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Degradation of organic matter in estuarine sedimentsIn marine sediments, organic matter is mainly degraded through bacterial processes, by which N and P bound in organic compounds as a result of primary production are released once again. Up to half of the bacterial degradation taking place in the sediments proceeds through oxygen respiration (i.e. aerobically). The remaining degradation takes place anaerobically through respiration of nitrate, iron, manganese or sulphate (Jørgensen 1996). Besides the release of CO2, NH4+ and PO43-, anaerobic degradation results in the formation of waste products (such as hydrogen sulphide from sulphate respiration) that are ultimately oxidised, thereby consuming the exact amount of oxygen that would have been used if the entire degradation process had been aerobic. Therefore, a measurement of the amount of oxygen consumed within the sediment in darkness will correspond quite closely to the total metabolism of organic matter going on within the sediment, since this measurement is the sum of aerobic and anaerobic degradation. OXYGEN NITROGEN From January to May, estuarine sediments take up dissolved inorganic nitrogen (DIN=NH4++NO3-), primarily in the form of NO3- (Figure 2.29). During this period, the water-column concentration of NO3- is high. Therefore, the sediments NO3- uptake may be a result of a lower pore-water concentration of NO3-, perhaps governed by denitrification activity and a high NO3--assimilation by active benthic microalgae. Not until the summer months (June, July and August) does the estuarine sediment release nitrogen in the form of NH4+ (Figure 2.30). Low water-column NO3-concentrations and a high sediment oxygen demand resulting in less favourable oxygen conditions cause both nitrification and denitrification to decline markedly or stop altogether. At the same time, NO3- assimilation ceases. Thus, in the summer months, NH4+ constitutes the major part of the DIN flux from sediment to water column. The flux of nitrogen from the sediment to the water column continues into autumn; but gradually NO3- becomes dominating in the release of nitrogen (Figure 2.29 A and B). During autumn, the sediment oxygen demand decreases (Figure 2.28), and this causes the oxygen content of the sediment to increase. This means that nitrification gains more and more importance, and that more and more NH4+ is oxidised to NO3- (nitrified) within the sediment. As benthic microalgae only assimilate insignificant amounts of NO3- during autumn because of lowered activity (see Figure 2.29 A), NO3- accumulates within the sediment, even though some denitrification is taking place, and eventually diffuses from the sediment, since water-column NO3- concentrations, at least until November, are low. PHOSPHORUS Eq. 1 It is the pool of oxidised iron (Feox) that binds part of the phosphate found in estuarine sediment (Jensen & Thamdrup 1993). Good oxygen conditions within the sediment increases the Feox pool, which is able to retain either phosphate or hydrogen sulphide (through a reaction forming ferrous sulphide; FeS – see Eq. 2). On the other hand, poor oxygen conditions cause the pool of oxidised iron to grow gradually smaller. As the Feox concentration decreases, phosphate is released from the sediment to the water column. Eq. 2
INTERNAL NUTRIENT LOAD It is in the summer months that water-column concentrations of nitrogen and phosphorus are low. Thus, growth of algae within the estuaries depends on a steady supply of nutrients to the water column from land (streams etc.), atmosphere, adjacent seas or through degradation of organic matter in the water column or the sediment. In June, July and August 2000, the total nutrient supply from land constituted 10% of the annual supply. Thus, a relatively low external nutrient supply during summer means that release of nitrogen and phosphorus through organic matter degradation in the sediment (the internal supply) may stimulate primary production in the estuaries at this time of year. The internal and external supplies of nitrogen (NO3-+NH4+) and phosphorus (PO43-) to four estuaries representing four different types of area, Roskilde Fjord, Odense Fjord, Horsens Fjord and Ringkøbing Fjord, in the period June-August 2000 are shown in Table 2.8, assessed as the total supply to the entire estuary. It is evident that fluxes from the sediment of both nitrogen and phosphorus contributed significantly to the nutrient supply reaching the estuaries in the summer of 2000. In this period, between 36 and 93% of the supply of phosphorus to the estuaries came from the sediment, and in the case of nitrogen, the internal supply was almost as large (10-78%). Table 2.8
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Danish Environmental Protection Agency & National Environmental Research Institute • updated: |