Phlorotannin concentration in kelp affects consumption by mussels: Gretchen McCarthy PhD student

Phlorotannins are long polymers of phloroglucinol which provide antiherbivore deterrence from grazers and are found exclusively in the class Phaeophyceae. Little is known about how these compounds may affect consumption rates of kelp-derived detrital material by suspension feeders. In the present study we compared seasonal phlorotannin concentrations in eleven species of Phaeophyceae among five kelp communities along a wave exposure gradient in the Otago Harbour region. We hypothesized that communities in the inner harbour would have higher phlorotannin concentrations in response to the increased grazing exposure that sheltered conditions allow. Outer harbour communities, however, would have lower average phlorotannin concentrations as abiotic factors including wave action provide refuge from grazers. Our initial results from summer 2021 and spring 2022 demonstrated that phlorotannin concentrations significantly varied between species (emmeans p < 0.01) when species and season were fixed effects, and site and harbour zone were nested random effects. In addition, inner and outer harbour sites had significantly different phlorotannin average concentrations (emmeans p < 0.01) when season was held as fixed effect. On average, inner harbour kelp communities had highest phlorotannin concentrations in the summer compared to spring (74.19 M g DTW ^-1 ± 8.16 and 58.97 M g DTW ^-1 ± 7.76 respectively) while outer coast communities had highest phlorotannin concentrations in the spring compared to summer (29.27 M g DTW ^-1 ± 4.02 and 17.34 M g DTW ^-1 ± 3.84 respectively). In addition, we tested the idea that species with lower phlorotannin concentrations would produce detrital material that was more readily palatable to the green-lipped mussel (Perna canaliculus). We hypothesized that macroalgae with lower phlorotannin concentrations (Macrocystsis pyrifera and Undaria pinnatifida) would be more readily consumed than species with high phlorotannin concentrations (Cystophora retroflexa and Cystophora torulosa). From these experiments, we hypothesized older kelp material would be easier to consume as phlorotannin concentrations degrade overtime.