THE NITROGEN BIOREMEDIATION POTENTIAL OF Saccharina latissima CULTIVATION AND HARVEST IN THE GULF OF MAINE

K elp uses dissolved inorganic nitrogen (DIN) from temperate marine environments to grow. When used as a management tool, assimilation and extraction of anthropogenic DIN by strategically-sited kelp farms is known as  nitrogen bioremediation (Fig. 1) . Here, we provide the first estimates of  N bioremediation by Saccharina latissima in the western Gulf of Maine (GOM). Wet biomass produced, total tissue N  and C  throughout the growing season, origin of tissue N (anthropogenic or marine), and nitrate reductase activity was measured in kelp farmed at four sites from 2016 - 2019 . Ambient  DIN measured at the farms was compared to nitrate reductase and total tissue N in the kelp tissue to determine quantity and rate of nitrogen assimilation by the kelp. Origin of the assimilated nitrogen was estimated by measuring nitrogen isotope values (δ15 N) in the kelp tissue.

The mean kelp biomass measured at the end of the growing season (late May) was 9.84 (± 2.53) - 14.84 kg (WW) m-1 . Mean total tissue N was 1.04% - 3.82 % DW (± 0.22).  These results suggest that harvesting 1 ha of S. latissima from the region in late May removes 18.5 - 55.6 kg N ha -1 with 6 m spacing between longlines.  Nitrogen isotope ratios in the sampled tissue do not show a clear pattern of anthropogenic nitrogen use. High variance was observed in nitrate reductase activity of individual laminae. We conclude that kelp aquaculture may complement other N mitigation strategies, but it is not a panacea for nutrient management in the region. Modifications to harvesting schedules, alternative species or co-culture, or more informed siting may increase total potential bioremediation by kelp farms in the GOM.