Denitrification in an oligotrophic estuary: a delayed sink for riverine nitrate

Abstract

Estuaries are often seen as natural filters of riverine nitrate, but knowledge of this nitrogen sink in oligotrophic systems is limited. We measured spring and summer dinitrogen production (denitrification, anammox) in muddy and non-permeable sandy sediments of an oligotrophic estuary in the northern Baltic Sea, to estimate its function in mitigating the riverine nitrate load. Both sediment types had similar denitrification rates, and no anammox was detected. In spring at high nitrate loading, denitrification was limited by likely low availability of labile organic carbon. In summer, the average denitrification rate was ~138 μmol N m. The corresponding estuarine nitrogen removal for August was ~1.2 t, of which ~93% was removed by coupled nitrification−denitrification. Particulate matter in the estuary was mainly phytoplankton derived (> 70% in surface waters) and likely based on the riverine nitrate which was not removed by direct denitrification due to water column stratification. Subsequently settling particles served as a link between the otherwise uncoupled nitrate in surface waters and benthic nitrogen removal. We suggest that the riverine nitrate brought into the oligotrophic estuary during the spring flood is gradually, and with a time delay, removed by benthic denitrification after being temporarily 'trapped' in phytoplankton particulate matter. The oligotrophic system is not likely to face eutrophication from increasing nitrogen loading due to phosphorus limitation. In response, coupled nitrification−denitrification rates are likely to stay constant, which might increase the future export of nitrate to the open sea and decrease the estuary's function as a nitrogen sink relative to the load.

Microbial community composition and functional potential in Bothnian Sea sediments is linked to Fe and S dynamics and the quality of organic matter

“…N in the ecosystem while anaerobic ammonium oxidation (anammox) was not detectable (Hellemann et al 2017). These activities are corroborated by the genetic potential of dominant taxa identified in this study.…”

Large variations in iron input to an oligotrophic Baltic Sea estuary: impact on sedimentary phosphorus burial

“…In this region, the spring flood (April, May) is the major annual hydrological event. The spring flood results in a brief period of enhanced water flow in streams and rivers (Hölemann et al, 2005;Björkvald et al, 2008) and export of Fe and terrestrial organic matter to the coastal zone (Rember and Trefry, 2004;Algesten et al, 2006). During high flow, total and dissolved Fe and Mn in streams can increase by a factor of up to 10, with concentrations being highly variable between years (Björkvald et al, 2008).…”

“…1b), with water depths varying from 10 to 33 m (Table 1). Sediments at all sites are fine-grained and rich in organic matter (Hellemann et al, 2017; Table 1). 63.54° N 63.52° N 63.5° N 63.48° N 63.46° N 19.7° E 19 75°E 19 8°E 19 85°E . .…”

Application of the isotope pairing technique in sediments: Use, challenges, and new directions

“…Using diffusive methods (here IPT) in an advective environment may lead to 15 NO 3 − not reaching the deep zone of NO 3 − reduction in the appropriate amount and within a reasonable timeframe, which is further complicated by the presence of three-dimensional redox zones (Huettel et al 1998). However, sandy sediments with insufficient permeability to facilitate pore water flow, e.g., at locations exposed only to very slow bottom water currents may not experience advective flow (Forster et al 2003) and the application of the conventional diffusive whole-core IPT is possible (Hellemann et al 2017). In general NO 3 − reduction process rates from permeable sands are relatively scarce compared to those from cohesive muddy sediments but several studies have made efforts to overcome specific issues related to the application of the IPT on permeable sediments.…”