Comparison of survival and growth in germlings of six fucoid species (Fucales, Phaeophyceae) at two different temperature and nutrient levels

During their initial post‐settlement period, fucoid germlings will be exposed to large variations in environmental factors such as temperature and nutrient concentration, which may influence survival and growth. Factorially designed culture experiments were used to compare the effects of temperature and nutrient concentration on the survival and growth of germlings in six fucoid species from the Norwegian Skagerrak coast. In four of the species (Ascophyllum nodosum, Fucus evanescens, F. serratus, and F. vesiculosus), survival did not differ between the two temperatures tested (7 and 17°C), whereas survival was reduced at 7°C in F. spiralis and Sargassum muticum. The effect of nutrient enrichment (seawater enriched with 32 μmol N and 2 μmol P per litre) on germling growth depended on the temperature regime, and differed little between species. The relative growth rate of germlings increased with increasing temperature in all six species and was higher in S. muticum (∼0.21 day ) than in the other fucoids (0.09–0.14 day ) at 17°C. At 7°C, F. serratus germlings had the highest relative growth rate (∼0.13 day ), whereas those of S. muticum and F. spiralis were reduced to ∼0.05 and ∼0.03 day , respectively. These results are in accordance with the reproductive season for the species, as germlings of the summer/autumn reproducers, F. spiralis and S. muticum, were less tolerant to low temperatures than the other Skagerrak fucoids whose reproduction occurs earlier in the year. Egg volume varied almost 10‐fold across species, from F. vesiculosus (smallest) to S. muticum (largest), and showed no correlation with relative growth rates or survival, wherease the size of germlings was positively related to egg volume after 15 days of cultivation under nutrient‐enriched conditions at 17°C. These results suggest that summer reproduction, larger eggs and a higher growth rate of germlings may give the introduced species, S. muticum, a competitive advantage.