Nitrogen dynamics in embayments and drowned river valleys

Block  diagram of nitrogen dynamics in embayments

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

Upwelling of nutrient-rich waters can introduce nitrogen to the embayment system, however typically in low quantities (Cresswell, 1994).

6. Atmospheric inputs

Input of particulate nitrogen from atmospheric sources such as smoke and ash are typically of low significance.

7. Large tidal prism and exchange of marine waters

A large tidal prism results in much of the dissolved and particulate nitrogen, including phytoplankton debris, being transported offshore (Bulthuis et al., 1984), and diluted by seawater.