Comparison of the structure of populations of Ascophyllum nodosum(Fucales, Phaeophyta) at sites with different harvesting histories

Abstract

Changes in the structure of the Ascophyllum nodosum population at Pubnico, southwest Nova Scotia, Canada,at an experimental site subjected to mechanical harvest and at two control sites never subjected to mechanicalharvest were monitored from 1991 to 1994. A bimodal population structure measured in terms of plant length wascharacteristic of all these sites before the experiment. The population structure of the experimental site becameunimodal immediately after experimental harvest by machine and remained unimodal for the subsequent two years.However, a bimodal population structure began to appear in the third year. A bimodal population structure remainedevident at the control site with bedrock as the substratum but was less evident at the other control site where thesubstratum is made up of boulders and cobbles. Movement of loose rocks with rockweeds still attached may havecontributed to the less distinct modal structure of this control site. Other sites with different harvesting historiesmonitored in the summer of 1992 showed some interesting patterns. A unimodal population structure was evidentin Argyle Sound and Pubnico Point South and at Charlesville, which had been harvested one and two years before,respectively. A bimodal population structure was more evident at Frenchman’s Point, which had been harvestedthree years prior. The rate of change from a unimodal to a bimodal population structure may depend on the intensityof harvest. Extensive canopy removal in intensively harvested areas may be conducive to an influx of recruits andto regeneration from the holdfast. Hence, plant length modal structure may be a useful measure of the relative stateof recovery of a harvested population.