P. H. NIENHUIS

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

TILES OF CONCRETE ! BASALT ASPHA 1 T CONCRETE BASAL T

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

LIMESTONE BASALT r I 1

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

LIMESTONE f GRANITE

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

a E N T O P H Y S A L l S DEUSTA BELT

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

~ F U C U S S P I R A L I S B E L T ,[, ~ R L I D I N G I A MINIM.4-ULOTHh'I%FrACCA BELT mASCOPHYLLUM NODOSUM BELT

PEL VETIA CANA UCULATA BE1 T FUCUS VESICULOSUS BELT

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 ~ ~~ ~~

70-;

6 0 ~ '

40-1

30-1

20-

10-

3I.H.W.

10-

20.-

30-

40-

50-

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 ~~ _ _ ~ ~

I

1 tiles of concrete very sheltered

NW

n H . i I . . I

! I I

limestone + doncrete very sheltered

E 15'

Fig. 2. Grevelingen (S. W.-Netherlands). Vertical amplitudes of some algal belts on diffe- rent types of substratum. I Entophysalis deusta belt; 11 Blidingia min ima - Clothrix flacca belt

The Significance of the Substratum for Intertidal Algal Growth 213

well distinguishable in autumn, whereas especially the upper limit has been pushed downwards by desiccation in summer.

The upper limit of the Pucus spiralis belt, not indicated in fig. 2 shows a slight shifting due to the influence of the changing weather conditions. In au- tumn the upper limit of this zone lies - generally spoken - lower than in spring (see also DEN HARTOG, 1959). It ought to be clear that the relations drawn here may show small differences, dependent on the intensity of the climatic fac- tors. The situations discussed here however, do not form an exception, but illu- strate a rule repeatedly observed by me in many places.

4. D i s c u s s i o n

The chemical nature of the substratum is hardly of importance to benthic algae; the physical structure is much more important. Not withstanding the fact that the size of the units of the substratum (pebbles, larger stones, massive rock; CHAPMAN 1964) has a distinct significance, the degree of hardness and the texture play a role. Stress is laid on these latter aspects of the problem. BOALCH (1957) working on the exposed coast of S. E.-Devon (England), traced the signi- ficance of different types of substrata for algal growth and concluded that there was no effect of the substratum on the local distribution of the algae. I come to the opposite conclusion. BOALCH worked on an exposed coast without Aseophyl- luin and practically without Fucus spiralis. The exposure factor could have been superimposed on the minor substratum factor. My work took place in shel- tered sea-arms where the importance of the substratum is distinctly pronounced. Moreover BOALCH studied the effect of the substratum on algal belts lying mainly below the mean high-water line a t neap tide. I examined the importance of the substratum principally in the supralittoral region and the upper eulittoral region. An influence of the substratum on the algal zonation cannot be noticed either in the Netherlands in the lower parts of the eulittoral region. My conclu- sion therefore has to be regarded as a completion of the one of BOALCH.

The importance of the substratum was also emphasized by DEN HARTOG (1959). He already noticed that this influence of the substratum is often recognizable by the position of the different algal belts. DEN HARTOG (1959) describes the exam- ple of Strijenham (S.W.-Netherlands) where the Pelvetia and Fucus spiralis belts are situated higher on limestone than on granite and were totally absent on ba- salt. LEWIS (1964) cites this example in his discussion about the role of the sub- stratum in local distribution problems of organisms. My contribution must partly be considered as an extension and more accurate analysis of the substrate- influences initially ascertained by DEN HARTOG (1959).

STEPHENSON (1961) cemented blocks of different kinds of rocks on the beach rock a t Heron Island (Australia) just above M.L.W.N., and then followed (a. 0.) the colonizing of algae. After a 12-month period he concluded t,hat the hardness of the rocksurface is related to algal growth. On the hard stones (basalt, granite) Ralj‘sia was a very conspicuous component, while on the softer (lime) stones green algae (a. 0. Blidingia minima) werde best developed. My observations in t,he Netherlands seem to offer indications to corroborate STEPHENSONS state- ment (I.c. p. 173) : “It must be concluded that the occurrence of Blidingia as a zone - forming dominant upon the beach is basically due to the nature of the substratum.”

214 P. H. NIENHUIS

Many authors have commented upon the causes of zonation (ZANEVELD, 1937; STEYHENSON, 1949; KNIGHT and PARKE, 1950: BARKMAN, 1950; DEN HARTOG, 1959). It is a complicated problem, beyond t'he scope of this article. I n general terms it is assumed that in the eulittoral, but especially in the sub- littoral region, t.he amount of light is determining, whereas more upwards, espe- cially in the supralittoral region, the tolerance for exposurc to t'he desiccating action of the atmosphere is determining. If we start from the principle that the hardness, the texture and tlie colour of the stones co-interfere witlh the percen- tage of moisture of the environmcnt, our observations underlinc the last part' of this hypothesis.

6 . S u m m a r y The significance of the physical structure of the substratum for intertidal algal growth

is demonstrated in different ways: 1 . The significance increascs from thc eulittoral to the suprelittoral region, directly coupled to tlie capacity for retaining wstcr. I n the lower part of the eulittoral region the zonation pattern on embankments appears to be independent from the nature of the substratum and higher on the dcpendence increases. 2. A relatively soft and rough substratum shows a more complete zonat'ion pattern than a hard and smooth surface. 3. The position of the algal belts wit'h regard to the N.H.\\r. line is st'rongly influenced by the physical characteristics of the substrittum.

6. A c k n o w l e d g e m e n t s The author wishes to exprcss his gratitude t o Mr. IV. S. SIJPEKDA (Institute of Gcology,

Free University, Amsterdam) for giving information about the mcthod of MOHS and for placing gauged minerals a t his disposal, to Dr. C. DEX HAFLTOG (Bijksherbarium, Leiden) for critically reading through tlie manuscript and to Dr. K. P. VA.*S (Dcltn Institute, Ycrseke) for kindly correcting the English text.

7. R e f e r e n c e s ANONYMOUS, 1966: Verandcringen in de getijbcweging vestelijk van tlc ($rcvclingendam.

BALLAKTINE, W. J., 19til: A biologically-dcfiucd exposure scale for thr comparative des-

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CICAPMAN, V. J., 1964: The Algae. McMillan Co. Ltd., London. EVANS, R. G., 1947: The intertidal ecology of Cardigan Bay. - J. Ecol. 34: 275-3309. HARTOG, C. DEN, 1959: The epilithic algal communities occurring along the c.oast of the

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- Driemaand. Ber. Ueltawerken, no. 3 t i : 283-292.

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Devon. - J.mar. biol. Ass. U. K. 36: 519-528.

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ratus L. - J. mar. biol. Ass. U. K. 29: 439-514.

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biol. Ass. U. K. 44: 499-542.

Staatsctruk., 's-Gravenhage.

The Significance of t'he Substratum for Intertidal Algal Growth 215

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Drs. P. H. NIENHUIS Hydrobiologicel Institute division Delta-Research Vierstraat 28 Yerseke The Netherlands