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

Abstract

Abstract. Estuaries worldwide act as “filters” of land-derived nitrogen (N) loads, yet differences in coastal environmental settings can affect the N filter function. We investigated microbial N retention (nitrification, ammonium assimilation) and N removal (denitrification, anammox) processes in the aphotic benthic system (bottom boundary layer (BBL) and sediment) of two Baltic Sea estuaries differing in riverine N loads, trophic state, geomorphology, and sediment type. In the BBL, rates of nitrification (5–227 nmol N L−1 d−1) and ammonium assimilation (9–704 nmol N L−1 d−1) were not enhanced in the eutrophied Vistula Estuary compared to the oligotrophic Öre Estuary. No anammox was detected in the sediment of either estuary, while denitrification rates were twice as high in the eutrophied (352±123 µmol N m−2 d−1) as in the oligotrophic estuary. Particulate organic matter (POM) was mainly of phytoplankton origin in the benthic systems of both estuaries. It seemed to control heterotrophic denitrification and ammonium assimilation as well as autotrophic nitrification by functioning as a substrate source of N and organic carbon. Our data suggest that in stratified estuaries, POM is an essential link between riverine N loads and benthic N turnover and may furthermore function as a temporary N reservoir. During long particle residence times or alongshore transport pathways, increased time is available for the recycling of N until its eventual removal, allowing effective coastal filtering even at low process rates. Understanding the key controls and microbial N processes in the coastal N filter therefore requires to also consider the effects of geomorphological and hydrological features.

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

“…In the photic, mixed zone, benthic denitrification rates often peak in early spring, when NO 3 − concentrations are high, bottom water temperatures quickly increase, and microbial competition with microphytobenthos for NO 3 − and ammonium (NH 4 + ) is still low (Sørensen 1984, Jørgensen & Sørensen 1985, Nielsen et al 1995, Rysgaard et al 1995. In contrast, 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).…”

“…In contrast, 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). Peak denitrification rates in early autumn seem to result from high availability of N and labile organic carbon from mineralized phytoplankton, and the annually highest bottom water temperatures at the breakdown of water column stratification (Hietanen & Kuparinen 2008, Jäntti et al 2011, Bonaglia et al 2014.…”

Lagrangian Residence Time in the Bay of Gdańsk, Baltic Sea

“…2016), determined very high ratios, up to 7.8 times higher than the Redfield ratio of 16 (Figure 3). Further data obtained during the cruise are reported in Bartl et al (2018Bartl et al ( , 2019. The high and unbalanced Redfield ratios in the coastal zone may alter nitrate cycling (Lunau et al, 2012).…”

“…The longer surface-water LRT in the near-coastal area may be conducive to the sedimentation of organic matter deriving from primary production on the near-coastal seafloor. This would fuel benthic nutrient turnover, especially nutrient retention via fluxes (Thoms et al, 2018) and nitrification (Bartl et al, 2019), in turn facilitating permanent nutrient-removal processes such as denitrification and burial. Thus, the surface-water LRT may have an indirect impact on the permanent removal of nutrients from this coastal system, which functions on time scales much longer than the mean water residence time (Asmala et al, 2017).…”

“…The high and unbalanced Redfield ratios in the coastal zone may alter nitrate cycling (Lunau et al, 2012). To understand the efficiency of nitrate removal, nutrient retention capacity and the complexity of the coastal filter function in the BG (Thoms et al, 2018;Bartl et al, 2019) requires knowledge of the water residence time. Based on the LOICZ (Land Ocean Interactions in the Coastal Zone) model of Gordon et al (1996); Witek et al (2003) computed a flushing time of 15 days in the BG, which they named "water exchange time" or "water residence time".…”