Rate, hydrographic and related environmental data for: Nitrogen retention-removal balance in illuminated coastal sandy sediments is governed by microphytobenthos and physical forcing (Version 1.0)

Gentsch, Kaja; Ahmerkamp, Soeren; Holtermann, Peter; Marzocchi, Ugo; Liskow, Iris; Selle, Torge; Voss, Maren

Abstract. Permeable sands are known to support substantial nitrogen (N) transformations. It is however, unclear to what extend physical forcing and seasonality influence oxygen dynamics, N transformations and subsequently the coastal N-filter. In this study we sampled monthly in an annual cycle at a sandy, illuminated shallow station in the southern Baltic Sea open coast. We measured denitrification and DNRA rates with an adapted revised isotope pairing technique to simulate advective pore water flow during the incubations. Benthic primary production and N-assimilations were estimated based on in situ oxygen profiles. Denitrification rates followed an inverse seasonal cycle with rates up to 228.5 µmol m-2 d-1 in winter and low to non-detectable rates in summer, while DNRA rates showed no seasonality with an annual mean rate of 26.6 ± 48.8 µmol m-2 d-1. To explore the mechanisms underlying these patterns, we integrated the observations into a coupled two-dimensional reaction–transport model that simulates the effects of microphytobenthos (MPB) activity and bottom-water flow on N-cycling in permeable sediments. Together with the in situ measurements, the model reveals a dynamic interplay between MPB activity and hydrodynamic forcing that regulates the partitioning between denitrification and DNRA. This interaction drives a seasonal shift from a N-retention dominated system in summer to a N-removal dominated system in winter. Together, our results show that coastal N-cycling, and specifically the partitioning between DNRA and denitrification, emerges from the dynamic coupling of physical transport and microbial processes.

Scientific purpose: to be published along with publication as demanded by journal.

Keywords: Nitrogen, denitrification, DNRA, southern Baltic Sea, annual cycle

Citation

Gentsch, K., Ahmerkamp, S., Holtermann, P., Marzocchi, U., Liskow, I., Selle, T., Voss, M. (2026). Rate, hydrographic and related environmental data for: Nitrogen retention-removal balance in illuminated coastal sandy sediments is governed by microphytobenthos and physical forcing. Leibniz Institute for Baltic Sea Research Warnemünde. doi:10.12754/data-2026-0003

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