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Date Permissions Signed

6-14-2015

Date of Award

Summer 2015

Document Type

Masters Thesis

Degree Name

Master of Science (MS)

Department

Environmental Sciences

First Advisor

Shull, David, 1965-

Second Advisor

Yang, Sylvia

Third Advisor

Love, Brooke

Abstract

Eelgrass, Zostera marina, provides critical habitat for many marine species in the Pacific Northwest. Although most eelgrass beds in the Salish Sea are stable, a few areas are experiencing decline. One region experiencing eelgrass decline is Hood Canal, a region that also has frequent hypoxic events. Hypoxia has the potential to be a stressor to eelgrass as it can lead to tissue anoxia at night. These tissues then must undergo anaerobic metabolism, which is less energetically efficient and can produce toxic byproducts. Hypoxia may also work in synergy with other stressors, such as sediment pore-water sulfide. Hypoxia can facilitate the intrusion of sulfide, a known phytotoxin, into eelgrass tissues. Pore-water sulfide has been found in elevated concentrations in the Puget Sound associated with wood waste and reductions in eelgrass density. Additionally, elevated pore-water sulfide has been found at sites in the San Juan Archipelago where eelgrass has declined. Furthermore, hypoxic events and elevated pore-water sulfide can co-occur as a result of eutrophication. In order to better understand the processes that lead to eelgrass decline across Puget Sound, this study examined the interaction between sulfide and hypoxia on the growth and photosynthetic efficiency of Zostera marina.

Eelgrass shoots were collected from Padilla Bay, Washington and placed into seawater tanks in 18 oz. cups of sediment with a disk of agar at the bottom to simulate organic enrichment and to stimulate sulfide production. The growth rate and photosynthetic efficiency of the eelgrass shoots were monitored weekly for six weeks. After week three, the water columns of six of the tanks were reduced to hypoxic conditions (L-1). During week 6, the oxygen concentration was dropped further to near anoxic conditions. After week 6, eelgrass tissue samples were collected for measurement of total sulfur, carbon, and nitrogen.

The results indicated that hypoxia had a significant negative effect on Z. marina shoots, which was evidenced by strong reductions in growth rates and photosynthetic efficiencies. While there was no direct effect of pore-water sulfide on the shoots, it was evident that hypoxia enhanced sulfide intrusion into the shoots. However, intrusion was limited to below-ground tissues and the total sulfur content in the below-ground tissues was not correlated with the measured pore-water sulfide concentrations. These findings indicate that hypoxia in conjunction with sediment organic enrichment harms eelgrass health and enhances the intrusion of sulfide into plant tissues, over a wide range of pore-water sulfide concentrations.

Type

Text

Publisher

Western Washington University

OCLC Number

914236005

Digital Format

application/pdf

Geographic Coverage

Salish Sea (B.C. and Wash.)

Genre/Form

Academic theses

Language

English

Rights

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

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