Grazer identity is crucial for facilitating growth of the perennial brown alga Fucus vesiculosus

Abstract

The positive top-down effects on macrophytes of the removal of fouling epiphytes by herbivores versus negative bottom-up effects of enhanced epiphyte growth through nutrient enrichment are known from previous works. However, few studies have differentiated effects within the suite of herbivore species, nor have they investigated the potential negative effects of these grazers on macrophytes. We conducted a mesocosm experiment to investigate the impact of different grazer compositions (no grazers, the isopod Idotea baltica, the gastropod Theodoxus fluviatilis, and a mixture of both species) and nutrient levels (ambient and enriched) on growth of the brown alga Fucus vesiculosus. Herbivore facilitation depended on grazer identity, as only the treatments that included T. fluviatilis increased the growth rate of F. vesiculosus. Since I. baltica reduced the epiphyte population by the same amount as T. fluviatilis, we suggest that the grazing activities of this isopod on F. vesiculosus itself may have counteracted any positive effect of epiphytic removal. The same explanation may account for the similar growth rates in the treatments with T. fluviatilis alone and with both herbivorous species together, despite the lower epiphytic load in the latter treatment. In contrast, earlier published data suggested that T. fluviatilis facilitation is greater than that of I. baltica because of different epiphytic algal preferences between the grazers. Nutrient addition treatments, despite significantly increasing epiphyte load, had no effect on the growth rate of F. vesiculosus. © Inter-Research 2008.