COS 85-1 - Impact of nutrient enrichment and herbivory pressure on phenolic anti-herbivory defenses of a temperate intertidal seaweed

Abstract: Silvetia compressa is a brown macroalga and an important habitat forming species in the rocky intertidal habitats of California. It has recently been on the decline and the focus of habitat restoration activity in Southern California. One human impact that causes changes to these seaweed communities is nutrient enrichment, which can impact the prevalence of anti-herbivory defenses in brown algae.

S. compressa contains a variety of anti-herbivory defenses including production of a class of phenolic compounds called phlorotannins. Phlorotannins bind to the gut of herbivores to reduce the palatability of the macroalga. Our experiment examined the impact of increasing nutrient concentrations on the presence of phlorotannins in S. compressa. Algae were grown in for 10-days in tanks under 4 differing nutrient regimes, ranging from 1-5 µM of NO₃^- and 0.1-0.5 µM of PO₄^-. Following 10-day exposure to nutrient enrichment, algae were exposed to treatments of zero, low, and high grazing pressure by crabs to examine herbivore response to nutrient enrichment and phlorotannin concentration. The herbivore experiment was again run for 10-days exposing replicates that that been exposed to each level of nutrients to zero, 15, or 35 herbivorous crabs. Algal growth was measured before and after each experiment by weighing each algal replicate. Phlorotannin concentrations were measured initially and after the experiment concluded using a reduction reaction called the Prussian Blue Assay and measuring absorbance following the reaction with a spectrophotometer.

In this way, our experiment can be analyzed to determine the impact of nutrient concentration on phlorotannin concentration and the resulting response of herbivores. Phlorotannin concentrations were highest at an intermediate level of nutrients and crab herbivores preferred algae grown under high nutrient concentrations. This suggests that allocation of nutrients to phlorotannin production may not sufficiently deter herbivores and herbivory on this alga may increase as human-caused nutrient enrichment through run-off increases in rocky intertidal environments. This information can be applied to S. compressa restoration and conservation efforts as lower nutrient concentrations may favor persistence of this seaweed overall, while increased nutrient concentrations favor more ephemeral algal species.