A Mass Balance of Nitrogen in a Large Lowland River (Elbe, Germany)

Stephanie Ritz,Helmut Fischer

doi:10.3390/w11112383

Abstract

Nitrogen (N) delivered by rivers causes severe eutrophication in many coastal waters, and its turnover and retention are therefore of major interest. We set up a mass balance along a 582 km river section of a large, N-rich lowland river to quantify N retention along this river segment and to identify the underlying processes. Our assessments are based on four Lagrangian sampling campaigns performed between 2011 and 2013. Water quality data served as a basis for calculations of N retention, while chlorophyll-a and zooplankton counts were used to quantify the respective primary and secondary transformations of dissolved inorganic N into biomass. The mass balance revealed an average N retention of 17 mg N m−2 h−1 for both nitrate N (NO3–N) and total N (TN). Stoichiometric estimates of the assimilative N uptake revealed that, although NO3–N retention was associated with high phytoplankton assimilation, only a maximum of 53% of NO3–N retention could be attributed to net algal assimilation. The high TN retention rates in turn were most probably caused by a combination of seston deposition and denitrification. The studied river segment acts as a TN sink by retaining almost 30% of the TN inputs, which shows that large rivers can contribute considerably to N retention during downstream transport.

Highlights

In the past centuries, human activities massively altered the natural biogeochemical cycle of N.In particular, the development of industrial N fixation with the Haber–Bosch process in the early20th century, which allowed the large-scale production of fertilizer, led to a dramatic, continuous accumulation of reactive N in our ecosystems [1,2]
Stoichiometric estimates of the assimilative N uptake revealed that, NO3 –N retention was associated with high phytoplankton assimilation, only a maximum of 53% of NO3 –N retention could be attributed to net algal assimilation
To clarify whether the quantified retention is temporal or permanent, we provide estimates about the relative role of phytoplankton assimilation against other NO3 –N retention pathways by relating the production of chlorophyll-a and zooplankton to the concurrent retention of NO3 –N and total N (TN)

Summary

Introduction

Human activities massively altered the natural biogeochemical cycle of N. 20th century, which allowed the large-scale production of fertilizer, led to a dramatic, continuous accumulation of reactive N in our ecosystems [1,2]. In-stream processes can substantially alter the reactive N load transported from catchments to coastal areas. Assimilation, for instance, reduces the dissolved N fraction from the river water by conversion to organic matter, and deposition of N compounds in the riverbed alters the total N (TN) pool during downstream transport. Due to possible re-mineralization and re-suspension, reactive N principally remains in the ecosystem and only contributes to a temporal retention of N. Denitrification, in contrast, leads to a permanent removal of reactive N from river water by gaseous losses of N2 (and small amounts of N2 O)

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