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Date Permissions Signed

7-2019

Date of Award

Summer 2019

Document Type

Masters Thesis

Department or Program Affiliation

Environmental Science, Marine and Estuarine Science Program

Degree Name

Master of Science (MS)

Department

Environmental Sciences

First Advisor

Shull, David, 1965-

Second Advisor

Love, Brooke

Third Advisor

Matthews, Robin A., 1952-

Abstract

Recent decades have seen changes to biogeochemical cycles in the Salish Sea, including alterations in water column nutrients, an expansion of hypoxic zones, and bottom water acidification. Marine sediments can be a major contributor to these biogeochemical cycles by exchanging solutes with bottom water. In an effort to understand the sediment biogeochemistry of the Salish Sea further, benthic fluxes of dissolved oxygen (DO), dissolved inorganic carbon (DIC), pH, total alkalinity (TA), and nutrients (ammonium, nitrate+nitrite, phosphate, silicate) between the sediment and the overlying water were directly measured using incubated flux cores at 42 sites in April and early May 2018. Explanatory variables describing bottom water conditions (salinity, temperature, DO, DIC, pH, and TA), sediment properties (grain size, total organic carbon, and C:N ratio), and site depth at the time of sampling were also collected. Denitrification estimates for each station were made using the benthic fluxes and estimates of C:N ratios of organic matter suspended in bottom water near each station.

Benthic fluxes varied considerably across the Salish Sea and high variability was observed between core replicates. In general, DO fluxes were larger than DIC fluxes and we attributed this to the oxidation of reduced compounds stored in sediments. The ratio phosphorus to DIC fluxes were less than the Redfield ratio, indicating phosphorus storage in the sediments. At all but two sites denitrification was present, with a median rate of 1 mmol m-2 d-1. Using canonical redundancy analysis, the spatial variation observed in benthic fluxes and denitrification rates was determined to be primarily driven by water column depth and bottom water DO.

Spatial variation was also observed in the estimations of sediment contribution to water column biogeochemistry in Puget Sound basins. Denitrification rates were estimated to remove 11% of the dissolved inorganic nitrogen (DIN) supplied to the bottom water in Hood Canal, while in the Central Basin only 1% was removed. This difference was due to the long bottom water residence time in Hood Canal, where the denitrification can have a larger impact on bottom water DIN concentrations as the water slowly moves through the basin. Nearly all of the particulate phosphorus that reached the seafloor during the early spring was estimated to be stored in the sediment. Sediment oxygen uptake was found to account for approximately 80% of the bottom water DO removal in the South Basin compared to 21 – 35% in the other basins. The sediments play a large role in the South Basin because it is shallow.

This spatial study illuminates the varying contributions that sediments make to the functioning of Salish Sea biogeochemistry. Seasonal benthic flux measurements should be made to further understand and quantify the influence that sediments have on water column biogeochemistry in the Salish Sea.

Type

Text

Keywords

benthic fluxes, sediment, biogeochemistry

Publisher

Western Washington University

OCLC Number

1112781496

Subject – LCSH

Benthos--Effect of sediments on--Salish Sea (B.C. and Wash.); Biogeochemical cycles--Salish Sea (B.C. and Wash.); Marine sediments--Salish Sea (B.C. and Wash.); Denitrification--Salish Sea (B.C. and Wash.)

Geographic Coverage

Salish Sea (B.C. and Wash.)

Format

application/pdf

Genre/Form

masters theses

Language

English

Rights

Copying of this document in whole or in part is allowable only for scholarly purposes. It is understood, however, that any copying or publication of this document for commercial purposes, or for financial gain, shall not be allowed without the author’s written permission.

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