Seasonal oxygen, nitrogen and phosphorus benthic cycling along an impacted Baltic Sea estuary: regulation and spatial patterns

Abstract

The regulatory roles of temperature, eutrophication and oxygen availability on benthic nitrogen (N) cycling and the stoichiometry of regenerated nitrogen and phosphorus (P) were explored along a Baltic Sea estuary affected by treated sewage discharge. Rates of sediment denitrification, anammox, dissimilatory nitrate reduction to ammonium (DNRA), nutrient exchange, oxygen (O 2 ) uptake and penetration were measured seasonally. Sediments not affected by the nutrient plume released by the sewage treatment plant (STP) showed a strong seasonality in rates of O 2 uptake and coupled nitrification-denitrification, with anammox never accounting for more than 20 % of the total dinitrogen (N 2 ) production. N cycling in sediments close to the STP was highly dependent on oxygen availability, which masked temperaturerelated effects. These sediments switched from low N loss and high ammonium (NH 4 ? ) efflux under hypoxic conditions in the fall, to a major N loss system in the winter when the sediment surface was oxidized. In the fall DNRA outcompeted denitrification as the main nitrate (NO 3 -) reduction pathway, resulting in N recycling and potential spreading of eutrophication. A comparison with historical records of nutrient discharge and denitrification indicated that the total N loss in the estuary has been tightly coupled to the total amount of nutrient discharge from the STP. Changes in dissolved inorganic nitrogen (DIN) released from the STP agreed well with variations in sedimentary N 2 removal. This indicates that denitrification and anammox efficiently counterbalance N loading in the estuary across the range of historical and present-day anthropogenic nutrient discharge. Overall low N/P ratios of the regenerated nutrient fluxes impose strong N limitation for the pelagic system and generate a high potential for nuisance cyanobacterial blooms.

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

“…Estuarine sediments are often efficient in removing riverine NO 3 − via denitrification (Seitzinger 1988, Nixon et al 1996, Trimmer et al 1998, Bonaglia et al 2014 due to the abundant river discharge of the substrates NO 3 − and OC. At the same time, high N loading can increase the production of nitrous oxide (N 2 O), a potent greenhouse gas (Seitzinger & Nixon 1985, Middelburg et al 1995.…”

“…The application of measurement methods that are based on the diffusion of a tracer along vertical redox zones, such as the isotope pairing technique (IPT, Nielsen 1992), has led to a high number of denitrification measurements in fine-grained, cohesive, muddy sediments (e.g. Rysgaard et al 1993, Dong et al 2000, Hietanen & Kuparinen 2008, Jäntti et al 2011, Bonaglia et al 2014. Measurements in sandy sediments are rare, although ~50% of the inner continental shelf is covered by sand (Hall 2002).…”

Nutrient fluxes from reduced Baltic Sea sediment: effects of oxygenation and macrobenthos

“…Particulate nutrients deposited in costal sediments may either recycle back into the water column or become permanently retained or transformed in the seabed. Their fate depends on several physicochemical and biological factors, particularly the oxygen gas (O 2 ) concentration in the bottom water (Kemp et al 2005, Viktorsson et al 2012, Bonaglia et al 2014). …”

“…For example, conversion of bioavailable nitrogen (N) into gaseous forms via canonical (heterotrophic) denitrification and anaerobic ammonium oxidation (anammox) are major sink mechanisms in the nitrogen cycle (Galloway et al 2004, Dalsgaard et al 2005. However, in organic-rich sediments overlain by hypoxic−anoxic but nitratecontaining bottom water, these processes are often replaced by dis similatory nitrate−nitrite reduction to ammonium (DNRA), in which bioavailable ammo-nium (NH 4 ), rather than gaseous forms of N, is the metabolic end product (McCarthy et al 2008, Thamdrup & Dalsgaard 2008 and references therein, Jäntti & Hietanen 2012, Bonaglia et al 2014). Furthermore, oxidation of NH 4 to NO 3 via nitrification does not take place in anoxic systems (Thamdrup & Dalsgaard 2000).…”

“…Furthermore, oxidation of NH 4 to NO 3 via nitrification does not take place in anoxic systems (Thamdrup & Dalsgaard 2000). Thus, bottom water oxygen depletion often increases net N recycling rates to the water column by stimulation of benthic NH 4 fluxes (Hall et al 1992, Joye & Anderson 2008, McCarthy et al 2008, Bonaglia et al 2014, and oxygenation of an oxic bottom water has been demonstrated to reduce the benthic dissolved inorganic N (DIN) release in a marine fjord by roughly half (De Brabandere et al 2015).…”