Abstract

Estuaries are nutrient filters and change riverine nutrient loads before they reach coastal oceans. They have been extensively changed by anthropogenic activities like draining, deepening, and dredging to meet economic and social demand, causing significant regime changes like tidal amplifications and in some cases to hyper-turbid conditions. Furthermore, increased nutrient loads, especially nitrogen, mainly by agriculture cause coastal eutrophication. Estuaries can either act as a sink or as a source of nitrate, depending on environmental and geomorphological conditions. These factors vary along an estuary, and change nitrogen turnover in the system. Here, we investigate the factors controlling nitrogen turnover in the hyper-turbid Ems estuary (Northern Germany) that has been strongly impacted by human activities. During two research cruises in August 2014 and June 2020, we measured water column properties, dissolved inorganic nitrogen, dual stable isotopes of nitrate and dissolved nitrous oxide concentration along the estuary. Overall, the Ems estuary acts as a nitrate sink in both years. However, three distinct biogeochemical zones exist along the estuary. A strong fractionation (~ 26 ‰) of nitrate stable isotopes points towards nitrate removal via water column denitrification in the hyper-turbid Tidal River, driven by anoxic conditions in deeper water layers. In the Middle Reaches of the estuary nitrification gains in importance turning this section into a net nitrate source. The Outer Reaches are dominated by mixing with nitrate uptake in 2020. We find that the overarching control on biogeochemical nitrogen cycling, zonation and nitrous oxide production in the Ems estuary is exerted by suspended particulate matter concentrations and the linked oxygen deficits.

Highlights

  • Estuaries can significantly alter riverine nutrient loads before they reach adjacent coastal oceans (Bouwman et al, 2013; Crossland et al, 2005)

  • We find that the overarching control on biogeochemical nitrogen cycling, zonation and nitrous oxide production in the Ems estuary is exerted by suspended particulate matter concentrations and the linked oxygen deficits

  • In the upper riverine part of the estuary, nitrate concentration fell below the conservative mixing line, indicating nitrate removal, followed by a zone with nitrate concentration slightly above the mixing line that acted as a net nitrate source

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Summary

Introduction

Estuaries can significantly alter riverine nutrient loads before they reach adjacent coastal oceans (Bouwman et al, 2013; Crossland et al, 2005). Estuaries have been extensively altered by humans and anthropogenic activities to meet economic and social demands. Draining, damming, diking, deepening and dredging lead to significant regime changes including tidal amplification, hyper-turbid conditions and loss of habitats High nutrient loads from agriculture, waste water and urban runoff have induced eutrophication (Galloway et al, 2003; Howarth, 2008; Van Beusekom et al, 2019), one of the greatest threats to coastal ecosystems worldwide (e.g. Howarth and Marino 2006; Voss et al 2011)

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