This thesis seeks to distinguish the main environmental parameters controlling the growth of Lobelia dortmanna and, by consideration of field conditions, describe which of these controls is most important in determining the zonation of the species, both horizontal and vertical, in Scottish lochs. A detailed analysis of growth across one season was undertaken in a Sutherland lochan, studying populations on sites of different nutrient status. Continuous recordings of temperature and light and monthly measurements of photosynthesis, soluble carbohydrate content and leaf production show continuous growth across the year. However, major increases in growth are strongly correlated to light increases, and only weakly with temperature. No differences occur between sites, so nutrient limitation is not thought to be important in growth. Germination studies reveal that seeds are absolute requirers of light for germination, are red-light promoted and require a cold stratification period before germination. Seeds germinate in low light, under conditions that seedlings subsequently cannot survive in. Examination of the light regime in Scottish lochs reveals that there is sufficient light for germination below the depth limit of zonation. Thus zonation is not controlled by a light requirement for germination. Studies of photosynthesis, pigment and carboxylase variations with depth in L. dortmanna reveal some ability to respond to shading, particularly increasing chlorophyll levels. However, chlorophyll/carboxylase ratios do not change, so indicating the plant is not typical of shade-adapting species. It is concluded that light control of photosynthetic production is the most important factor in controlling growth and zonation.