Spring-neap tidal cycles modulate the strength of the carbon source at the estuary-coast interface

Abstract. Estuaries are dynamic environments with large biogeochemical variability modulated by tides, linking land to the coastal ocean. The carbon cycle at this land-sea interface can be better constrained by increasing the frequency of observations and by identifying the influence of tides with respect to the spring-neap variability. Here we use FerryBox measurements from a Ship-of-Opportunity travelling between two large temperate estuaries in the North Sea and find that the spring-neap tidal cycle drives a large percentage of the biogeochemical variability, in particular in inorganic and organic carbon concentrations at the land-sea interface in the outer estuaries and the adjacent coastal region. Of particular importance to carbon budgeting is the up to 74 % increase (up to 43.0 ± 17.1 mmol C m^-2 day^-1) in the strength of the estuarine carbon source to the atmosphere estimated during spring tide in a macrotidal estuary. We describe the biogeochemical processes occurring during both spring and neap tidal stages, their net effect on the partial pressure of carbon dioxide in seawater, and the ratios of dissolved inorganic to dissolved organic carbon concentrations. Surprisingly, while the two example outer estuaries in this study differ in the timing of the variability, the metabolic state progression and the observed phytoplankton species distribution, an increase in the strength of the potential carbon source to the atmosphere occurs at both outer estuaries on roughly 14-day cycles, suggesting that this is an underlying characteristic essential for the correct estimation of carbon budgets in tidally-driven estuaries and the nearby coastal regions. Understanding the functioning of estuarine systems and quantifying their effect on coastal seas should improve our current biogeochemical models and therefore future carbon exchange and budget predictability.