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

Abstract

Effects of bottom water oxygenation and macrofaunal colonisation on benthic fluxes of nitrogen (N), phosphorus (P) and silicon (Si) from long-term anoxic Baltic Sea bottom sediment were investigated. Sediment boxcosms from an anoxic site at 150 m depth in the open Baltic proper were incubated in the laboratory to follow the development of benthic nutrient fluxes during 74 d exposure to flow-through of oxygen-rich water. In contrast to traditional end-point experimental designs, our repeated measurement approach allowed for separation of transient and long-term effects of oxygenation and bioturbation on benthic nutrient recycling. The composition, but not the rate, of the benthic total dissolved N efflux changed by oxygenation from being dominated by NH 4 in situ to being mostly composed of NO 2 + NO 3 and dissolved organic N (DON) under oxic conditions. Oxygenation in the boxcosms decreased the benthic efflux of dissolved silicate (DSi) and essentially shut off the in situ flux of dissolved inorganic phosphorus (DIP). After 20 d of oxygenation, 2 bottom macrofauna taxa, the polychaete Marenzelleria spp. and the amphipod Monoporiea affinis, were introduced to a subset of the boxcosms. Bioturbation by either taxa increased the efflux of dissolved inorganic N (DIN), DON and DSi to the overlying water. The Prich benthic flux under in situ anoxic conditions roughly approached Redfield N:P stoichiometry after oxygenation in the sediment boxcosms. Upon addition of macrofauna, bioturbation generated even higher N:P flux ratios.

Influence of Natural Oxygenation of Baltic Proper Deep Water on Benthic Recycling and Removal of Phosphorus, Nitrogen, Silicon and Carbon

“…The composition and elemental ratios of benthic solute fluxes under anoxic conditions thus change upon oxygenation as suggested by several studies. Ekeroth et al (2016a) found a very low DIN:DIP molar ratio (2.8) of the benthic flux measured under anoxia, but when these reduced Baltic proper sediments were experimentally oxygenated, the DIN:DIP ratio of the flux increased to about 12, which was due to a retention of DIP in the oxygenated azoic sediment rather than to a stimulation of the DIN flux. Viktorsson et al (2012Viktorsson et al ( , 2013a reported average DIC:DIP molar ratios of anoxic benthic fluxes of about 30 in the Gulf of Finland, and about 70 in the EGB and in west-Swedish fjords.…”

“…Oxygenation of surficial sediments effects transformation, retention and removal of various elements differently (e.g., Sundby et al, 1986;Ekeroth et al, 2016a). The composition and elemental ratios of benthic solute fluxes under anoxic conditions thus change upon oxygenation as suggested by several studies.…”

“…The influence of oxygenation of long-term anoxic benthic systems on biogeochemical processes in sediments and solute exchange at the sediment-water interface is a function of duration of oxygenation and the actual bottom water oxygen concentration level (e.g., De Brabandere et al, 2015; Ekeroth et al, 2016a). The present study investigated the situation in the EGB in early July 2015, i.e., when the EGB had been oxygenated for 3-4 months by the 2014-2015 MBI.…”

“…In addition, it was recently shown that the rate of sedimentary deposition and degradation of organic matter controls the benthic P flux under long term anoxic conditions in Baltic Sea basins and in westSwedish fjords (Viktorsson et al, 2012(Viktorsson et al, , 2013a. Furthermore, the benthic solute flux under anoxic conditions in these basins was found to be very P rich in relation to both C and N. Hence, oxygen does not only control the magnitude of the dissolved bioavailable N, P (e.g., Bonaglia et al, 2013) and sometimes even Si flux (Ekeroth et al, 2016a), but may also have a strong influence on the N:P, C:P, and Si:P ratios in benthic fluxes, and thus indirectly on abundance and composition of phytoplankton and cyanobacteria communities in surface waters.…”

“…Mineralization of organic matter containing N, P, and C, and dissolution of biogenic Si, in sediment produces nutrients and DIC, which are dissolved in the sediment pore water and then released to the water column as a benthic flux. Degradation of organic matter in sediments also produces dissolved organic N (DON), P (DOP), and C (DOC), which may be released to the water column and contribute to the benthic flux Ekeroth et al, 2012Ekeroth et al, , 2016a. A fraction (in some sediments significant) of the particulate organic matter containing N, P, and C, and of biogenic Si, which is deposited from overlying waters on the sea-floor, is not mineralized and recycled, but will undergo burial in the sediment (reviewed by Burdige, 2007).…”