Spatiotemporal variability in benthic-pelagic coupling on the Oregon-Washington shelf

Abstract

Continental shelf sediments are sinks for dissolved oxygen (DO) and sources of many major and minor nutrients required for oceanic surface primary production resulting in a strong coupling between benthic and pelagic biogeochemical cycling. In this study, we present paired benthic flux and bottom water biogeochemical data collected from two Oregon shelf sites sampled approximately quarter-annually between 2017 and 2019, and from nine other shelf sites, located off central Oregon to southern Washington, and sampled in either July or September 2022. The benthic fluxes were determined using a novel set-up for ex situ core incubations. When fluxes were normalized to the respective measured sediment DO flux, ratios aligned well with ratios of past flux estimates from the region which were determined using in situ benthic chambers; however, the ex situ flux magnitudes are generally lower. Our findings demonstrate sediments acting as net sinks for DO and nitrate, and sources for phosphate, silicate, and ammonium. Shelf-wide estimates of the relative contribution of sediment-remineralized phosphate and silicate to surface waters on the Oregon shelf, indicate that shelf sediments supplied at least 5 ± 7 % and 37 ± 7 % of the available phosphate and silicate during recent summer upwelling seasons, with similar, respective estimates of 2 ± 9 % and 35 ± 11 % during the spring. Remineralization ratios of C:N:P:O₂ corroborate increased denitrification during the summer and weak denitrification during the winter due to a more oxygenated water column in support of previous studies. A multi-tracer water mass analysis also exhibited an increased water-column nitrate deficit during the summer and fall. Benthic denitrification rates, estimated from benthic fluxes, were between 0.2 and 1.8 mmol N m^-2 day^-1 and in the range of past assessments during the upwelling season. A simple model, applied to further constrain the contributions to bottom water fixed nitrogen (N) loss under assumptions of benthic boundary layer height and residence time, showed that although sediment denitrification could readily account for total bottom water N losses during the summer, additional water-column denitrification is indicated by the strength of early fall deficits at some stations. Constraining water-column and benthic contributions to fixed N deficits is important for understanding how N-limited primary productivity in this region will respond to projected ocean deoxygenation under anticipated global warming. These results demonstrate the interplay of sediment and water-column remineralization processes across the OR-WA shelf. As in most shallow marine systems, the two are integral to the ecosystem dynamics and responses to environmental change.