Short term effects of hypoxia and anoxia on sediment biogeochemistry
Hypoxia and anoxia alter the behaviour, diversity and habitat of the benthic communities and cause accumulation of organic matter and nutrients. Most studies have focused on the effect of anoxia on benthic organisms and little information is available on nutrient sediment-water fluxes and rates of key biogeochemical processes. This study aimed to evaluate the oxygen concentration at which harmful effects start to occur, the rates that the system responds to different levels of oxygen saturation in the overlying water and the recovery times of system processes when re-aeration are restored. Sampling was carried out at Breydon Water, Great Yarmouth in June, 2010 and key biogeochemical processes were monitored in sediment/artificial seawater mesocosms maintained at constant temperature in the laboratory. The experimental work assessed the short term effect of different levels of oxygen by manipulating oxygen into five different treatments; oxic (96% saturation in the overlying water), hypoxia (25% saturation), one day of anoxia (24 hours without oxygen followed by re-aeration of up to 6 days), four days of anoxia (4 days without oxygen and subsequent re-aeration for 3 days) and seven days of anoxia (0% saturation). The effect of these oxygen concentrations on different biogeochemical processes (solute transport, oxygen uptake, nutrient fluxes, porewater chemistry, and enzyme activities) was quantified. Bromide used in solute transport assessments were analysed using inductively coupled plasma mass spectrometry and oxygen was measured using an oxygen minielectrode. Nutrient analysis was carried out using the SKALAR autoanalyser and enzyme activities were measured using microplate based assays. With decreasing oxygen supply, ammonium and phosphate are rapidly released into the water column, and nitrate concentrations decrease, reflecting the reducing conditions. In oxic conditions, rapid transport of water and oxygen into the sediments occurs through irrigation fluxes. Slower transport was observed from the porewater of anoxic treatments, as only molecular diffusion occurs. Vertical profiles of alkaline phosphatase, cellobiohydrolase, β-glucosidase and chitinase activities in the sediment reflect organic matter inputs decreasing with depth. Urease activity was enhanced following anoxic incubations of 7 days, with an inverse relationship with porewater ammonium concentrations and increased total organic carbon input. The biogeochemical resilience of the system is dependent on the duration of anoxic events. The effect of 1 day anoxia is relatively reversible and the system recovers to normal conditions within the 7 days of treatment. However, hypoxia (25% oxygen saturation) or longer duration (4 days) anoxia followed by re-aeration are enough to cause negative effects on the biogeochemical functioning of the system. So, sediment biogeochemistry has some resilience towards short term anoxic events, but more prolonged hypoxia or anoxic events that are continued for only a few days can have major effects on ecosystem function.
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