Seasonal variation in kelp phlorotannins in relation to grazer abundance and environmental variables in the Alaskan sublittoral zone

Abstract

Phlorotannins are common metabolites produced in kelps that can have deterrent functions against grazers. The factors dictating seasonal patterns of phlorotannin content in northeastern Pacific kelps are not well understood. This study assessed density and grazing of the gastropod Lacuna vincta on the annual canopy-forming kelp Nereocystis luetkeana and the perennial understory species Agarum clathratum, Saccharina latissima and S. groenlandica in Kachemak Bay, Alaska. In addition, we assessed seasonal patterns of environmental variables as possible drivers of phlorotannin concentrations. Phlorotannins occurred in all species, with overall lowest levels in N. luetkeana, and with different seasonal patterns among the four species. Lacuna vincta was most dense on N. luetkeana thalli in the summer and had highest grazing rates on this low-phlorotannin species. However, correlations between L. vincta density and phlorotannin content of each kelp species were not significant. Except for N. luetkeana, there were no correlations between phlorotannin levels and environmental variables. We suggest that kelp life history traits may be more important for phlorotannin patterns in these kelp species than grazers or environmental drivers.