Rapid direct and indirect effects of a single nutrient pulse in a seaweed-epiphyte-grazer system

In the Baltic Sea, rye tested how short nutrient pulses of different length s and frequencies affect macroalgae, epiphytes, grazers and their interacti ons. We hypothesized that even small-scale variations in nutrient supply ma y have significant impacts by favoring fast-growing epiphytes which can cau se large-scale declines of canopy-forming macroalgae. In a factorial field experiment single plants of the canopy-forming macroalga Fucus vesiculosus with and without epiphytes were exposed to pulses of elevated nutrients (N and P) over 25 d. Five 1 h pulses given every 5 d had no significant effect s. A single 5 h pulse increased the epiphyte load but not F. vesiculosus gr owth rate. In contrast, increasing epiphyte load caused F. vesiculosus grow th rate to decline and attracted higher densities of gastropod grazers. The se results indicate that a single nutrient pulse can have rapid direct and indirect effects on macroalgae and their associated epiphytes and grazers. Temporal variability of nutrient supply (five 1 h vs one 5 h pulse) plays a significant role in determining the response of primary producers and cons umers to elevated nutrients.