Algal propagule banks modify competition, consumer and resource control onBaltic rocky shores

We hypothesized that supply from macroalgal propagule banks may influence t he relative abundance of annual and perennial algae and that this may alter the effects of grazers and nutrients on species composition. In a factoria l field experiment in the Baltic Sea littoral system we tested the effects of manipulating propagule banks, the abundance of crustacean and gastropod grazers, and nutrient supply on recruitment and growth of macroalgae over a year. Moreover, we determined seasonal patterns of macroalgal propagule di spersal at the experimental site and quantified algal abundance and recruit ment at 25 locations throughout the Baltic Sea. Experimental manipulations had minor effects on adults of the dominating perennial alga, Fucus vesicul osus. Instead, we found that species composition was determined by processe s operating at early life stages. Propagule supply from a propagule bank st rongly favored the fast-growing annual alga Enteromorpha spp. which then bl ocked settlement and recruitment of Fucus. Grazers reduced the abundance of annual algae and indirectly favored Fucus recruitment. There was an appare nt tradeoff between gains from the propagule bank and losses to herbivory i n five of seven colonizing species. Nutrient enrichment overrode grazer con trol of annual algae and accelerated the decline of Fucus only when annual algae had already achieved high densities through the propagule bank. Corro borating the experimental findings, field surveys across the Baltic showed that Fucus recruit densities can be predicted from the cover of annual alga e during the period of Fucus reproduction and settlement, Recruitment inhib ition by annual algae, which is driven by the abundance of annuals in the p ropagule bank, increasing nutrient levels, and declining consumer control, is suggested as a mechanistic explanation of the current decline of perenni al algae in the Baltic Sea.