Date of Award

2017

Document Type

Open Access Dissertation

Department

Marine Science

Sub-Department

College of Arts and Sciences

First Advisor

Ronald Benner

Abstract

Dissolved organic matter (DOM) in aquatic ecosystems is a large reservoir of reduced carbon that is mostly resistant to degradation. A small fraction of DOM cycles relatively quickly and is biologically utilized on timescales of days to months. This bioavailable DOM (BDOM) supports aquatic food webs, drives major elemental cycles, and is coupled to atmospheric CO2. Despite wide-ranging importance, bioavailability of DOM and its linkages to ecosystem properties (e.g., primary production, nutrients) are poorly characterized, particularly at the ecosystem level. Bioassay experiments are commonly used to determine BDOM, but this approach alters conditions and has limited spatial and temporal coverage. In this dissertation, biochemical indicators of DOM bioavailability were developed and implemented in a wide range of ecosystems to reveal large-scale distributions of BDOM in the Arctic (Chapter 1) and Antarctic Oceans (Chapter 2), locate seasonal biological hotspots in a subtropical ocean margin (Chapter 3), and to trace transport and fate of BDOM from surface to ground waters (Chapter 4).

Measurements of amino acids, a major bioactive component of BDOM, were compared between the high (Chukchi Sea) and low (Beaufort Sea) productivity regions of the western Arctic Ocean. Bulk concentrations of dissolved organic carbon (DOC) were similar in the two systems despite their contrasting productivity, but DOM bioavailability as indicated by amino acid yields was much higher in the more productive Chukchi Sea. Seasonal trends of amino acids revealed elevated production and rapid off-shelf transport of BDOM in the Chukchi Sea during a season with reduced sea-ice cover.

The use of amino acids as BDOM indicator was further tested in the Southern Ocean during austral winter when primary production is light-limited and minimal. The sampling encompassed ice-covered and ice-free waters along a latitudinal gradient in the region of the Antarctic Peninsula, one of the fastest warming regions on Earth. Unlike the DOC that varied irregularly, amino acid-based indices illustrated a significant northward increase in DOM bioavailability from ice-covered to open waters. Overall, the observations in the polar oceans indicate a correspondence between DOM bioavailability and nutrient- and light-driven changes in ecosystem productivity.

A novel approach using two biochemical indicators (amino acids and carbohydrates) of BDOM was developed during a large-scale seasonal survey of the river-influenced Louisiana margin. These indicators revealed patchy distributions of compositionally distinct types of BDOM hotspots that varied with phytoplankton biomass and nutrient levels, and they further indicated a diel variability in sources of BDOM, with zooplankton grazing at night and phytoplankton extracellular release during daytime under nutrient limitation.

Biochemical indicators were extended to surface and groundwater in South Carolina. Amino acids, lignin phenols, and chromophoric DOM were monitored monthly over two years. Linking groundwater DOM with surface water DOM and precipitation revealed differential transport of hydrophilic and hydrophobic molecules through soils and depletion of BDOM in groundwater. These observations guided a development of the Regional Chromatography Model to illustrate processes regulating the composition and bioavailability of DOM during transport through the soil column to the saturated zone.

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