Investigating Benthic Marine Sediments near Giant Kelp Forests as a Nutrient Source to the Overlying Water Column

During the period from July to November, the water column in nearshore regions of the Santa Barbara Channel becomes warm, stratified, and low in dissolved inorganic nitrogen (DIN). During this otherwise low-nutrient period, permeable marine sediments located in proximity to giant kelp forests might contribute a measurable efflux of DIN to the overlying water column and be a net source of nitrogen to support primary production. Sediment cores and seawater were collected at three kelp forest sites over three consecutive summers (2017-2019) and studied using flow-through sediment bioreactors to measure nutrient fluxes. Surficial sediments were sectioned from the cores, placed in a closed, flow-through bioreactor setup, and unfiltered seawater was pumped from a reservoir through the sediment for three hours’ time. Across all sites and years sampled, kelp forest sediments were a significant net source of ammonium (NH₄⁺) but not nitrate (NO₃^-) to the surrounding seawater. Based on sediment porewater NH₄⁺ concentrations and generation rates, it takes approximately 9 hours for microbes to regenerate in situ porewater NH₄⁺ concentrations. These results suggest that due to organic matter inputs, advective flow of porewater, and microbial communities, permeable sediments located near giant kelp forests in the Santa Barbara Channel are a net source of DIN to the overlying water column in stratified conditions.