Date of Award

Winter 2022

Project Type

Thesis

Program or Major

Oceanography

Degree Name

Master of Science

First Advisor

Robert T Letscher

Second Advisor

Elizabeth Harvey

Third Advisor

Kai Ziervogel

Abstract

Net biologically produced organic matter integrated over an annual cycle in the euphotic zone of the global ocean is equal to annual net community production (ANCP). ANCP is limited by the rate of delivery of essential nutrients (nitrogen (N), phosphorus (P), and iron (Fe)) to the sunlit surface ocean and the efficiency with which these nutrients are either metabolized or returned to the ocean’s interior. In subtropical oceans, which are regions of large-scale downwelling and consequently characteristic of nutrient-depleted surface waters, ANCP remains comparable to more nutrient replete ecosystems (~2 – 5 mol C m-2 yr-1). To understand what may fuel this productivity, analyses of dissolved organic matter (DOM) within the upper 500 meters of the subtropical North Pacific gyre were performed. Observed meridional gradients suggest the consumption of dissolved organic phosphorus (DOP) and bioavailable dissolved organic nitrogen (bDON) may contribute bioavailable P and N on the order of 8.8 ± 2.2 mmol P m-2 y-1 and 14.3 ± 3.7 mmol N m-2 y-1 along the northward transit of waters from the southern edge towards the gyre core. Additionally, bioassay incubation experiments were performed within two vertically distinct layers of the euphotic zone to quantify the magnitude and rate of heterotrophic DOP remineralization in surface waters (5 m) and the deep chlorophyll maximum (DCM) (125 m) at two stations in the subtropical North Pacific; Station ALOHA (22.75°N) and 31°N along a transect on 158°W, north of Oahu, HI. Evidence for a measurable pool of labile DOP present in surface waters on the order of 25 – 60 nM was found to be consumed in ~5 days near the southern edge of the gyre at Station ALOHA. This consumption was ~1/3 of the latitudinal gradient in surface waters to 31°N, the core of the gyre. Additionally, a new method was deployed to estimate the fraction of the bulk dissolved organic nitrogen (DON) pool composed of bioavailable DON (bDON). A similar meridional gradient in surface ocean bDON was observed on the order of 230 nM N across the gyre, with a DON pool that was comprised of ~13.3 ± 3.5% bDON. The meridional gradients observed in the size of the surface ocean labile DOP and DON pools found in this study largely affirms the importance of upper-ocean lateral organic nutrient transport on supplying North Pacific subtropical gyre surface waters with bioavailable phosphorus and nitrogen and provides important observational data to validate existing models of marine organic nutrient cycling.

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