Eutrophication is the process of nutrient enrichment in aquatic ecosystems such that productivity is no longer limited by nutrient availability. Typically, nitrogen or phosphorus is limiting, and their availability can be increased by human activities, e.g. via sewage deposition or fertilizer drainage.

Bioavailable nitrogen and phosphorus are both released during the breakdown of organic matter, but their ultimate fate depends on the environmental conditions present.

Eutrophication can have several undesirable consequences, including depletion of O2 in the water body, nuisance or harmful algal blooms, and decline in submerted aquatic vegetation.

Mitigation processes can include immobilization of dissolved species, or conversion into a chemical form that is no longer bioavailable. For phosphorus, this includes mainly sorption on oxide phases, or the formation of P-containing solid phases. Nitrogen, in contrast, exhibits a complex redox chemistry. Hence, nitrogen removal is not only due to sorption of ammonium, but also its transformation into a gaseous product (N2O and N2). This is achieved by denitrification, the conversion of nitrate into nitrogen gas, a process performed by facultative anaerobic microorganisms.

When reactive organic matter is introduced into the environment, a plume with different redox zones develops. Oxygen utilization decreases as distance from the organic matter source increases; near the source all O2 has been used up, while further away, the added organic matter has not consumed all of it.

Schematic of redox zonation in a plume of high dissolved organic matter concentrations. Red denotes the anoxic region, yellow the suboxic region and green the extent of the plume with O2 present.

The central role of organic matter, whose breakdown fuels subsurface biogeochemistry, is illustrated in the following figure. Around this energy source, the cycling of substrates depends strongly on redox conditions, in particular the presence or absence of O2.

Schematic of the reaction network. Primary reactions involved in the breakdown of organic matter are denoted with circles, and include 1 - aerobic respiration, 2 - denitrification , 3 - dissimilatory nitrate reduction to ammonium, 4 - manganese oxide reduction, 5 - iron oxide reduction, 6 - sulfate reduction, 7 - methanogenesis. Secondary reactions, where metabolites are oxidized are denoted with squares: 1 - manganese oxidation, 2 - iron oxidation, 3 - nitrification, 4 - sulfide oxidation, 5 - FeS oxidation, 6 - aerobic methane oxidation, 7 - iron oxidation with manganese oxides, 8&9 - sulfide oxidation with manganese and iron oxids, respectively and 10 - anaerobic methane oxidation. Mineral precipitation/dissolution are labeled with hexagons: 1 - MnCO3, 2 - FeCO3, 3 - FeS, 4 - CaCO3 and 5 - CaSO4 (Figure by . W. Porubsky).

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