Reproduction, recruitment and mortality in two species of Laminaria in southwest Nova Scotia

Abstract

In a southwestern Nova Scotia kelp forest the average density of Laminaria longicruris Pyle. remained constant through 1 yr at ≈1.2 plant·m −2. The density of L. digitata (Huds.) Lamour. was more variable through time, but averaged 3.2 plants·m −2. At these densities those plants which reproduced developed 8.9 × 10 9 and 20.02 × 10 9 spores·m −2·yr −1, respectively for L. longicruris and L. digitata. From the spores produced, 8.89 × 10 6 ( L. longicruris) and 0.98 × 10 6 ( L. digitata) microscopic benthic stages were recruited per m 2 per yr. From this bank of microscopic stages, one sporophyte of L. longicruris and two of L. digitata grew to visible size per m 2 per yr. Recruitment of visible sporophytes was enhanced ten-fold by removal of a red algal turf. Removal of macroscopic kelp plants had no effect on recruitment of visible stages. The rate of mortality among macroscopic individuals of L. longicruris (but not of L. digitata) was related to initial plant size and distance to nearest congeneric neighbour. Mortality rates of both species were unrelated to type and size of substratum or season of the year.