Benthic nitrogen regeneration under oxygen and organic matter spatial variability off Concepción (∼36°S), central Chile

The effects of dissolved oxygen (DO) and organic matter (OM) on several geochemical parameters and nitrogen cycling were evaluated in the upwelling region off central Chile (∼36°S). The sediments of the study area presented: (i) nearly anoxic conditions (2.68 μM) and high chlorophyll-a content (1112 mg g−1) within the Concepción Bay (29 m water depth); (ii) suboxic to oxic conditions (10–129 μM), and from moderate to low chlorophyll-a levels (47–13 mg g−1), in a depth transect outside the bay (88–820 m depth); and (iii) suboxic conditions (5.75–8.9 μM) and low chlorophyll-a content (∼13 mg g−1) in the Itata Canyon area (209 and 362 m depth). Nitrogen processes (in mmol m−2 d−1) varied with depth, in association with changes in fresh OM input and DO content at the transect stations: i.e. decreased ammonification (from 12.9 to 1.0), NH4+ efflux (from 4.8 to 1.2) and denitrification (from 1.3 to 0.3), and increased potential nitrification (from 1.0 to 2.4). Concepción Bay and one of the Itata Canyon stations (209 m) were notably different from the others, indicating heterogeneity of benthic nitrogen cycling in this upwelling region. The bay presented the highest ammonification and lowest denitrification rates. Itata Canyon sediments showed high urea influxes at both sites, although high NH4+ influxes and the highest potential nitrification were only found at the 209 m station. Patterns such as the occurrence of dissolved organic nitrogen effluxes (4.9–75) and higher ammonification rates were found at stations <30 km offshore; influxes (9.7–20.5) and lower ammonification rates were observed at stations >52 km offshore. All the stations presented NO3−+NO2− influxes (0.6–4.3). A nitrogen-cycling imbalance in the NH4+ benthic cycle was found at four stations: NH4+ excesses (30–>100%) at two transect and one of the canyon stations, and a deficit (>100%) at the other canyon station. A small proportion of the NO3−+NO2− influxes (<42%) was explained by the processes studied herein. These imbalances indicated potential incubation biases and the presence of other N processes not accounted for in this study.