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RESEARCH ARTICLE (Open Access)

Constraining nitrogen sources to a seagrass-dominated coastal embayment by using an isotope mass balance approach

Douglas G. Russell A , Adam J. Kessler https://orcid.org/0000-0003-4753-9292 A B , Wei Wen Wong A , Dick Van Oevelen C and Perran L. M. Cook https://orcid.org/0000-0002-0444-3488 A *
+ Author Affiliations
- Author Affiliations

A Water Studies Centre, School of Chemistry, Monash University, Clayton, Vic. 3800, Australia.

B School of Earth, Atmosphere and Environment, Monash University, Clayton, Vic. 3800, Australia.

C Department of Estuarine and Delta Systems, Royal Netherlands Institute for Sea Research (NIOZ-Yerseke), Yerseke, Netherlands.

* Correspondence to: perran.cook@monash.edu

Handling Editor: Thomas Wernberg

Marine and Freshwater Research 73(5) 703-709 https://doi.org/10.1071/MF21320
Submitted: 9 November 2021  Accepted: 17 January 2022   Published: 25 February 2022

© 2022 The Author(s) (or their employer(s)). Published by CSIRO Publishing. This is an open access article distributed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND)

Abstract

Nitrogen (N) is often the key nutrient limiting primary production in coastal waters. Quantifying sources and sinks of N is therefore critical to understanding the factors that underpin the productivity of coastal ecosystems. Constraining nitrogen inputs can be difficult for some terms such as N fixation and marine exchange as a consequence of uncertainties associated with scaling and stochasticity. To help overcome these issues, we undertook a N budget incorporating an isotope and mass balance to constrain N sources in a large oligotrophic coastal embayment (Western Port, Australia). The total N input to Western Port was calculated to be 1400 Mg N year−1, which is remarkably consistent with previous estimates of sedimentation rates within the system. Catchment inputs, N fixation, marine sources and atmospheric deposition comprised 44, 28, 28 and 13% of N inputs respectively. Retention of marine-derived N equated to ~3 and ~10% of total N and NOx flushed through the system from the marine end-member. The relatively high contribution of N fixation compared with previous studies was most likely to be due to the high proportion of nutrient-limited intertidal sediments where N is mediated by seagrasses and sediment cyanobacteria.

Keywords: 15N, denitrification, isotope, nitrogen, nitrogen budget, nitrogen fixation, seagrass, tidal flat.


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