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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.
Highlights
Coastal ecosystems commonly exhibit a tight benthic–pelagic coupling due to their shallow water depth (Middelburg & Soetaert 2004), which connects the elemental cycles of sediment and water column, linking benthic organic matter mineralization to pelagic organic matter production
The seasonal pattern of bottom water temperature, dissolved oxygen, and NO2− + NO3− concentrations was related to the reduced water mixing during density stratification (Fig. 3)
dissimilatory nitrate reduction to ammonium (DNRA) rates could not be calculated in April, May, measurable denitrification rates in June (Figs. 3 & 5); they remained low throughout the summer and and November due to the insignificant 29,30N2 production in these months and the subhence could not strongly support denitrification sequent lack of 29/30N2 ratio needed for the calculations
Summary
Coastal ecosystems commonly exhibit a tight benthic–pelagic coupling due to their shallow water depth (Middelburg & Soetaert 2004), which connects the elemental cycles of sediment and water column, linking benthic organic matter mineralization to pelagic organic matter production. In the deeper, aphotic, stratified zone, denitrification rates are often limited in spring and first peak in early autumn (Hietanen & Kuparinen 2008, Jäntti et al 2011, Bonaglia et al 2014, Bartl et al 2019) This limitation has been associated with seasonally low availability of labile organic carbon (Hietanen & Kuparinen 2008, Jäntti et al 2011, Bartl et al 2019); as the main share of landderived particulate and dissolved organic matter (POM, DOM) often remains in shallower areas, deeper areas are largely dependent on the supply of POM from pelagic autochthonous primary production, which starts in spring and, contrary to DOM, can sink across stratified water layers (Bartl et al 2019). Roughly 30% of the Baltic coastal sediments consist of sand and are located to a large part in the aphotic stratified zone (Al-Hamdani & Reker 2007), little is known about the seasonal pattern of denitrification there
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