Suman Acharya

Suman Acharya - Postgraduate Student

Dissolved Organic Nitrogen (DON) in natural waters as a source of bioavailable nitrogen

Dissolved nitrogen in natural water (rivers, lakes and wetlands) is composed of dissolved inorganic nitrogen (DIN; NO3-, NO2- and NH4+) and a very heterogeneous matrix of dissolved organic nitrogen (DON) compounds. DON is part of the wider pool of dissolved organic carbon (DOC), a group of large and poorly-characterised macromolecules, present in freshwaters and formed through decomposition of natural biomolecules. It is well established that the DIN pool is readily utilized by phytoplankton and strongly contributes to eutrophication. The role of DON has received less attention which is likely due to the large array of compounds that comprise DON, thus leading a greater complexity in understanding its potential biological function. DON quantity and composition can vary through hydrological events due to processes that control the production of DON and its delivery to freshwaters. In general, DON concentrations increase during pulse events due to mobilization of multiple sources of DON (e.g. riverine inputs, terrestrial runoff and biogeochemical processes) within a catchment. However, there is still little known about the composition and bioavailability of DON with respect to hydrological conditions. Characterizing DON at the molecular level and determining its bioavailability are crucial steps in understanding its importance to microorganisms and its potential influence on aquatic food webs.

Characterisation techniques for DOC and DON tend to be global (non-specific) methods that do not provide detailed speciation, limiting our understanding of nutrient uptake from these sources. In this project we will investigate the association of identifiable organic nitrogen (e.g. amino acids, N-acetyl glucosamine, nucleic acids) with DOC, over a range of DOC size classes, using DOC from a variety of natural and reference sources. The analytical methods developed in this work will be used to determine DON quantity and composition in riverine and peatland aquatic ecosystems and investigate the relative mobility of DON compounds during the natural disturbance events (e.g. storms and/or floods). Such data will be invaluable to understanding the contributions of different hydraulic components of stream flow to DON quantity and composition and the interpretation of landscape-scale processes.


University

La Trobe University

Supervisors

Primary Supervisor

Associate Professor Ewen Silvester (La Trobe University)

Co-Supervisor

Dr Aleicia Holland (La Trobe University)

Duration

July 2019 to July 2023