Intensive and extensive nitrogen loss from intertidal permeable sediments of the Wadden Sea

Nitrogen (N) loss rates were determined in permeable sediments of the Wadden Sea using a combination of stable N isotope incubation experiments and model simulation approaches during three seasons. Three different incubation methods that employed the isotope pairing technique were used: intact core incubations simulating either (1) diffusive or (2) advective transport conditions and (3) slurry incubations.

N loss rates from core incubations under simulated advective transport conditions exceeded those rates measured under diffusive transport conditions by 1–2 orders of magnitude, but were comparable to those observed in slurry incubations. N loss rates generally showed little seasonal and spatial variation (207 ± 30 μmol m−2 h−1) in autumn 2006 and spring and summer 2007.

Utilizing an extensive time series of nutrient concentrations and current velocities obtained from a continuous monitoring station, nitrate and nitrite (i.e., NO x − ) flux into the sediment was modeled over a full annual cycle.

Fluxes were sufficient to support the experimentally derived N loss rates. Combining the measured rates with the modeled results, an annual N removal rate of 745 ± 109 mmol N m−2 yr−1 was estimated for permeable sediments of the Wadden Sea. This rate agrees well with previous N loss estimates for the Wadden Sea based on N budget calculations.

Permeable sediments, accounting for 58–70% of the continental shelf area, are an important N sink and their contribution to the global N loss budget should be reevaluated.