Nitrogen dynamic processes in embayments and drowned river valleys
1. Catchment nitrogen sources
Nitrogen, both particulate and dissolved or total nitrogen, enters the embayment system from point- and non-point sources from within the catchment. River flow and nutrient input varies regionally, depending on local catchment and climatic conditions (Harris, 2001).
2. Processing of nitrogen by phytoplankton
The catchment-derived dissolved inorganic nitrogen is transported into the embayment, where it is rapidly processed and assimilated by phytoplankton and benthic micro-algae, if temperature and light levels are suitable (Elosegui et al., 1987, Nicholson et al., 1999, Longmore et al., 1999).
3. Deposition and burial of particulate nitrogen
Some deposition and burial of particulate nitrogen occurs on flanking environments, due to wave-induced landward sediment transport, depending on exposure to oceanic swells, and tidal deposition aided by the baffling effects 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.
4. Uptake of nitrogen by marine algae
Seagrasses and macrophytes take up dissolved inorganic nitrogen from the water column. Nitrogen-fixation occurring in the root-zone of seagrasses contributes additional dissolved inorganic nitrogen to this pool. Denitrification is also an important process in seagrass meadows (Pollard et al., 1991). Sandy sediment is permeable, hence can be ventilated by oxygen-rich overlying waters resulting in efficient remineralisation of organic debris (mostly by denitrification) with little preservation of organic matter.
5. Upwelling of nutrient-rich waters
6. Atmospheric inputs
Input of particulate nitrogen from atmospheric sources such as smoke and ash are typically of low significance.