ABSTRACT: In the Baltic Sea, we tested how short nutrient pulses of different lengths and frequencies affect macroalgae, epiphytes, grazers and their interactions. We hypothesized that even small-scale variations in nutrient supply may have significant impacts by favoring fast-growing epiphytes which can cause 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 effects. A single 5 h pulse increased the epiphyte load but not F.vesiculosus growth rate. In contrast, increasing epiphyte load caused F. vesiculosus growth rate to decline and attracted higher densities of gastropod grazers. These 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 consumers to elevated nutrients.

KEY WORDS: Competition · Grazing · Eutrophication · Nutrient supply · Bottom-up control · Macroalgae · Fucus vesiculosus

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Published in MEPS Vol. 202 (2000) on August 28
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 2000