Spring-neap tidal cycles are major drivers for biogeochemical variability in two large outer estuaries

The land-ocean continuum is a key component of the biogeochemical cycling of carbon and nutrients. At the same time, the rapidly changing conditions at the river-sea interface make the nearshore regions highly dynamic and variable. For better understanding of the drivers of this variability, high temporal and spatial frequency observations are needed. Here, we use ship-of-opportunity measurements in the outer estuaries of the Humber (UK) and Elbe (Germany) rivers to show that essential ocean variables in these regions vary with a periodicity that matches the spring-neap tidal cycles. The tidal disturbance was observed up to 10 km offshore from the river mouth. While at neap tide, the conditions at this location were typical of the open North Sea, during spring tide, the salinity dropped below 15, the turbidity was high (>50 FTU), dissolved oxygen dropped to under-saturated values and the partial pressure of carbon dioxide (pCO₂) in seawater rose above 700 µatm. Since neap tide seawater pCO₂ values were below 400 µatm, lower than atmospheric levels, the near-shore area shifts between being a sink and a source of carbon dioxide to the atmosphere on roughly one-week timescales. This behaviour is currently not resolved in some biogeochemical models. Correctly integrating the drivers of this variability into regional assessments is essential for the accurate evaluation of shelf sea carbon budgets.