The lower limit of the Laminaria hyperborea (Gunn.) Foslie zone is probably not always determined only by lack of sufficient light, even when a suitable substratum is present (Kain, 1962). It also appears that the establishment stage is critical although growth conditions deteriorate with depth (Kain, 1963). It therefore seemed desirable to determine exactly what the light requirements of the various different early stages were, and relate these to what could be found out about the light penetration in the natural habitat. The effect of temperature on these stages is also of interest, in relation to the seasonal cycle as well as to the geographical distribution of the species. In this study two different techniques were used: culturing the gametophytes on cover-slips and also studying changes in oxygen concentration as measured by the Winkler method.The gametophytes of the different members of the Laminariales show a similar general course of development. A zoospore from a fertile frond of a Laminaria plant loses its motility, rounds off, swells and produces a tube with another sphere at the end into which all the cell contents pass (Killian, 1911). This process can be carried through in the dark but further development depends on light (Schreiber, 1930). In this genus the gametophytes are dioecious (Sauvageau, 1918; Williams, 1921) and equal numbers of male and female plants are produced (Schreiber, 1930). The female gametophyte may become fertile before dividing further (Sauvageau, 1918), resulting in a single egg, or may form a luxuriant filamentous structure which may produce many oogonia over a period of time (Ikari, 1921; Myers, 1925; Schreiber, 1930).

A method of determining the age of Laminaria hyperborea by examination of a longitudinal median section of the lower stipe and holdfast was found to be reliable for at least the first 4 years.

The effects of density on the growth rate and survival of individual plants as well as changes in population structure (hierarchy) and biomass accumulation (self-thinning) were experimentally evaluated in two brown macroalgae. Laminaria digitata (Hudson) Lamouroux and Fucus serratus Linnaeus populations were constructed at five (650-5156 plants·m Ϫ2 ) and four densities (650-2668 plants·m Ϫ2 ), respectively, and were cultivated in tanks. The relative growth rates and survivorship of individuals and the populations' biomass and density (estimated dry mass) were periodically measured. To investigate how plant population size hierarchies influence conspecifics, single density populations of L. digitata were constructed of up to three sizes of plants in equal proportions, and these parts of the populations were compared through time with plants of the three sizes grown singly. At higher density, L. digitata plants grew more slowly, while F. serratus populations showed a similar trend that was never statistically significant. Survival of plants of both species was lower at high densities, and mortality selectively removed smaller plants. Plants of both species exhibited zero growth rates before death, when parts of the fronds were lost, but meristems (apical in F. serratus, at the base of the frond in L. digitata) were preserved until the death of the plant. All singly grown L. digitata plants survived, but survivorship was low in the fractions of small plants in mixedsize populations compared with that of the largest size plant fractions. Small L. digitata plants grew relatively faster than did large ones singly, but in mixed-size populations, small plants grew relatively slowly. Plant sizes became progressively more unequal (Gini and skewness coefficients) until self-thinning started reducing the size variability. The seaweeds followed self-thinning (density-biomass) trajectories predicted by the self-thinning ''rule'', and self-thinning appeared to be seasonal-rather than species-dependent, as it occurred at a time of year when ambient light levels start to fall in the Isle of Man. Culture studies of this kind, despite their considerable potential, are a tool as yet underexploited by marine ecologists as a means of assessing intraspecific competitive interaction among seaweeds.

KURZFASSUNG: Algenkolonisation im untergetauchten Gezeitenbereich nach Entfernung von Echinus. An der Seeseite des Port Erin Wellenbrecherdammes kommen keine gr/51~erenAlgen 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 NThe ruined breakwater at Port Erin is composed of concrete blocks and boulders and forms a suitable substratum for algae from mid-tide 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 normally absent from about 8 to 11 m depth and smaller species are sparsely distributed on the blo&s and boulders. Not 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 part of the breakwater is the grazing pressure exerted by the sea urchin Echinus esculentus L. This animal is present all over the breakwater 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 had observed that following the wreck of an oil tanker off Baja California there was heavy mortality among sea ur&ins and other

Although a dense cover of epiphytes is generally considered to be harmful for some coralline algae (Corallinaceae, Rhodophyta), crustose corallines in the littoral zone seem to be preserved from bleaching when covered by canopy plants and epiphytes during summer. This study aimed to test the responses of coralline crusts to epiphytes and canopy algae and their interaction with grazing limpets. Growth rates and color changes were followed in two crust species in areas with or without canopy algae in the Isle of Man, British Isles. Limpets were excluded, to allow epiphytes to grow upon crusts. Responses were measured both on pieces of crusts upon acrylic plates and on crusts growing naturally on the shore. Fucus canopy and epiphytic Enteromorpha significantly influenced the crusts' growth, depending on season. Epiphytes reduced the light levels beneath by up to 78%, more than the canopy algae (62%). Crusts exposed outside the canopy bleached in summer, but gradually restored their color once they were covered by epiphytes. The fast‐growing Phymatolithon lenormandii (Aresch.) Adey recovered its coloration more quickly than the slow‐growing P. purpureum (P. et H. Crouan) Woelkerling et Irvine. However, neither crust species could restore its color when epiphytes were reduced by grazing limpets, Patella vulgata L. Bleaching did not kill the crusts, but seemed to interfere with crusts' growth. Restoration of pigmentation was quantified for the first time on bleached coralline crusts. Epiphyte and canopy algae were experimentally shown to be beneficial, probably by providing shade and also protecting crusts from desiccation.