Particulate organic matter controls benthic microbial N retention and N removal in contrasting estuaries of the Baltic Sea

Abstract

<p><strong>Abstract.</strong> Estuaries worldwide are known to act as <q>filters</q> of land-derived N loads, yet their variable environmental settings can affect microbial nitrogen (N) retention and removal and thus the coastal filter function. We investigated microbial N-retention (nitrification, ammonium assimilation) and N-removal (denitrification, anammox) in the aphotic benthic systems (here defined as: bottom boundary layer [BBL] and sediment) of two Baltic Sea estuaries that differ in riverine N loads, trophic state, bottom topography, and sediment type. Contrary to our expectations, nitrification rates (5–227 nmol L<sup>−1</sup> d<sup>−1</sup>) in the BBL neither differed between the eutrophied Vistula estuary and the oligotrophic Öre estuary, nor between seasons. Ammonium assimilation rates were slightly higher in the oligotrophic Öre estuary in spring but did not differ between estuaries in summer (9–704 nmol L<sup>−1</sup> d<sup>−1</sup>). In the sediment, no anammox was found in either estuary and denitrification rates were higher in the eutrophied (349 ± 117 µmol N m<sup>−2</sup> d<sup>−1</sup>) than in the oligotrophic estuary (138 ± 47 µmol N m<sup>−2</sup> d<sup>−1</sup>). Irrespective of their differences, in both estuaries the quality of the mainly phytoplankton-derived particulate organic matter (POM) – evaluated by means of C : N and POC : Chl.<i>a</i> ratios – seemed to control N-cycling processes through the availability of particulate organic N and C as substrate sources. Our data suggest, that in stratified estuaries, phytoplankton-derived POM is an essential link between riverine N loads and benthic N cycling and may function as a temporary N reservoir via long particle residence time or coastal parallel transport. Even at low process rates, effective coastal filtering would thus be achieved by the increased time available for the recycling of N via microbial retention processes until its permanent removal via denitrification.</p>

Enhanced benthic nitrous oxide and ammonium production after natural oxygenation of long‐term anoxic sediments

“…A as determined from 15 N isotope incubations (Fig. 5), in agreement with previous studies of permanently oxic sites in the Baltic Sea (Hellemann et al 2017, 2020; Bartl et al 2019). A small N 2 O peak was visible in the oxic part of the sediment in 2018 (Fig.…”

“…Since the bottom water at Sta. A was well‐oxygenated with low concentrations, a higher sedimentary OC content would likely have stimulated denitrification, as reported from other sites in the Baltic Sea (Jäntti et al 2011; Bartl et al 2019).…”

Biogeochemical functioning of the Baltic Sea

“…However, new results from the south-western Baltic Sea region show that both oxygen uptake and sulfate reduction rates are similar in sandy sediments compared to nearby muds (Lipka et al, 2018). Similarly, Bartl et al (2019) detected comparable rates of denitrification between permeable and non-permeable sediments offshore from the Vistula estuary. Extrapolation of location-specific process rate estimates to the basin-scale remains fraught with difficulty, but advances in high-resolution sediment typology (e.g.…”

“…Most likely, denitrification will be stimulated in lagoons and estuaries where the availability of labile OM is high, whereas denitrification will be reduced in archipelagos that receive low inputs of nutrients and OM. Thus, the resulting outcome of temperature increase on denitrification varies among coastal ecosystems and the seasonality in nutrient and organic-matter inputs and processing (Bartl et al, 2019). The productive period is prolonged with increasing temperatures, already indicated by the occurrence of earlier spring and later autumn phytoplankton blooms (Wasmund et al, 2019).…”

Factors regulating the coastal nutrient filter in the Baltic Sea

“…In the Ö re River estuary, denitrification was the largest in summer and fuelled by degradation of organic material from the spring bloom (Hellemann et al 2017), whereas in the Curonian lagoon denitrification was large immediately after the spring bloom but low during the summer cyanobacteria bloom (Zilius et al 2018). How organic matter production in surface waters fuels sediment removal process thus depends on a variety of site-specific interactions (Bartl et al 2019).…”