Small-Scale Gradients in Big River: Implications for a State-of-the-Art Research Platform in the Elbe
Authors: Bold Sina , van Beusekom Justus E.E. , Voynova Yoana G. , Cysewski Marius , Van Dam Bryce , Stresser Michael , Carrasco Alvarez Ruben , Horstmann Jochen , Dähnke Kirstin , Sanders Tina , Friedrich Jana , Pröfrock Daniel , Thomas Helmuth 
Published: March 2021

&lt;p&gt;Estuaries are crucial in transforming matter fluxes from land to sea. To better understand and quantify these processes and respective fluxes, it is important to determine the input into an estuary accurately. To allow for such studies in the Elbe estuary in Germany, a state-of-the-art research platform is currently being set-up just upstream of the weir in Geesthacht at the entrance of the estuary. Here, we report on small-scale spatial dynamics of organic matter and associated processes from several cross and longitudinal profiles around the planned location and the implications for the set-up of the aforementioned research platform.&lt;/p&gt;&lt;p&gt;Based on preliminary data obtained in August 2020 during a period of relatively low discharge, we present the following results: (1) In three cross profiles along a 10 km transect of the Elbe upstream of the weir, we observed considerable small-scale gradients regarding currents and various biogeochemical parameters. In comparison to the fairway, water from the riverbanks was depleted in suspended particulate matter, chlorophyll a, dissolved oxygen, and nitrate, and enhanced in ammonium, phosphate and silicate, as well as total alkalinity and dissolved inorganic carbon paralleled by decreasing pH. This suggests that in the summer, organic matter is deposited and remineralised at the riverbanks, resulting in the release of ammonium, phosphate and silicate, and in the removal of nitrate, presumably by denitrification. (2) Along the 10 km transect towards the weir, we observed that concentrations of suspended particulate matter, chlorophyll a, dissolved oxygen, nitrate and pH were decreasing. In contrast, we found that ammonium, phosphate and silicate, total alkalinity and dissolved inorganic carbon increased towards the weir. This suggests an increased sedimentation and subsequent remineralisation due to the reduced flow velocities in front of the weir. (3) An analysis of a 10-year time series from the weir supports this by showing higher ammonium concentrations when discharges were relatively low. The implications of these findings for the set-up of the research platform in this area, as well as for optimising estimates of budgets are discussed. The research platform will contribute to understand further such variations in biogeochemical parameters at the entrance of the Elbe estuary over time.&lt;/p&gt;&lt;p&gt;The research platform is set-up in cooperation with the Helmholtz initiative MOSES (&amp;#8220;Modular Observation Solutions for Earth Systems&amp;#8220;) and will be incorporated in the Elbe-North Sea Supersite of DANUBIUS-RI (&amp;#8220;International Centre for Advanced Studies on River-Sea Systems&amp;#8220;). Funding is provided by European Regional Development Funds, the federal state of Schleswig-Holstein, the Helmholtz Association and the Helmholtz-Zentrum Geesthacht. The research platform, planned to be operational in autumn 2021, will also be open for users e.g. to develop and test new methods and technologies. Data will be made available through the &amp;#8220;Helmholtz Coastal Data Centre&amp;#8221; (HCDC).&lt;/p&gt;