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The effects of hypolimnetic aeration on oxygen penetration depth and oxygen flux into the sediment (DOU) were studied during two successive years. The study was conducted by comparing data from the aerated Enonselkä basin with the non-aerated Kajaanselkä basin of Lake Vesijärvi (southern Finland). Aeration increased the average O2 concentration in near-bottom layers. This was mainly due to shortened stratification period in the aerated basin, which was again attributed to the positive effect of aeration on hypolimnetic water temperature. Aeration did not prevent hypoxia during summer stratification and oxygen penetration depth in the sediment and DOU did not increase. During summer stratification DOU was even lower at the aerated than at the non-aerated basin. The weak effect of aeration on DOU and oxygen penetration was mostly due to the minor effect on hypolimnetic oxygen concentrations. The positive effect of aeration on hypolimnetic O2 concentration was negated by the positive effects of aeration on hypolimnetic temperature and turbulence that increase oxygen consumption in the water column. Due to the generally low effect on DOU, the present aeration effort in Enonselkä does not have positive effects on sediment quality. [ABSTRACT FROM AUTHOR]

Hypoxia has occurred intermittently over the Holocene in the Baltic Sea, but the recent expansion from less than 10 000 km before 1950 to >60 000 km since 2000 is mainly caused by enhanced nutrient inputs from land and atmosphere. With worsening hypoxia, the role of sediments changes from nitrogen removal to nitrogen release as ammonium. At present, denitrification in the water column and sediments is equally important. Phosphorus is currently buried in sediments mainly in organic form, with an additional contribution of reduced Fe-phosphate minerals in the deep anoxic basins. Upon the transition to oxic conditions, a significant proportion of the organic phosphorus will be remineralized, with the phosphorus then being bound to iron oxides. This iron-oxide bound phosphorus is readily released to the water column upon the onset of hypoxia again. Important ecosystems services carried out by the benthic fauna, including biogeochemical feedback-loops and biomass production, are also lost with hypoxia. The results provide quantitative knowledge of nutrient release and recycling processes under various environmental conditions in support of decision support tools underlying the Baltic Sea Action Plan. [ABSTRACT FROM AUTHOR]

Ecosystem functioning is threatened by an increasing number of anthropogenic stressors, creating a legacy of disturbance that undermines ecosystem resilience. However, few empirical studies have assessed to what extent an ecosystem can tolerate repeated disturbances and sustain its multiple functions. By inducing increasingly recurring hypoxic disturbances to a sedimentary ecosystem, we show that the majority of individual ecosystem functions experience gradual degradation patterns in response to repetitive pulse disturbances. The degradation in overall ecosystem functioning was, however, evident at an earlier stage than for single ecosystem functions and was induced after a short pulse of hypoxia (i.e., three days), which likely reduced ecosystem resistance to further hypoxic perturbations. The increasing number of repeated pulse disturbances gradually moved the system closer to a press response. In addition to the disturbance regime, the changes in benthic trait composition as well as habitat heterogeneity were important for explaining the variability in overall ecosystem functioning. Our results suggest that disturbance-induced responses across multiple ecosystem functions can serve as a warning signal for losses of the adaptive capacity of an ecosystem, and might at an early stage provide information to managers and policy makers when remediation efforts should be initiated. [ABSTRACT FROM AUTHOR]

Steep redoxclines form between oxic surface water and the stagnant, sulfidic hypolimnion in the eutrophied, brackish water Baltic Sea. Nitrification, denitrification, and anammox were measured at and below the redoxcline to quantify the role of water-column nitrogen processes in the overall magnitude of nitrogen removal in the Baltic Sea. Rates of nitrification were very high (up to 85 nmol N L-1 d-1) at the oxic-anoxic interface, but, surprisingly, nitrification was separated from the processes reducing nitrate to N2 (up to 810 nmol N L-1 d-1) by tens of meters in depth. N2 production was dominated by chemolithotrophic denitrification, with anammox playing only a negligible role, and limited to the water layers in which nitrite or nitrate coexisted with sulfide. The alternating oxygen concentrations in the basin induce irregular bursts of nitrogen removal. Nitrification takes place after mixing of ammonium-rich deep water with oxic water, and denitrification uses the formed nitrite and nitrate once anoxic conditions re-establish. Although removal rates can be high, conditions allowing such rates are likely short-lived. While the sedimentary denitrification rates at the shallower, oxic areas are lower, they are more constant in time, highlighting the need to avoid hypoxia, which would prevent sedimentary denitrification. [ABSTRACT FROM AUTHOR]