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

Abstract

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.