Gametophyte reproduction and development of Undaria pinnatifida under varied nutrient and irradiance conditions

To facilitate predictions of how environmental heterogeneity affects invasion success and range expansion in the kelp Undaria pinnatifida (Harvey) Suringar, it is essential to understand how physical and chemical dynamics influence development and reproduction. Although the productivity of mature U. pinnatifida sporophytes is known to be strongly influenced by the availability of light and dissolved nutrients, both of which show considerable spatial and temporal variability, no previous studies have evaluated their interactive effects on gametophytes or microscopic sporophytes. To evaluate these effects, we used a crossed-factorial laboratory experiment to expose U. pinnatifida zoospores to different irradiance (3 levels: 8.52, 27.57, and 144.85 μmol quanta m− 2 ∙ s− 1) and nutrient regimes (3 levels: dissolved inorganic nitrogen (DIN): 12.3, 45.3, and 429.7 μM) for 22 days. Over this period we monitored zoospore settlement density, gametophyte length, gametophyte surface area, egg formation, zygote formation, sporophyte formation, and sporophyte surface area. Under low irradiance conditions, gametophyte growth stalled within 2 days of zoospore settlement, and development did not increase with increased nutrient concentrations. Under medium and high irradiance levels, gametophyte growth and reproduction rate were generally reduced at low nutrient levels. Differences in development between higher nutrient treatments were slight during the early stages of development, but additional nutrients had a strong positive effect on development and growth of young sporophytes, especially in high irradiance conditions. Our results suggest that the development time from zoospore to sporophyte (and hence the rate of invasion) is likely to be most rapid in environments where irradiance and nutrients are jointly abundant (e.g., where shading has been reduced via the removal of canopy forming species and anthropogenic nutrient additions occur).