Joanna M. Kain

The seasons in the subtidal

In the context of this paper «subtidal» refers to the algae inhabiting the substratum below lowest astronomical tide; the plants are virtually always submerged. By «seasons» is meant not four of them but a continuum of change from one extreme to the other

Aspects of the Biology of Laminaria Hyperborea III. Survival and Growth of Gametophytes

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).

Aspects of the Biology of Laminaria Hyperborea : II. Age, Weight and Length

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.

Subtidal algal colonization following the removal ofEchinus

KurzfassungAn der Sesseite des Port Erin Wellenbrecherdammes kommen keine größeren Algen im Bereich der unteren 3 m vor. Hier herrscht eine große Besiedlungsdichte des SeeigelsEchinus esculentus (3,6 Individuen pro m2). Über eine Zeitspanne von 3 Jahren wurden nun sämtlicheE. esculentus von 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 jungenLaminaria hyperborea auf dem von Seeigeln freigehaltenen Felsstreifen 22,7/m2. In den folgenden Jahren wurden zwei- und dreijährigeL. hyperborea nur auf diesem Felsstreifen oder in der Nähe seiner Begrenzungen gefunden. Andere Algenarten reagierten in ähnlicher Weise. Aus den Befunden wird gefolgert, daß die untere Verbreitungsgrenze vonL. hyperborea zumindest zum Teil durch Seeigelfraß bestimmt wird, und daßE. esculentus die Siedlungsdichte der anderen Algenarten beeinflußt.Summary1. 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.

A review of the life history, reproduction and phenology of Gracilaria

The basic life history of the red alga Gracilaria is of the three-phase Polysiphonia type but a number of species show deviations. Plants can bear both gametangia and tetrasporangia, either on separate parts of the thallus or on the same. Explanations include the in situ germination of tetraspores (allowing gametophytic thalli to be epiphytic on tetrasporophytes), the coalescence of spores or developing discs (resulting in chimaeras), mitotic recombination during cell division in the mature diploid thallus (resulting in patches of diploid male and female cells on the tetrasporophyte), a mutation eliminating the repression of female expression allowing haploid male plants to be bisexual and initial failure of cell walls to form during the development of tetraspores. Polyploids can be produced from plants with diploid gametangia. The sexes and phases are usually morphologically identical but gametophytes or their parts may be smaller. The growth rates of the sexes may differ and diploid juveniles may survive better than haploid. Neither polyploidy nor hybridization results in superior growth. The sex ratio is probably 1:1 but females may appear to be more abundant. Diploid and haploid phases are usually either about equal or diploids predominate, often depending on the type of substratum. At high latitudes reproduction peaks in late summer whereas in the tropics it may be high all year. In temperate regions growth rate is fastest and biomass highest in late summer; in the tropics peak biomass is mainly in the winter. Spermatia are effective for only a few h. Spores vary in size around 25 \m, diploid ones usually being larger. Cystocarps or tetrasporangia in the field may not currently be releasing spores. In the laboratory spore release shows a diurnal rhythm, peaking during the night or day according to the species. All the above attributes are potentially important in planning and executing Gracilaria cultivation.

The Biology of Laminaria hyperborea. V. Comparison with early stages of competitors

The responses to light and temperature variations of the gametophytes and early sporophytes of Laminaria hyperborean were compared in culture with those of its main competitors in Britain, namely L. digitata, L. saccharina and Saccorhiza polyschides . Some undeveloped gametophytes of all species are able to survive in the dark for at least 80 days.

AN EXPERIMENTAL EVALUATION OF DENSITY AND PLANT SIZE IN TWO LARGE BROWN SEAWEEDS

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−2) and four densities (650–2668 plants·m−2), 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 mixed‐size 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.

The biology of Laminaria hyperborea VII. Reproduction of the sporophyte

There are two critical stages in the life history of Laminaria hyperborea (Gunn.) Fosl. The first is the attachment of the zoospores (meiospores) to suitable rock surfaces and the second is the successful fusion of gametes. The second clearly partly depends on the first which is in turn affected by the efficiency of zoospore production by the sporophyte and the time of year that this takes place. Because of lack of information on these aspects the present study of reproduction in the sporophyte was undertaken.

The movement of sand waves on Warts Bank, Isle of Man

Abstract Warts Bank, near the Isle of Man, Irish Sea, was surveyed by echo sounder and the sand waves on its surface were observed by diving. Their rate of travel averaged 5–10 cm/day during 2 summer months while one crest moved 74 cm during a single flood tide. Bottom samples and near-bed current data made it possible to compare observed and theoretical values of volume transport.

The effects of some pollutants on the survival, growth and respiration of Laminaria hyperborea

Abstract The effects of seventeen pollutants including metals, herbicides and detergents were measured in the laboratory by culturing the young stages and measuring the respiration rates of discs of mature frond tissue of Laminaria hyperborea . In culture the survival of germinating gametophytes was reduced by the following pollutants (with the toxic concentration in mg/dm 3 ); mercury (0.01), copper (0.1), sodium dodecyl benzene sulphonate or SDBS (0.5), zinc (5.0), mixed detergent A (5.0), mixed detergent B (5.0) and sodium lauryl ether sulphate or SLES (10). The pollutants which were not toxic to these stages were (maximum concentration tested in mg/dm 3 ); atrazine (1.0), cadmium (10), coconut fatty acid diethanolamide or CFD (10), DOBS 055 (10), dalapon (100), MCPA (100), 2,4-D (100), phenol (100) and pluronic (propylene oxide/ethylene oxide condensate) (100). The production of sporophytes from gametophytes was delayed by several pollutants while the growth of sporophytes in culture was prevented or inhibited by mercury (0.05), copper (0.01), atrazine (0.01), cadmium (0.1), zinc (0.5), SDBS (1.0), DOBS 055 (1.0), sodium pentachlorophenate (1.0), phenol (75) and MCPA (100). Mixed detergent A (5), mixed detergent B (10), SLES (10), CFD (10) and pluronic (100) were not toxic at the maximum concentrations tested (given in brackets). In culture the zoospores and gametophyte stages were found to be more tolerant than the sporophyte stage to zinc, cadmium, atrazine, MCPA, phenol, DOBS 055 and copper. The reverse was true for mixed detergent A, mixed detergent B and SLES. The respiration rate was reduced by mercury (5.0), copper (25), phenol (100), zinc (250), MCPA (1000) and CFD (1000). Pollutants having no adverse effect at the maximum concentration tested (1000) were; atrazine, dalapon, 2,4-D, mixed detergent B, SLES and pluronic. Considerably higher concentrations were required to reduce respiration than were inhibitory to the growth of the young stages of the plant in culture. Results are within the ranges covered by other workers using a variety of marine algae. The growth of the young stages was not affected when cultured in samples of polluted seawater collected from the coast of N.E. England.