Effects of wave exposure on population structure, demography, biomass and productivity of the kelp Laminaria hyperborea

Patterns of potential recruitment, survival, age-structure, density, biomass and primary production were studied in the kelp Laminaria hyperborea along a wave-exposure gradient in western Norway. The distribution of L. hyperborea is positively related to wave exposure, and the present work aimed to study how demography and population dynamics are affected by wave exposure. Populations at 9 sites representing 3 different levels of wave exposure were investigated. The biomass and production of L. hyperborea doubled along the gradient from low- to high-exposure sites. This increase was caused by an increase in plant density and individual plant size. Recruits and sub-canopy plants made up the majority of all individuals at all sites, but potential recruitment increased more than mortality rate among recruits and young sub-canopy plants as wave exposure increased, leading to a higher density of these small individuals at high-exposure sites. Despite their large numbers, recruits and sub-canopy plants were not important for total biomass and production, and variations in those parameters with wave exposure were largely driven by variations in the density and size of adult canopy plants. Canopy plants suffered higher rates of mortality at low wave exposure, leading to shorter longevity and lower density than at high-exposure sites. The density and individual size of canopy plants both increased by ca. 50% with increasing wave exposure, explaining the higher biomass and productivity at high-exposure sites. We suggest that the high mortality rate, low density and small size of canopy plants at low-exposure sites is caused by a combination of high epiphytic load and self-shading, which may impair light conditions, affect nutrient uptake and increase drag on the blades during extreme wind events. The reason why plants at high-exposure sites reach a larger individual size remains unknown.