Strong bottom-up and top-down control of early life stages of macroalgae

In contrast to most pelagic primary producers, benthic macrophytes pass thr ough morphologically distinct life stages, which can be subject to differen t ecological controls. Using factorial field experiments, we investigated h ow grazing pressure (three levels) and nutrient supply (four levels) intera ct in controlling the passage of marine macroalgae through an apparent recr uitment "bottleneck" at the germling stage. In comparative experiments, we asked whether relative bottom-up and top-down effects on early life stages (<4 week germlings) vary (1) between the eutrophic Baltic Sea and the oligo trophic NW Atlantic, (2) across seasons in the NW Atlantic, and (3) among a nnual and perennial macroalgae. In both systems nutrient enrichment favored and grazers suppressed recruitment of green and brown annual algae; howeve r, enrichment effects were much more pronounced in the Baltic, whereas graz er effects dominated in the NW Atlantic. Grazers induced a shift from graze r-susceptible green to more resistant brown algae in the Baltic without red ucing total germling density. In the NW Atlantic, grazers strongly reduced overall recruitment rate throughout all seasons. Effects on perennials were similar in both systems with moderate losses to grazing and no effects of nutrient enrichment. Recruit densities: and species composition shifted wit h season in the NW Atlantic. We conclude that the relative effects of graze rs and nutrient enrichment depended on the nutrient status of the system, a lgal life history strategy, and season. Strong bottom-up and top-down contr ols shape benthic community composition before macroalgae reach visible siz e.