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 (O2) 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 O2 uptake and coupled nitrification–denitrification, with anammox never accounting for more than 20 % of the total dinitrogen (N2) production. N cycling in sediments close to the STP was highly dependent on oxygen availability, which masked temperature-related effects. These sediments switched from low N loss and high ammonium (NH4 +) 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 (NO3 −) 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 N2 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.
Highlights
The Baltic Sea has a long history of eutrophication and water anoxia (Zillen et al 2008; Conley et al 2009)
The regional and spatial coverage of benthic flux data for the Baltic is low and most of the existing studies have not investigated the key regulators of benthic N cycling such as temperature, oxygen availability and demand, presence/absence of macrofauna, sedimentary organic matter content, or variable nutrient loading from human activities
This study presents a comprehensive set of seasonal measurements carried out in an estuary on the western border of the Baltic Sea, which has a 40 year-long record of anthropogenic organic matter and nutrient loading
Summary
The Baltic Sea has a long history of eutrophication and water anoxia (Zillen et al 2008; Conley et al 2009). It has been suggested that the reduction of nutrient levels alone may not be able to neutralize or reverse the impact of eutrophication (Duarte et al 2009) This points to the importance of internal dynamics for sustaining long-term eutrophication and emphasizes the need for temporally and spatially resolved benthic studies focusing on respiration and nutrient regeneration. Disappearance/decrease of the oxic sediment surface layer following anoxia/hypoxia makes the path of NO3- to the anoxic horizon thinner (Jensen et al 1994; Rysgaard et al 1994) This can either stimulate denitrification if NO3- is abundant in the bottom water, or inhibit nitrification, which is generally thought to be the major NO3- source within
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