Coupling and decoupling of carbon, oxygen, and nitrogen in the Elbe Estuary

<p>The Elbe Estuary and its biogeochemistry are strongly influenced by tidal cycles of the North Sea, high nutrient and organic matter loads from the catchment area, and dredging of the navigation channel to maintain the connection between the North Sea and Germanys largest seaport in Hamburg.</p><p>Due to large phytoplankton blooms upstream of the port, the input of organic matter is high and provides high metabolic activity within and downstream the Hamburg port.</p><p>Here, we combined carbon, oxygen, and nitrogen data to elucidate their relationship and distribution along the Elbe Estuary. We used a box model approach to balance the budgets of dissolved inorganic carbon (DIC), oxygen (O₂), and nitrogen in form of nitrate (NO₃^-). To complete carbon and oxygen, we included atmospheric exchange of carbon dioxide (CO₂) and O₂.</p><p>DIC generation and O₂ consumption reveal the highest metabolic activity in the Hamburg port area, decreasing downstream. In contrast, NO₃^- budgets are stable along the estuary, indicating a strong decoupling of carbon and nitrogen in the Elbe Estuary. This decoupling can be explained by anaerobic processes such as denitrification in the port area, but it also implies lateral nitrogen sources further downstream.</p>