Lauren E. Bell1, Kristy J. Kroeker1
1Department of Ecology and Evolutionary Biology, University of California Santa Cruz, 130 McAllister Way, Santa Cruz, CA 95060
Marine climate change in high latitude oceans will overlay onto the distinct seasonality of these environments. Our knowledge of how such environmental variability and change will interact to affect kelp productivity in these regions is limited. This lack of data is particularly evident for the macroalgal communities associated with the globally abundant giant kelp Macrocystis pyrifera, which has been studied extensively at lower latitudes but rarely considered at the polar edges of its range. To assess the current production potential of giant kelp forests at their high latitude fringe, we conducted a multi-year field study of the growth and turnover dynamics of M. pyrifera and co-occurring understory kelps Hedophyllum nigripes and Neoagarum fimbriatum in Sitka Sound, Alaska. We then grew these kelps in the lab within seasonally relevant scenarios of light and nutrient availability under current vs. end-of-century projections for temperature and pCO2. We found that Macrocystis beds in southeast Alaska produce an estimated ~150 g C · m-2 · yr-1 and turnover an estimated 2.1 times per year, on average substantially lower rates than have been observed at lower latitudes. However, carbon production of high latitude giant kelp was unaffected by end-of-century temperature and pCO2 conditions, regardless of season. In contrast, the more poleward-occurring understory kelps exhibited reduced growth and nutritional content under elevated future summer temperatures. Incorporating seasonally-specific global change scenarios for macroalgal populations across the breadth of their range will be critical to understanding future impacts to temporal and spatial variation in kelp species’ production.