Intraspecific Competition in Sargassum muticum (Phaeophyceae) Germlings under Various Density, Nutrient and Temperature Regimes

Interactive effects of settlement density, nutrient concentration and temperature on growth and survival of Sargassum muticum germlings were recorded over 45 days in laboratory cultures, using a factorial experimental design. Germling growth was stimulated by high temperature (17 °C compared to 7 °C) and nutrient enrichment (seawater enriched with 32 μmol nitrogen and 2 μmol phosphorus per litre), and negatively related to settlement density (in the range of 10-100 germlings cm - 2 ). Survival was reduced when high settlement density was combined with high temperature and nutrient enrichment. After 45 days the size-distribution of germlings became positively skewed in some of the high-density stands, probably as a result of a few large individuals gaining dominance and suppressing many small plants. Intraspecific competition, defined as the relative reduction of germling growth and survival in high-density cultures compared with low-density cultures, increased during the course of the experiment, and depended on temperature and nutrient regime. As biomass accumulated in the stands competition increased more per biomass-unit at 7 °C than at 17 °C, which indicates a reduced tolerance to competition in germlings at the lower temperature. These results suggest the presence of intraspecific competition in stands of young S. muticum germlings, and a reduced growth and competition ability of germlings at low temperatures.