Responses of growth and phlorotannins in Fucus vesiculosus to nutrient enrichment and herbivory

Abstract

Discharge from anthropogenic sources may modify both macroalgal growth patterns and resource allocation to carbon based secondary compounds, thereby affecting their susceptibility for herbivory. We tested the effect of eutrophication in terms of nutrient enhancement on growth and phlorotannin concentration of Fucus vesiculosus by conducting manipulative experiments in the field and mesocosms. In the field experiment we utilised fish farms as nutrient sources and in the mesocosm-experiment we manipulated ambient nutrient levels and occurrence of the herbivorous isopod Idotea baltica. Vicinity of a fish farm affected neither growth nor the phlorotannin concentration of Fucus but increased the amount of epiphytes growing on Fucus. Other organisms such as epiphytic filamentous algae and periphyton, which are more capable of quickly utilizing excess nutrients, may restrain the direct effects of nutrient enhancement on Fucus. In a manipulative mesocosm experiment, neither nutrient enrichment nor occurrence of herbivores affected phlorotannin concentration implying lack of induced defences, at least in terms of increasing phlorotannin concentration. Feeding of thallus decreased the growth rate of algae, but the number of reproductive organs, receptacles, was not affected by herbivory. The negative effect of herbivory on the amount of apical tips tended to be stronger under nutrient enriched conditions. We conclude that eutrophication processes, in terms of nutrient enrichment, does not have strong direct effect on growth or phlorotannin production of F. vesiculosus. However, there may be important indirect consequences. First, herbivory may be targeted more to apical parts of the thallus under eutrophicated conditions. Second, the result that Fucus growing close to nutrient sources were smaller than those in control areas may reflect differences in mortality schedules of algae between eutrophicated and control areas.

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

“…This is somewhat surprising, given the documented laboratory (Jormalainen et al 2001, Hemmi et al 2005) and field studies (e.g. Engkvist et al 2004) demonstrating the negative effects of the grazing activities of Idotea on Fucus growth rate and cover.…”

“…Chapman 1989, Duffy 1990, Hemmi et al 2005. Likewise, there are field observations of negative correlations between the cover of the large, perennial brown alga Fucus vesiculosus and the abundance of the herbivorous isopod Idotea baltica (e.g.…”

The responses of brown macroalgae to environmental change from local to global scales: direct versus ecologically mediated effects

“…2; Vogt & Schramm 1991) however, recent evidence suggests it is rather a multifactorial phenomenon and references therein): in the Western Baltic, Fucus is presently found between 0 and 2m depth (Wahl et al, own obs.). Eutrophication enhances grazing (Hemmi et al 2005) and leads to denser plankton in spring and summer (BACC 2010) that, through shading, limits the growth of Fucus below 4-5 m, but enables it above 4m (Rohde et al 2008). However, between 2 and 4 m, reduced irradiation ("1" in Fig.…”

Divergent responses in growth and nutritional quality of coastal macroalgae to the combination of increased pCO2 and nutrients

“…By contrast, we predicted that extreme levels of pCO 2 will decrease the growth rate of Fucus, which is likely to occur due to potential pH sensitivity of its CCM (Axelsson et al, 2000;Gutow et al, 2014), but increase tissue quality (as seen in Gutow et al, 2014). We hypothesized that Fucus growth would be unaffected by nutrient loading as this species has lower nutrient uptake rates and is typically adapted to lower nutrient environments (Savage and Elmgren, 2004), and nutrients may indirectly reduce growth rates by promoting the growth of competitors (Hemmi et al, 2005;Pedersen and Borum, 1996;Worm and Lotze, 2006). Growth rates and tissue quality of…”