Response of grazers to sudden nutrient pulses in oligotrophic versus eutrophic conditions

Herbivores can consume more nutrient rich algae than nutrient poor algae, and such foraging behaviour may counter the negative effects of elevated nutrients on algal habitats. Understanding this phenomenon may be useful in understanding why some localities have greater persistence and resilience than others in the face of sudden increases in nutrient loads (e.g. run-off from storm events). We used 3 successive field experiments to test, and subsequently accept, the hypotheses that under intense regimes of herbivory (1) molluscs reduce a greater percentage cover of opportunistic algae (turfs) exposed to elevated nutrients, and consequently these algae have less biomass than those exposed to ambient nutrients, (2) turfs exposed to elevated nutrients attract greater densities of herbivores, and (3) grazers exposed to sudden increases in nutrient rich algae reduce algal biomass more when background nutrient loads are normally low (ambient nutrient conditions) than when they are high (enriched nutrient conditions). Critically, these effects were greater under oligotrophic conditions, suggesting that the response of grazers to sudden nutrient events would be greater in systems where nutrient concentrations are usually low. However, grazers were not able to control increased algal growth when nutrient enrichment occurred over a longer period (i.e. eutrophic conditions). These observations support the idea that grazers may provide useful functions in systems susceptible to human activities that reduce water quality over short periods (i.e. short-term increase in nutrient availability), but that this mechanism may not be sufficient to reduce the longterm effects of eutrophication.