Subtidal algal colonization following the removal ofEchinus

Helgoland Marine Research, Jul 1967

1. On the seaward face of Port Erin breakwater large algae do not occur on the lowest three metres. Here the density ofEchinus esculentus is high. 2. Over a three year period allEchinus were removed from a 10 m wide strip at about monthly intervals. The total number of individuals removed approached 3000. 3. One year after initial clearance the mean density ofLaminaria hyperborea sporelings on the strip was 22.7/m2 and 5.1/m2 to one side of it. By the winter these had almost disappeared outside the strip and in the succeeding years second and third year plants were found only on the strip or close to its edge. Other algae were similarly affected. 4. It is concluded that the lower limit ofL. hyperborea is determined at least in part by the grazing pressure ofEchinus and that the urchins exercise some control over the numbers of other algae.

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Subtidal algal colonization following the removal ofEchinus

Subtidal algal colonization following the removal of Echinus S. JONES a n d J O A N N A 0 0 Marine Biological Station , Port Erin, Isle of Man , U. K KURZFASSUNG: Algenkolonisation im untergetauchten Gezeitenbereich nach Entfernung von Echinus. An der Seeseite des Port Erin Wellenbrecherdammes kommen keine gr/51~eren Algen im Bereich der unteren 3 m vor. Hier herrscht eine groffe Besiedlungsdichte des Seeigels Echinus esculentus (3,6 Individuen pro m~). 1Jber eine Zeitspanne yon 3 Jahren wurden nun s~imtliche E. esculentus yon einem 10 m breiten und 12 m langen Felsstreifen allmonatlich entfernt; insgesamt wurden dabei etwa 3000 Individuen abgesammelt. Ein Jahr nach Beginn des Absammelns betrug die mittlere Siedlungsdichte der jungen Laminaria hyperborea auf dem yon Seeigeln freigehaltenen Felsstreifen 22,7/m e. In den folgenden Jahren wurden zwei- und dreij~ihrige L. hyperborea nut auf diesem Felsstreifen oder in der N~ihe seiner Begrenzungen gefunden. Andere Algenarten reagierten in ~ihnlicher Weise. Aus den Befunden wird gefolgert, dal~ die untere Verbreitungsgrenze yon L. hyperborea zumindest zum Teil durch Seeigelfraf~ bestimmt wird, und da~ E. escuIentus die Siedlungsdichte der anderen Algenarten beeinfluf~t. - I N T R O D U C T I O N The ruined b r e a k w a t e r at P o r t Erin is composed of concrete blocks and boulders and forms a suitable substratum for algae from m i d - t i d e level down to 11 m below extreme low water springs, where it meets the sandy bottom. On the seaward face Larninaria hyperborea (GuNN.) FOSL. forms a forest from ELWS (extreme low water springs) to 6 m below but larger algae are n o r m a l l y absent from about 8 to 11 m depth and smaller species are sparsely distributed on the blo&s and boulders. N o t far away, however, where there is deeper rock, k. hyperborea extends down to 15 m or more (KAIN 1962) . A possible reason for the absence of L. hyperborea from the lower p a r t of the b r e a k w a t e r is the grazing pressure exerted by the sea urchin Echinus esculentus L. This animal is present all over the b r e a k w a t e r but appears to congregate on the lower parts near the sand. It was therefore decided to clear the Echinus from a strip down a profile of the rock, to maintain the strip relatively clear, and to observe the effects on the growth of algae. There are several other records of the effects of grazing by echinoids. NeUSHUL (1958 ) reported that NORTH h a d observed that following the wreck of an oil t a n k e r off Baja C a l i f o r n i a there was heavy m o r t a l i t y among sea ur&ins and other grazers. This resulted in the colonization by Macrocystis and other algae of a number of small areas where they had previously been scarce. LEIGHTON, JONES & NORTH (1966) discussed the effects of grazing by sea urchins and stated that seaweeds invariably reappeared after urchin numbers were reduced sufficiently. Experimental destruction of sea urchins with quicklime showed that algal reestablishment took place when Strongylocentrotus franciscanus (AGAssiz) was reduced to 1 individual per ms, S. purpuratus (STIMVSON)to 10 per m s, and Lytechinus anamesus CLARKtO 10 per m'-'. KITCHIN6 & EBLING (1961), working at Lough Ine, Ireland, induced a rich growth of Enteromorpha and other algae by the removal of large numbers of Paracentrotus lividus (LAMARCK)from certain rocky areas in shallow water. NEILL & LARIIU~a(1966 ) removed Arbacia lixula Lov~N and Paracentrotus lividus from squares with 5 m sides and observed that a thick fur of algal sporelings had developed aRer a month. M E T H O D S The seaward face of the ruined breakwater at Port Erin is about 200 m long. A wire rope was laid out at right angles to the line of the breakwater, a little way north from the base, from low water mark down to the sand at 11 m and fixed at either end. In July 1963 all individuals of Echinus esculentus were removed from squares of 5 m side alongside the wire successively from above downwards. The Echinus from each square were placed in a bag and counted in the laboratory. Later a further strip was cleared on the north side of the wire at its lower end, and an area 12 m long by 10 m wide has been subsequently cleared at intervals of about four weeks until the present. RESULTS The strip, when first cleared, was covered from low water to about 7 m depth, for a distance of about 22 m horizontally, by a forest of Laminaria hyperborea with occasional plants of Saccorhiza polyschides (LIcHTF.) BATT. and a few Larninaria saccharina (L.) LAX*OUR(Fig. 1). From about 7 to 9 m depth there were only occasional L. hyperborea and more Saccorhiza, and below this a zone bare of larger algae with only occasional smaller specimens. Figure 1 also shows the density of Echinus on the strip in July 1963 and the density on a transect some distance to the north of the strip in July 1965. In both cases there was a considerably greater concentration of these animals on the lower boulder (...truncated)


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Norman S. Jones, Joanna M. Kain. Subtidal algal colonization following the removal ofEchinus, Helgoland Marine Research, 1967, pp. 460-466, Volume 15, Issue 1-4, DOI: 10.1007/BF01618642