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RESEARCH ARTICLE

Dissolved organic carbon and dissolved organic nitrogen concentrations and exports upstream and downstream of the Dallas–Fort Worth metropolis, Texas, USA

J. A. Aitkenhead-Peterson A C and M. K. Steele B
+ Author Affiliations
- Author Affiliations

A Department of Soil and Crop Sciences, Texas A&M University, College Station, Texas, TX 77843, USA.

B Department of Crop and Soil Environmental Sciences, Virginia Tech, Blacksburg, VA 24061, USA.

C Corresponding author: Email: jacqui_a-p@tamu.edu

Marine and Freshwater Research 67(9) 1326-1337 https://doi.org/10.1071/MF15280
Submitted: 28 July 2015  Accepted: 17 February 2016   Published: 3 June 2016

Abstract

Concentrations and export of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) from terrestrial landscapes to near-coastal zones vary with land use. Information on (DOC) and (DON) concentrations and exports from urban ecosystems is sparse; thus, their source from within urbanised watersheds such as soil or vegetation or from permitted sewage discharge is unknown. We examined DOC and DON concentrations and exports in four gauged subwatersheds in the humid subtropical, upper Trinity River basin, upstream and downstream of the Dallas–Fort Worth metropolis in Texas, USA. Annual average DOC concentrations ranged from 5.7 ± 0.4 to 6.4 ± 0.8 mg L–1 and DON concentrations ranged from 0.31 ± 0.05 to 0.33 ± 0.14 mg L–1. Dissolved organic carbon exports, which included permitted sewage discharge, ranged from 522 kg km–2 year–1 above Dallas–Fort Worth to 3637 kg km–2 year–1 below Dallas–Fort Worth. Permitted effluent discharge contributed between 1 and 35% of DOC loading above and below the Dallas–Fort Worth metropolis. DON exports ranged from 27 to 179 kg km–2 year–1 above and below Dallas–Fort Worth respectively. There was difficulty apportioning permitted effluent-discharge contribution to DON because of the transformations among nitrogen-species. A moderate but significant relationship was found between DOC and sodium concentrations (R2 = 0.45; P < 0.0001; n = 40) and between DOC and potassium concentrations (R2 = 0.45; P < 0.0001; n = 40). Dissolved organic nitrogen also displayed a significant relationship with sodium (R2 = 0.33; P < 0.001; n = 40) and potassium (R2 = 0.59; P < 0.001; n = 40), suggesting that increases in these cations to aquatic ecosystems may induce increases in DOC and DON concentrations. Although DOC export was significantly correlated with medium-density urban land use (r = 0.96; P < 0.05: n = 4), DON export was not (r = 0.93; P > 0.05; n = 4), suggesting that land-management practices and permitted point-source discharges have a significant effect on aquatic DOC and DON concentrations and exports derived from urban watersheds.

Additional keywords: anions, cations, DOC, DON, rivers, urban.


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