Ecophysiological traits explain species dominance patterns in macroalgal blooms

Macroalgal blooms are a growing environmental problem in eutrophic coastal ecosystems world wide. These blooms are dominated typically by only one out of several co-occurring opportunistic species, which are all favored by in creased nutrient loads. We asked whether pronounced dominance of filamentou s Pilayella littoralis Kjellm. (Phaeophyceae) over foliose Enteromorpha int estinalis L. (Chlorophyceae) in the Baltic Sea can be explained by interspe cific physiological differences. In laboratory experiments, we analyzed upt ake kinetics of nitrate, ammonium, and phosphate and the time dependency of uptake rates for both species. We further examined growth rates and nutrie nt assimilation in relation to single and combined enrichment with nitrate and phosphate, and three different nitrogen sources. Overall, we did not de tect distinct differences in uptake, growth, and assimilation rates between P. littoralis and E. intestinalis. Minor differences and the related advan tages for single species are discussed. Highest maximal uptake rates were f ound for ammonium, followed by nitrate and phosphate. Strong time dependenc y of uptake occurred, with the highest rates during the first 15 to 30 min. Nitrate enrichment had far more of an effect on growth than phosphate. Enr ichment with urea, ammonium, and nitrate significantly increased growth rat es without interspecific differences. A larger surface area to volume (SA/V ) ratio in Pilayella compared with Enteromorpha did not translate into grea ter physiological capacity. We conclude that species dominance patterns in macroalgal blooms are not always a direct result of different ecophysiologi cal traits among species. Ecological traits such as susceptibility to herbi vory are important factors in determining species distribution in the field .