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In the framework of investigations on possible effects of eutrophication on the macrophytobenthos in Kiel Bay (Western Baltic), a large-scale survey of the distribution and occurrence of the genus Fucus was carried out in 1987/88. For large-scale quantitative mapping, underwater television was employed or direct observations from the water surface were made. Species composition and quantitative biomass data were obtained by dredging and SCUBA-diving. Comparison with the results of earlier investigations revealed a drastic decline in Fucus biomass from between 40 000 and 45 000 t wet wt down to only 2400 t wet wt in 1987/88, which means a decrease by 94 to 95 %. Whereas Fucus vesiculosus as well as F. serratus were still frequent at depths below 2 m down to 13 m in the seventies, during our investigation Fucus spp. was not found in water depths greater than 2 m. Possible causes for the observed changes are discussed, and it is concluded that decreased light levels or increased epiphyte growth as a result of eutrophication, and the reduction of substrate for algal growth due to stone fishing and sand deposition, are the main causes for the decline of Fucus spp.

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Although blooms of opportunistic fast-growing macroalgae now occur frequently in coastal ecosystems affected by eutrophication, their initiation and control is little understood. Most previous studies have focused on the ecophysiology of adult algae only. We show that spores and/or germlings may represent critical stages in the life cycles and mass-developments of co-occurring opportunistic macroalgae in the Baltic (Pilayella littoralis and Enteromorpha spp.). We investigated the overwintering of spores, timing of germination, subsequent growth, and grazing on spores and germlings, in order to explain the initiation of mass blooms and species dominance patterns. In the field, Enteromorpha spp. showed 10- to 50-fold higher abundances of overwintering microscopic forms (up to 330 individuals cm) than P. littoralis. Moreover, we found continuous production of spores (up to 1.2 million settling spores m h) from April to October in Enteromorpha spp., while there was evidence of only a short reproductive period in Pilayella. However, in spring, germlings and adults of P. littoralis appeared earlier in the field and reached a 10-fold higher biomass than Enteromorpha spp. In factorial laboratory experiments including temperature and light, there were clear differences in timing of germination. P. littoralis germinated at 5°C whereas Enteromorpha spp. required temperatures of 10-15°C for germination. In contrast, we detected only minor differences in growth response among adults of P. littoralis and Enteromorpha spp. Germination, not growth of adults, appeared to be the ecophysiological bottleneck for initiating mass spring development. Following the spring Pilayella bloom, Enteromorpha germlings occurred massively in the field (April-September), but rarely developed into adults. In laboratory feeding experiments we tested whether crustacean mesograzers common in summer may control development of Enteromorpha germlings. Both germination of settled spores and growth of germlings were reduced by 93-99% in the presence of grazers (Idotea chelipes and Gammarus locusta). Thus in addition to ecophysiological constraints, grazers, if present, may play a decisive role in the early life stages of macroalgal mass developments. These results mirror patterns of overwintering of seeds, germination control, seed and seedling predation in terrestrial plant communities.

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 increased nutrient loads. We asked whether pronounced dominance of filamentous Pilayella littoralis Kjellm. (Phaeophyceae) over foliose Enteromorpha intestinalis L. (Chlorophyceae) in the Baltic Sea can be explained by interspecific physiological differences. In laboratory experiments, we analyzed uptake kinetics of nitrate, ammonium, and phosphate and the time dependency of uptake rates for both species. We further examined growth rates and nutrient assimilation in relation to single and combined enrichment with nitrate and phosphate, and three different nitrogen sources. Overall, we did not detect distinct differences in uptake, growth, and assimilation rates between P. littoralis and E. intestinalis. Minor differences and the related advantages for single species are discussed. Highest maximal uptake rates were found for ammonium, followed by nitrate and phosphate. Strong time dependency 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. Enrichment with urea, ammonium, and nitrate significantly increased growth rates without interspecific differences. A larger surface area to volume (SA/V) ratio in Pilayella compared with Enteromorpha did not translate into greater physiological capacity. We conclude that species dominance patterns in macroalgal blooms are not always a direct result of different ecophysiological traits among species. Ecological traits such as susceptibility to herbivory are important factors in determining species distribution in the field.Abbreviations: K m , half-saturation constant; RGR, relative growth rate; SA/V, surface area to volume; V max , maximal uptake rate; ␣ , initial slope ( V max / K m ) 1