LAUSR

ABSTRACT Lowman, H.E.; Hirsch, M.E.; Brzezinski, M.A., and Melack, J.M., 2023. Examining the potential of sandy marine sediments surrounding giant kelp forests to provide recycled nutrients for growth. Journal of Coastal Research, 39(3), 442–454. Charlotte (North Carolina), ISSN 0749-0208. Permeable marine sediments are biogeochemically active and may contribute dissolved nutrients to support primary production in coastal regions. This study examined the potential of permeable marine sediments near giant kelp forests in the Santa Barbara Channel, California as a source of ammonium (NH4+) to the overlying water column to support the observed growth of kelp during summer months when nitrate availability is low. Several nearshore sites located in coastal California in <20 m water depth were sampled for porewater nutrient concentrations, flushing rates, and nutrient fluxes in addition to diel fluctuations in nutrient concentrations of the overlying water column. Time-series analyses of porewater temperatures indicate that porewater flushed to a depth of 15 cm approximately every two hours, and mean NH4+ concentrations of porewater at these depths was 40 µM. The results of flow-through bioreactor incubations indicate that the top 2 cm of sediment are a net source of dissolved nitrogen to the overlying water column and are capable of supplying from 0.05 to 0.90 mmol NH4+ m–2 day–1. Diel water sampling demonstrates that kelp forests may be exposed to NH4+ concentrations greater than 1 µM for multiple hours (four–eight) over a day. These measured reservoirs and exchange rates of NH4+ suggest sandy marine sediments provide a significant source of nitrogen to the water column and may help meet the nitrogen demand by giant kelp during summer in the Santa Barbara Channel.