Control of macroalgal blooms at early developmental stages: Pilayella littoralis versus Enteromorpha spp.

Although blooms of opportunistic fast-growing macroalgae now occur frequent ly in coastal ecosystems affected by eutrophication, their initiation and c ontrol is little understood. Most previous studies have focused on the ecop hysiology of adult algae only. We show that spores and/or germlings may rep resent critical stages in the life cycles and mass-developments of cooccurr ing opportunistic macroalgae in the Baltic (Pilayella littoralis and Entero morpha spp.). We investigated the overwintering of spores. timing of germin ation, subsequent growth, and grazing on spores and germlings, in order to explain the initiation of mass blooms and species dominance patterns. In th e field, Enteromorpha spp. showed 10- to 50-fold higher abundances of overw intering microscopic forms (up to 330 individuals cm(-2)) than P. littorali s. Moreover, we found continuous production of spores (up to 1.2 million se ttling sports m(-2) h(-1)) from April to October in Enteromorpha spp., whil e there was evidence of only a short reproductive period in Pilayella. Howe ver, in spring. germlings and adults of P. littoralis appeared earlier in t he field and reached a 10-fold higher biomass than Enteromorpha, spp. In fa ctorial laboratory experiments including temperature and light, there were clear differences in timing of germination. P. littoralis germinated at 5 d egrees C whereas Enteromorpha spp, required temperatures of 10-15 degrees 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 in itiating mass spring development. Following the spring Pilayella bloom. Ent eromorpha germlings occurred massively in the field (April-September), but rarely developed into adults. In laboratory feeding experiments we tested w hether crustacean mesograzers common in summer may control development of E nteromorpha germlings. Both germination of settled spores and growth of ger mlings were reduced bq 93-99% in the presence of grazers (Idotea chelipes a nd Gammarus locusta). Thus in addition to ecophysiological constraints, gra zers, if present, may play a decisive role in the early life stales of macr oalgal mass developments, These results mirror patterns of overwintering of seeds, germination control, seed and seedling predation in terrestrial pla nt communities.