Graduate Thesis Or Dissertation
 

Spatiotemporal Variability in Benthic-Pelagic Coupling on the Oregon-Washington Shelf: An Investigation of Bottom Water and Benthic Flux Data

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  • Continental shelf sediments are sinks for dissolved oxygen 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. However, the influence and spatiotemporal variability of benthic remineralization on bottom-water chemistry and the supply of nutrients to surface waters has received minimal study on the Oregon-Washington (OR-WA) shelf. To expand knowledge of these areas, bottom water chemistry and benthic flux measurements were made approximately quarter-annually at inner-shelf and mid-shelf sites on the Newport Hydrographic (NH) Line at 44.6 ˚N between December 2017 and July 2019, and again between 44.5˚ and 46.5˚N along the mid-shelf during July and September of 2022. The benthic fluxes were determined using a novel set-up for ex situ core incubations. When these ex situ benthic fluxes were normalized to the respective sediment dissolved oxygen (DO) flux, the ratios align 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. These new findings demonstrate sediments acting as net sinks for DO and nitrate, and sources for phosphate, silicate, and ammonium. Along the NH Line, the average ratio of nitrate fluxes to DO fluxes ranged between 0.2 during summer and -0.01 during winter months, indicating increased denitrification during the summer and little denitrification during winter due to a more oxygenated water column. An examination of C:N:P:O2 remineralization ratios based on both water column and sediment incubation measurements indicated seasonal variability along the NH line. Both bottom water and flux ratios of phosphate to DO were generally greater than the expected ratio for aerobic Redfield remineralization during the summer and fall and lower during the winter. Ratios of nitrate to phosphate for bottom waters appeared lower than the nominal Redfield ratio of 16:1, while benthic flux ratios exhibited high seasonal variability. Estimates of shelf-wide percent contributions of phosphate and silicate to surface waters, computed from measured flux data and the Biologically Effective Upwelling Transport Indices (BEUTI) (Jacox et al., 2018), indicated that during the summer upwelling season, Oregon shelf sediments supply up to 7% and 26% of respective upwelled phosphate and silicate, with these estimates increasing to 44% and 47% of respective upwelled phosphate and silicate during the spring. A multi-tracer water mass analysis, run to constrain the contributions of water-mass mixing and water-column remineralization to observed bottom water chemistries, revealed the presence of an additional year-round sink for oxygen, appearing strongest in the winter and spring. These observations are consistent with supporting measurements of increased sediment DO uptake during the winter and spring when turbulence and water column oxygen are elevated. An increased water-column nitrate deficit during the summer and fall is added evidence of enhanced sediment denitrification. Benthic denitrification rates, estimated with a mass balance of nitrogen, were between 0.21 to 1.82 mmol N m-2 day-1 and in the range of past estimates during the upwelling season of between 1.5 to 4.4 mmol N m-2 day-1 (Fuchsman et al., 2015). Simple models were also applied to further constrain the contributions of sediment remineralization reactions to bottom water, fixed N and DO losses, under assumptions of benthic boundary layer (BBL) height, residence time, and water column remineralization rate. The fixed N model showed that although summer sediment uptake N rates, associated with sediment denitrification, could account for total bottom water N losses, early fall data from the OR-WA shelf suggested the presence of an additional N sink, such as water column denitrification associated with aggregates of phytodetritus. The DO uptake model revealed that for the maximum DO fluxes observed in this study and a BBL height of 5 m, sediments could contribute up to 69% of the respiratory DO loss in bottom waters retained on the shelf. From these many assessments, this study demonstrates 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.
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  • New Orleans, Lousiana, USA
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  • This work was funded by NSF grant numbers OCE-1634319 and OCE-2126112.
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  • Pending Publication
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  • 2024-02-16 to 2024-04-02

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