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Marine Ecology Progress Series

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MEPS 637:15-28 (2020)  -  DOI:

Seasonal cycle of benthic denitrification and DNRA in the aphotic coastal zone, northern Baltic Sea

D. Hellemann1,2,*, P. Tallberg1,2, S. L. Aalto3,4, M. Bartoli5, S. Hietanen1,2

1Ecosystems and Environment Research Program, Faculty of Biological and Environmental Sciences, University of Helsinki, 00014 Helsinki, Finland
2Tvärminne Zoological Station, University of Helsinki, 10900 Hanko, Finland
3Department of Biological and Environmental Science, University of Jyväskylä, 40014 Jyväskylä, Finland
4Department of Environmental and Biological Sciences, University of Eastern Finland, 70211 Kuopio, Finland
5Department of Chemistry, Life Sciences and Environmental Sustainability, University of Parma, 43124 Parma, Italy
*Corresponding author:

ABSTRACT: Current knowledge on the seasonality of benthic nitrate reduction pathways in the aphotic, density stratified coastal zone of the Baltic Sea is largely based on data from muddy sediments, neglecting the potential contribution of sandy sediments. To gain a more comprehensive understanding of seasonality in this part of the Baltic Sea coast, we measured rates of benthic denitrification, anammox and dissimilatory nitrate reduction to ammonium (DNRA) monthly in the ice-free period of 2016 in both sandy and muddy aphotic sediments, northwestern Gulf of Finland. No anammox was observed. The seasonal cycle of denitrification in both sediment types was related to the hydrography-driven development of bottom water temperature. The seasonal cycle of DNRA was less clear and likely connected to a combination of bottom water temperature, carbon to nitrogen ratio, and substrate competition with denitrification. Denitrification and DNRA rates were 50-80 and 20% lower in the sandy than in the muddy sediment. The share of DNRA in total nitrate reduction, however, was higher in the sandy than in the muddy sediment, being (by ~50%) the highest DNRA share in sandy sediments so far measured. Our data add to the small pool of published studies showing significant DNRA in both cold and/or sandy sediments and suggest that DNRA is currently underestimated in the Baltic coastal nitrogen filter. Our results furthermore emphasize that the various environmental conditions of a coastal habitat (light regime, hydrography, and geomorphology) affect biogeochemical element cycling and thus need to be considered in data interpretation.

KEY WORDS: Nitrate reduction · Sandy sediment · Coastal filter · Benthic-pelagic coupling · Water column density stratification · Organic matter · Geomorphology · Northern Gulf of Finland

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Cite this article as: Hellemann D, Tallberg P, Aalto SL, Bartoli M, Hietanen S (2020) Seasonal cycle of benthic denitrification and DNRA in the aphotic coastal zone, northern Baltic Sea. Mar Ecol Prog Ser 637:15-28.

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