Temporal and spatial distribution of grazers and kelp phlorotannins in Kachemak Bay, Alaska

Abstract

The potential influence of grazers on the density and distribution of kelp thalli is substantial and may be mediated by kelp phlorotannins serving in a defensive role. The purpose of this study was to determine how temporal and spatial phlorotannin patterns in four kelp species in Kachemak Bay, Alaska, are influenced by density and distribution of the gastropod grazer Lacuna vincta and environmental variables. Temporal phlorotannin patterns from June 2004 through December 2005 were mainly explained by the life history of particular kelp species as well as strong correlations with light attenuation and nitrate concentration. The Optimal Defense Theory of resource allocation to differentiated tissues was supported by observations of higher proportional allocation to attachment structures and meristematic tissue in all kelp species. Lacuna vincta distribution was not clearly related to phlorotannin content and therefore, grazer habitat and food choices may be influenced by the combination of high phlorotannin content, tissue toughness and/or nutritional content. Strong physical disturbances such as currents and wave action may supersede these factors and drive grazer distribution. An understanding of the biological and physical factors affecting phlorotannin content and distribution in kelp thalli may suggest reasons for temporal and spatial kelp bed variability.