Dissolved organic nitrogen remineralization

Figure 10.5 Major processes involved in the biogeochemical cycling of N in estuaries and the coastal ocean (1) biological N2 fixation (2) ammonia assimilation (3) nitrification (4) assimilatory NC>3 reduction (5) ammonification or N remineralization (6) ammonium oxidation (speculative at this time) (7) denitrification and dissimilatory NO3 reduction to NH4+ and (8) assimilation of dissolved organic nitrogen (DON). (Modified from Libes, 1992.)...

The loss terms in N-cycle models that transform particulate and dissolved organic nitrogen into other forms can include a variety of processes (e.g., phytoplankton exudation, zooplankton grazing, sloppy feeding, phytoplankton and zooplankton mortality, bacterial remineralization, etc.). Different models may differ substantially in terms of which of these are included and their formulation (Christian and Anderson, 2002). Many N-cycle models now include significant phytoplankton exudation loss terms. This is often parameterized by simply specifying that some fixed fraction of the DIN uptake by phytoplankton is shunted directly to the DON pool (e.g., Anderson and Williams, 1998). Sloppy feeding by zooplankton can be similarly accounted for. Many models also include linear loss terms in the phytoplankton equation that represent either natural mortality or phytoplankton respiration (e.g.. Hood et ai, 2001). 

The impact of anthropogenic nutrient emissions in the coastal zone is heightened by its chemical speciation. Pollutant nitrogen and phosphorus are delivered to the coastal waters primarily in inorganic form, whereas most of the natural riverine dissolved nitrogen and phosphorus are components of organic compounds, i.e., DON and DOE Thus, the pollutant nutrients are delivered to the coastal waters in a chemical form that can be directly assimilated by coastal plankton, whereas the organically bound (natural) forms must first be remineralized. 

Figure 23.2 Conceptual model showing the possible fates of seagrass-hound nitrogen. Nitrogen temporarily immobilized in plant tissue can become available through exudation from live tissue, remineralization through decomposition, and grazing and subsequent excretion. Some material may be removed from the system through burial or export as dissolved or particulate organic matter.

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