Nutrient Dynamics in European Water Systems
Synthesis Results

3. Fate of nutrients in coastal areas (2 of 4)

High nitrogen concentrations in the water column, high organic loading, abundant bottom fauna and the rapid exchange of oxic water contributed to very high denitrification rates (avg. 140 kg N ha^-1 yr^-1) in the Danish Randers fjord (Nielsen et al., 2001). In this case denitrification rates could not be explained solely by NO₃^- concentrations in the water column and O₂ uptake by the sediment as suggested from other studies. The authors ascribed this to a system specific matrix of interactions between the denitrification process , algae and animal activity.

Until recently, denitrification was recognised as the only important process removing nitrogen from the fixed pool in natural environments. Recently however, it was discovered that ammonium is oxidized anaerobically in sediments in the presence of nitrate and that this alternative pathway contributes significantly to benthic N₂ production (Dalsgaard & Thamdrup, 2002 and Thamdrup & Dalsgaard, 2002).

Incubations with ₁₅N-labeled nitrate or ammonium demonstrated that during this process N₂ is formed through one-to-one pairing of nitrogen from nitrate and ammonium, which clearly separates the process from denitrification. Nitrite which accumulated transiently, was the likely oxidant for ammonium and the process is thus similar to the ANNAMOX process known from wastewater bireactors (NH₄⁺ + NO₂^- => N₂ + 2H₂O).

Newly evidenced processes (DNRA, ANNAMOX) from the NICE project are alternative pathways for denitrification (Welsh et al., 2001 & Dalsgaard & Thamdrup, 2002)as shown in Figure 3.1(c). Welsh et al., (2001) documented the DNRA from a seagrass meadow and stressed the importance of the process as a possible source of N₂O to the atmosphere. This process was also measured by Christensen et al. (2000) (NICE project) under trout cages in a Danish fjord. The authors show that significant DNRA only occurs under the heavy organic loading of the sediment, occurring right underneath the cages. Little is known about the factors determining the occurrence and rate of the DNRA process. This point needs further study.