Herbivory and harvesting: Effects on sexual recruitment and vegetative modules of Ascophyllum nodosum

Abstract

Populations of Ascophyllum nodosum (Phaeophyta) have the potential to grow from either sexual recruits or by vegetative propagation of modules (i.e., fronds). In stands that are harvested in southwestern Nova Scotia there are two major potential regulators of these sources of natality, herbivores and commercial harvesting activities. It was shown here that herbivory has a major effect on sexual recruitment. Nearly all of the zygotes (99.9 %) which settled naturally in 1989 were consumed within one year. Slow growth of the survivors and continued herbivore pressure indicated that remaining germlings would not survive in subsequent years. Our results also suggested that harvesting may enhance the production of zygotes but this effect may not result in increased recruitment as most germlings do not survive grazing.

Grazing did not affect module survivorship or breakage to any major extent. Hence our study suggested that increased size represents an escape from herbivory.

Because sexual reproduction is limited in natural populations, vegetative propagation is the most important component of natality.

Keywords:

• grazing
• algal recruitment
• harvesting
• Ascophyllum nodosum