Quantifying the impact of primary production and net ecosystem metabolism on carbon and nutrient cycling at the land-sea interface

Estuaries are typically net heterotrophic systems and a source of CO₂ to the atmosphere, while continental shelves tend to take up CO₂ from the atmosphere. Yet the primary production and net ecosystem metabolism (NEM) in these systems are variable, and this has implications for nutrient and carbon processing along the land-sea interface. To resolve this variability in a macrotidal system, high-frequency dissolved oxygen and ancillary biogeochemical data from a research station (equipped with a FerryBox) located at the outflow of a major temperate estuary into a shelf sea, were used to quantify the gross primary production (GPP) and NEM at the land-sea interface. Despite high nutrient concentrations in early and mid-spring in the outer Elbe River estuary in Germany, we find that low GPP rates (155 ± 46 mg C m^-2 d^-1 in April 2020 and 74 ± 24 mg C m^-2 d^-1 in March to April 2021) coincided with elevated turbidity (31 ± 9 NTU and 35 ± 7 NTU), suggesting light limitation, which was a function of turbidity and solar irradiance. Only when turbidity decreased (16 ± 5 NTU in 2020 and 19 ± 4 NTU in 2021), did we observe elevated GPP rates in late spring (May), and highest GPP rates in summer (July–August), with seasonal averages of 613 ± 89 mg C m^-2 d^-1 in 2020 and 558 ± 77 mg C m^-2 d^-1 in 2021. Primary production in the outer Elbe Estuary waters was most likely not nutrient-limited, since silicate, phosphate and nitrate concentrations exceeded the expected limiting levels of 5 µM Si, 0.5 µM PO₄^3- and 2 µM NO₃^-. Despite the high nutrient concentrations and estimated GPP rates, the system was in near trophic balance, with seasonal average NEM ranging between -2 ± 49 mg C m^-2 d^-1 and -149 ± 41 mg C m^-2 d^-1. The large errors resulted from (weekly to bi-weekly) fluctuations between net heterotrophic and net autotrophic state during the two-year observation period. A significant seasonal decrease in dissolved inorganic carbon (125 – 160 µmol kg^-1) from May to September, and in total alkalinity (TA) (116 – 128 µmol kg^-1) from December to August, was most likely driven by seasonal high primary production in the upper estuary and upstream regions some 142 km upstream of the outer estuary. This seasonal decrease opposes the previously documented seasonal increase in TA of up to 150 μmol kg^-1 in the coastal waters adjacent to the North Sea and to the intertidal flats of the Wadden Sea. This highlights the heterogeneity of carbonate system at the land-sea interface.