Nitrogen dynamics in wave-dominated deltas

Block diagram of nitrogen dynamics in wave-dominated deltas

Point and non-point N sources Point and non-point N sources Point and non-point N sources Point and non-point N sources Point and non-point N sources Total nitrogen (TN) catchment input Floodplain deposition and uptake of N Deposition and burial in saltmarshes Downstream displacement of N Downstream displacement and scouring Uptake of dissolved N by seagrasses Large export of N Minor import of N Uptake by marine phytoplankton

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Nitrogen dynamic processes in wave-dominated deltas

1. Total nitrogen catchment input

Nitrogen, both particulate and dissolved or total nitrogen, enters the deltaic channels from point- and non-point sources from within the catchment. River flow and nutrient input varies regionally, depending on local catchment and climatic conditions. Typically, a high proportion of catchment-derived nutrient material is transported into estuaries and deltas (Harris, 2001).

2. Deposition of particulate nitrogen

Limited deposition of particulate nitrogen occurs upon the floodplain during high flow events. This process is enhanced by the effects of 'baffling' by floodplain vegetation. Biological uptake of DIN also occurs on the flanks of the river channel.

3. Processing of nitrogen by phytoplankton

Limited deposition and burial of particulate nitrogen occurs in flanking environments, due to the baffling effect of saltmarsh and/or mangrove vegetation, depending on latitude. Burial and resuspension of particulate nitrogen and dissolved inorganic nitrogen can also occur within intertidal flats. Flanking environments typically play a much smaller role in deltas in comparison to estuaries.

4. Downstream displacement of nitrogen

The majority of the river-borne total nitrogen is transported through the delta by strong river flow or downstream displacement (Keil et al., 1997), especially during flood events (Eyre, 2000). Deposition typically does not occur in the channels due to scouring.

5. Large export of nitrogen

A large proportion of the total nitrogen load is exported through the mouth of the delta into the marine environment (Eyre, 2000, Nixon et al., 1996). Uptake of dissolved inorganic nitrogen by seagrasses may occur in the mouth of the delta. The nitrogen exported into the ocean is typically assimilated by marine phytoplankton.

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