- yellow black

    2. M a t e r i a l and m e t h o d s The position of the different algal belts under discussion is determined by

    measuring the distance from a arbitrary horizontal zero-line a t the top of the slope to the upper edge of the belts, on every meter (fig. 1) . The schemes are not strictly comparable, since only the relative altitudes of the zones have been measured and, moreover, other ecological factors are variable. On several placw the upper and lower limits of the algal belts are exactly fixed with regard to N.A.Y.,I) by means of levellings. These data have been combined with measu- rements of the average position of the different hydrological levels (viz. M.H.M.) as well with regard to N.A.P., carried out by the State Department of Roads and Waterways (RIJRSWATERSTAAT, 1964; ANONYMOUS, 1966; fig. 2).

    Table 1. Properties of some materials used for the revetment of dikes in the S.W .-Netherlands

    _ _ _ _ _ _ ~ _ - ~~ ~~ ____ - - Naterial 1 Hardness 1 Texture 1 Colour

    ~~~~~~ ~~ - ___ ~~~ __ ~~~~ I I I

    granite basalt concrete (tiles)

    concrete (mortar) limestone asphalt concrete

    7 6-7 4-5

    4-6 4

    dependent on temperature

    rough, rather coarse-grained rather smooth, fine-grained rather rough, rather fine- grained1 rough, coarse-grained rough, rather fine-grained rough, coarse-grained

    grey-brown dark grey-black grey

    The hardness of the materials used for the protection of the dikes was detcr- mined with the aid of the test of MOHS. According to this scale the physical hardness of a mineral can be expressed by its scratchability with a gauged mineral in a fixed number (1 - 10). The higher the number the harder the mine- ral. It should be noticed that the method of MOHS has been developed for deter- mining the hardness of minerals, while it is employed by us for natural and arti- ficial stones. Therefore the hardness-numbers (table 1) have a limited utility only. I n table 1 some properties of a number of materials, used for the construc- tion of dikes, are summed up.

    During the interpretation of the phenomena depicted in figures 1 and 2 special attention was paid to the age of the various dike revetments, as this factor is also of paramount importance.

    3. R e s u l t s In the intertidal zone, the border area bctween land and water, various eco-

    logical factors are operating, dctcrmining the local distribution of organisms. The major factors are the periods of immersion and emersion, wave action and tidal currents (exposure) and the topography of the area. As modifying factors can be mentioned the aspect of the coast (shade or sunlight, north- or south- facing slope), the angle of inclination and the physical nature of the substratum (hardness, surface texture and colour ; cf. LEWIS, 1964).

    Level (M.S.L.).

    - - ._

    l) Standard level used in the Netherlands; i t corresponds approximately to Mean Sea

  • The Significance of the Substratum for Intertidal Algal Growth 209

    On many rocky shores exposure and topography come to the fore to such a degree that the modifying factors are of secondary importance. This may be one of the reasons that the significance of the substratum is hardly discussed in many papers about zonation phenomena (a. Q . EVANS, 1947; BARKMAN, 1950; KAIN, 1958; LEWIS, 1965). The Dutch circumstances however, show a different image. Exposure and topography can be eliminated rather completely, since all data can be obtained in relatively sheltered sea-arms on slightly inclining slopes of dikes without disturbing obstacles. Applying the biologically defined expo- sure scales of BALLANTINE (1961) and LEWIS (1964), a large part of the area appears to be sheltered to very sheltered. For this reason the modifying factors are most conspicuous in the Netherlands. However, since we compare different substrata showing similar aspect and having similar angles of inclina- tion, even these factors can be eliminated, so as to leave the physical structure of the substratum as the important ecological factor.

    The physical structure of the substratum shows a direct relation to the capa- city for retaining water. Ascending in the littoral region, desiccation of the substratum stands an increasingly better chance of becoming the limiting factor of growth. The influence of the substratum can be illustrated best by the posi- tiw of the highest belts in the zonation pattern.

    To that end we confine ourselves to the situation in the sheltered parts of the sea-arms in the south-western part of the Netherlands and pay attention to the spring aspect. The highest situated algal zone is the black Entophysalis deustal) belt. Studying the species-composition in many localities leads to the following survey: Entophysalis deusta is dominating in nearly all cases. As com- panion species can act Prasiola stipitata, Bangia fuscopurpurea, Urospora peni- cilliformis, Rhizoclonium riparium, Enteromorpha torta, Calothrix scopulorum, Phormidium fragile, Plectonema battersii, together with a few other blue-green algae and some constituents of the next algal belt, to be discussed below.

    Right under the Entophysalis belt the Blidingia minima - Ulothrix flacca belt is situated. As a rule Blidingia min ima is dominating the whole year through, but especially in spring the short living Ulothrix flacca can be aspect-determi- ning on places covered by algae only a short period of the year. Besides most of the associates mentioned at the Entophysalis belt, the following species can be distinguished here : Blidingia marginata, Ulothrix pseudoflacca, U . subflaccida, Monostroma oxyspermum, Codiolum gregarium, Enteromorpha compressa, E . po l i fera , Pelvetia canaliculata, Bucus spiralis, Ralfsia verrucosa, Hildenbrandia prototypus, Porphyra umbilicalis, always in small quantities.

    In most places a Pelvetia canaliculata belt is lacking. In localities where this vegetation is still distinct, a considerable overlap takes place with the Blidingia- Ulothrix belt. Connected with the Blidingia- Ulothrix belt the Fucus spiralis belt appears, sometimes mixed with Pelvetia canaliculata. F . spiralis is the only dominant. Except many species of the Blidingia- Ulothrix belt companions are : Catenella repens, Rhodochorton purpureum, Ascophyllum nodosum, Fucus vesi- culosus, Entophysalis conferta, and Microcoleus tenerrimus. Downwards, adja- cent to the Fucus spiralis belt, we find a zone dominated by Ascophyllum nodo- sum at some places, and by Fucrus vesiculosus on other, usually more exposed localities. This lower situated zone plays a subordinate role in our argument, as

    l) Nomenclature according to PARKE and DIXON (1964).

  • 210 P. H. NIENHUIS


    a)ZANDKREEKDAM - EASTERN SCHELDT Dafe: 23-4- J968 Inclinafion: 20" Aspect: S


    Dafe: 78-3-7968 Inclinafion: 20" Aspect: S


    Date: 7-5- 7968 Inclinafion: 20" Aspect: SE Legenda: --


    Dater 78-3-7968 OFm InclinaiVon: ZOO Aspect: YE 1



    Fig. 1. Schemes of the zonation pattern of algae on different kinds of substratum (S. W.-Netherlands)

    we shall see. On the still lower lying Pucus serratus belt the hardness and the texture of the substratum have no discernable influence a t all.

    I n order to explain the importance of the substratum we compare the zonation patterns on parts of embankments built up of different materials, acljacent to one another. From fig. l a it appears that the Entophysalis deusta and PUCUS spiralis belt are lacking on a slope of basalt constructed in 1959. The Blidingia minima-Ulothrix flacca belt is only visible on a relatively low-lying level. The vertical amplitude of this pioneer vegetation extends from about the mean high-water line at spring-tide (M.H.W.S.) till below mean sea level (M.S.L.). Only the lowest part of the belt is conspicuous on basalt. At the same time it is evident that the Ascophyllum nodosum - Fucus vesiculosus vegetation lies 1 to 2 m lower on basalt than on tiles of concrete. On the less hard and more rough concrete the cornplete zonation pattern is sharply distinguished. From fig. 1 a it appears moreover that the substratum asserts itself only in the supralittoral

  • The Significance of the Substratum for Intertidal Algal Growth 211

    region (above the mean high-water line) and the upper part of the eulittoral region. In the lower eulittoral region we find the same vegetation on concrete as on basalt.

    I n fig. 1 b the situation on a slope consisting of asphalt concrete and basalt, completed in 1955, is reproduced. The coarse-grained, rather soft asphalt con- crete shows the same complete zonation pattern as the tiles of concrete of fig. l a . The more thermolabile mastic asphalt is used in reinforcing the dikes, the smaller the chance will become for perennial algae to maintain themselves du- ring summer. In fig. l c Vilvordian limestone is compared with basalt on the slope of a dike with a revetment of - a t least - ten years of age. On limestone the complete zonation pattern in present again, comparabele with concrete and aspalt concrete. At the same time it becomes clear from this profile that the occurrence of Pelvetia on limestone only shows quantitative differences com- pared with the distribution on basalt : the covering is the smallest on basalt. Although the vertical amplitude of the Pelvetia belt is smaller on basalt than on limestone - as a result of the quantitative differences - the Pelvetia belt shows no downward shifting. In fig. 1 d we find a slope of an embankment on which the very hard granite is adjacent to the softer limestone. The moderately developed Entophysalis, Blidingia- Ulothrix and Pucus spiralis belts, present on limestone, are lacking on granite. Moreover the upper limit of the Ascophyllum zone is situated about 1 m lower on granite than on limestone. Ascophyllum forms a closed vegetation cover in the lower eulittoral region on granite as well as on limestone.

    In the Grevelingen, a sea-arm in the south-western part of the Netherlands, we were able to collect some comparable data pertaining to the substratum phenomenon. Three localities separated by several kilometers, are shown in fig. 2. Applicating the exposure scale of BALLANTINE (1961) the three slopes can be reckoned among the very sheltered. Insolation and angle of inclina- tion show negligible differences. The only variable factor is the substratum, in all cases placed on the dike many years ago. The localities have been arranged, according to their substrata, from very hard to much less hard. The thick ver- tical lines represent the vertical amplitudes of the algal belts (cover more than 5 % ) ; the interruption of these lines means a decreasing coverage. Both the spring aspect and the autum aspect of the same slopes have been reproduced in the figure.

    As regards the spring aspect of the three slopes, we should like to make the following remarks. On basalt the Entophysalis belt is hardly perceptible. The belt becomes broader and, moreover, grows on a higher level, while a t the same time the covering increases as the stones become less hard and more rough. The Blidingia-Ulothrix belt too shows a shift towards a higher level, from basalt to limestone.

    The autumn aspect of the same transects shows a different picture. On basalt the Entophysalis as well as the Blidingia- Ulothrix belt have totally disappeared in the course of summer, probably owing to the slight capacity of basalt for retaining water, and also to the fact that the temperature of the boulders is raised owing to the absorbation of heat, a result of their black coloration. On the tiles of concrete the Entophysalis belt has disappeared and the upper part of the Blidingia- Ulothrix belt is not present anymore, while the lower border shifted downwards. On the rough limestone-concrete (mortar) slope both belts are

  • 212 P. H. NIENHUIS

    substratum exposure aspect inclination

    Scharendi j ke Bruinisse Harbour Jet ty I Locality

    basalt very sheltered

    NW 20"

    _ _ ~ date 1 12.4.1967

    Belt , I I1 ~ ~~ ~~


    6 0 ~ '











    cm. ____

    I I I I I I I I

    I I . m

    I I m

    ! I

    -. ~~-

    I I1 10. 1966 1 11 -

    Brouwershaven Harbourentrance

    12.4. 1967 1 7. 11. 1966 ~~ _ _ ~ ~


    1 tiles of concrete very sheltered


    n H . i I . . I

    ! I I



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