Abstract
Herbivore impacts on macrophyte growth vary with the identity of the herbivores and macrophytes, as well as under different abiotic conditions. This interaction is further complicated by anthropogenic alterations to the environment, such as eutrophication. In this study, we utilized in situ herbivore exclusion experiments and mesocosm feeding preference assays to examine the impacts of different herbivores on the growth of two morphologically similar, co-occurring macroalgal bloom Ulva species in a nutrient-rich environment. We found that herbivory had a measurable impact on Ulva biomass, though the rate of consumption rarely surpassed growth for either Ulva species. We determined that the primary herbivores within the blooms were amphipods and mud crabs, and that their effects varied among study sites and months. Our results also confirmed that, even with a diverse suite of consumers, Ulva blooms are capable of escaping herbivore control, particularly early in the growing season when growth rates peak and herbivore activity is limited. Furthermore, our experiments revealed species-specific feeding preferences among herbivores, as well as differences in growth rates and chemistry between the two Ulva species, which likely influence bloom dynamics.
Highlights
The structure of macrophyte communities is affected by the availability of resources and the strength of herbivory; the relative contribution of these opposing forces can fluctuate considerably among different habitats (Shurin et al, 2002; Hillebrand et al, 2007; Gruner et al., 2008)
We were unable to include an in situ herbivore exclusion cage control to assess total Ulva growth rates, since mesh sizes small enough to exclude amphipods severely limited Ulva growth
While the lack of a non-herbivore control prevented us from calculating total herbivory, we can confidently attribute significant differences in Ulva growth among cage types to the differences in the herbivore communities the cages created
Summary
The structure of macrophyte communities is affected by the availability of resources and the strength of herbivory; the relative contribution of these opposing forces can fluctuate considerably among different habitats (Shurin et al, 2002; Hillebrand et al, 2007; Gruner et al., 2008). Anthropogenic impacts, including nutrient pollution, can significantly alter the relative influence of resource availability and herbivory on macroalgal communities. Eutrophication can promote the growth of fast growing ephemeral macroalgal species at the expense of perennial macroalgae and seagrasses (Valiela et al, 1997; Hauxwell et al, 2001; Worm & Lotze, 2006). In many cases nutrient enrichment enables macroalgal growth to surpass herbivore control (Horne et al, 1994; Hauxwell et al, 1998; Morgan et al, 2003; Worm & Lotze, 2006; Fox et al, 2012), resulting in macroalgal blooms. Persistent eutrophication can further lessen herbivore control by decreasing herbivore functional responses to the higher algal nutrient concentrations that occur with nutrient pulses (Russell & Connell, 2007) and increasing the occurrence of hypoxic events that result in herbivore mortality (Hauxwell et al, 1998; Berezina et al, 2007)
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