Interspecific competition between Enteromorpha (Ulvales: Chlorophyceae) and Fucus (Fucales: Phaeophyceae) germlings: effects of nutrient concentration, temperature, and settlement density

The increased abundance of ephemeral algae and corresponding decline in perennial fucoids observed in eutrophic (nutrient-rich) waters could be caused by eutrophication-driven shifts in the competitive relationship between the 2 groups as early as the germling stage. During the initial post-settlement period germlings are exposed to large temporal variations in temperature, which may influence their performance and competitive interactions. Effects of temperature, nutrient concentration, and settlement density on the competitive relationship between germlings of Fucus serratus L. and F. evanescens C. Agardh and of the ephemeral alga Enteromorpha compressa (Linnaeus) Greville were examined using factorially designed culture experiments. In monocultures, germling growth of both Fucus species and E. compressa was generally stimulated by nutrient enrichment (32 μmol nitrate/ammonia and 2 μmol phosphate l–1 seawater) and high temperature (17°C compared to 7°C), and negatively related to settlement density (10 to 50 germlings cm–2 for Fucus, 30 to 130 germlings cm–2 for E. compressa). In mixed cultures, interspecific competition effects of E. compressa on F. serratus and F. evanescens germlings increased with increasing nutrient and temperature levels. In the nutrient-enriched cultures, interspecific competition from E. compressa suppressed the yield of Fucus spp. by 80 to 100% at 17°C, with F. evanescens being more suppressed than F. serratus, whereas the suppressive effects of E. compressa on Fucus germlings were <60% at 7°C. These results suggest that competition from ephemeral algae at early life stages may be a reason for the decline of fucoids in eutrophic waters and that reproduction at low temperatures, as in species like F. serratus and F. evanescens, may shield the vulnerable initial germling stages from the severest competition pressure by ephemeral algae.