Abstract
Terrestrially-derived dissolved organic carbon (DOC) and nitrogen (DON) transported by rivers have been recognised as contributors to aquatic nutrient burdens, and can be of importance in rivers and estuaries already impacted by anthropogenic inorganic nutrient discharges. The concentration of DOC and DON and the flux of both to the estuary and ultimately the coastal zone is dependent upon many factors including rainfall, catchment land use, and biological processes. DOC and DON concentrations together with nitrate plus nitrite and ammonium concentrations were measured in the anthropogenically-impacted estuary Christchurch Harbour (UK) and at sites in the lower reaches of its two source rivers, the Hampshire Avon and the Stour, at weekly intervals for a year during which time several extreme rainfall events occurred. A series of transects along the estuary were also performed after weekly sampling was completed. DOC concentrations were correlated between both rivers and the estuary and were positively related to increases in river flow, but DON concentrations revealed a more complicated picture. Peak instantaneous fluxes of DOC and DON exceeded 60,000 kg C d−1 and 7000 kg N d−1 respectively both in the Stour and the estuary during high flow periods. The sources of both and routes by which they enter the aquatic system may account for the differences in dynamics, with flushing of superficial soils being a key source of DOC and point sources such as sewage treatment works being proposed as sources of DON. Removal processes within the estuary were also of importance for DON concentrations whilst DOC behaved more conservatively with some evidence of local production within the estuary. Estimated annual loads of DON and DOC to the coastal zone from Christchurch Harbour were 118 kg N km−2 y−1 and 2296 kg C km−2 y−1.
Highlights
River flow and estuarine flushing time. Water temperature in both rivers and the estuary followed a seasonal cycle with minimum temperatures between 5.7 °C and 6.2 °C observed in winter and maximum summer temperatures reaching between 22.8 °C and 23 °C in July 2013
Flow data was unavailable at Knapp Mill for several days in early January 2014 after the gauging station was struck by lightning, but river flow rates both immediately before and after the loss of data were substantially elevated from background and so the flow was assumed to remain elevated across the period of missing data
Two periods of sustained rainfall with associated elevations in river flow were captured during the year of sampling with an initial wetting up period in late October 2013 being followed by a prolonged period of several months duration between December 2013 and March 2014. These events allow the dynamics of Dissolved inorganic nitrogen (DIN), dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) concentrations within the two rivers and the estuary to be examined under a range of hydrological conditions
Summary
Once considered to be passive transporters of terrestrially-derived DOM from soils to the sea, rivers are recognised as dynamic systems where both production and loss processes can potentially alter both the concentration and the composition of DOM during transport (Battin et al, 2008; Bertuzzo et al, 2017; Graeber et al, 2018; Harris et al, 2018) The balance of such processes can differ, even between reaches of the same river and the flux of both dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) is hard to predict (Wymore et al, 2018).
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