Differential meiospore size and tolerance of ultraviolet light stress within and among kelp species along a depth gradient

Kelp are differentially stratified along a gradient of UV exposure (as a function of water depth). The role of ultraviolet light in seaweed zonation has not been fully explored. This study found a significant meiospore size difference within and among the kelp species examined: Pterygophora californica Ruprecht (high to mid-subtidal), Laminaria groenlandica Bory (high to mid-subtidal), Laminaria groenlandica Rosenvinge (high-subtidal), Alaria marginata Postels and Ruprecht (low-intertidal) and Hedophyllum sessile Setchell (mid-intertidal). This size difference was correlated with the depth distribution of adult plants, with the largest meiospores originating from shallow-dwelling adult kelp exposed to high UV light. Under UV stress in the laboratory, meiospores from adults growing in high-UV environments displayed greater germination and survival rates than the progeny of adult kelp occupying lower-UV environments. This suggested that in Barkley Sound, British Columbia, Canada, differential tolerance to UV (possibly determined by meiospore size) may limit the upper settlement position of kelp species and individuals. Tolerance to UV may be an important determinant of kelp zonation on rocky shores.