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.

Effects of seabird nesting colonies on algae and aquatic invertebrates in coastal waters

“…Seabird islands with a high nest density and a low wave exposure may be seen as a natural experiment for studying the effects of eutrophication on marine communities. We found, consistent with former fertilisation experiments, increased algal nitrogen and phosphorus content and a higher epiphytic algae to Fucus ratio on cormorant islands with an increased nutrient availability (Pedersen & Borum 1996, Kraufvelin et al 2006, Kraufvelin 2007, Råberg & Kautsky 2008. Contrary to previous studies investigating the effect of nutrient enrichment, on grazers in the Baltic Sea (Kotta et al 2000, Kraufvelin et al 2006, Kraufvelin 2007, Korpinen et al 2010) and chironomids in Lake Michigan, USA (Blumenshine et al 1997), not all invertebrates responded with increased abundance.…”

“…Our results from the diet mixing model are of particular interest considering that previous studies on the diet preference of invertebrates of the Baltic Sea rarely utilized stable isotope analysis and furthermore disagree in the preferred diet source of Idotea (Karez et al 2000, Jormalainen et al 2001, Goecker & Kåll 2003. T. fluviatilis might, similarly to Idotea spp., suffer from decreased shelter but also from the increased filamentous algae load that competes with microalgae, an important food source for the scraping snails (Råberg & Kautsky 2008).…”

“…Golovkin & Garkavaya 1975, Bosman et al 1986, Bosman & Hockey 1988, Keatley et al 2009), and should produce strong positive responses of primary producers (Elser et al 2007). Many studies also show that fast growing filamentous algae and invertebrate consumers respond with increasing productivity, biomass, and abundance to nutrient enrichments (Pedersen & Borum 1996, Kraufvelin et al 2006, Korpinen et al 2007, Kraufvelin 2007, Råberg & Kautsky 2008. Similarly, algae and vascular plants near seabird colonies were shown to have increased abundances, growth rates, or nutrient contents (Golovkin 1967, Onuf et al 1977, Bosman et al 1986, Bosman & Hockey 1988, but see Wootton 1991.…”

“…Second, we investigated indirectly if cormorants increase the nutrient availability in coastal waters around their nesting island by comparing the P content of algae and the N and P content of algae and associated invertebrates from the same set of islands. Third, we tested if the ratio of epiphytic algae to Fucus was increased near cormorant islands and expected, based on previous studies on nutrient enrichment effects in marine systems, increased epiphytic loads in the waters near cormorant islands , Råberg & Kautsky 2008. Fourth, we investigated if aquatic invertebrates responded to the high nutrient loads around cormorant islands by comparing the abundance and biomass of invertebrate grazers and chironomids from the 3 cormorant island categories with reference islands.…”

Snail grazing facilitates growth of a bloom-forming alga

“…Herbivores can also positively affect macroalgal growth through the addition of nutrients (Fong et al 1997) and, in a situation similar to the removal of neighboring competitors, grazers can preferentially remove epiphytes that grow on algal blades (Dudley 1992, Littler et al 1995, Kamermans et al 2002, Jormalainen et al 2003, Råberg & Kautsky 2008. In our study, Ulva lactuca growth was greatest when snails grazed directly on the blades; this was true for experiments that simulated subtidal and low tide conditions.…”