Nitrogen budget for a high-energy ecosystem

A nitrogen budget is compiled for an exposed beach and surf zone ecosystem in South Africa. Phytoplankton (particularly diatoms) are the major users of nitrogen in this system as macrophytes are absent. Inputs of nitrogen occur via groundwater inflow and from an estuary with rain and airborne detritus being of minor importance. Loss of nitrogen to the sediment as a result of beach accretion or to the atmosphere by denitrification is considered negligible because of sediment reworking and the highly oxidised state of this high-energy surf zone. Nitrogen budgets for the major zooplankton, benthos and fish components are presented and the amounts of nitrogen recycled by the total macrofauna (5735 g N m-' yr-l), interstitial fauna (5200 g N m-' yr-') and microbial loop (4607 g N m-' yr-') trophic assemblages are estimated for a 1 m wide transect across the system (500 m long). The budget indicates that: (1) inorganic nitrogen inputs (groundwater, rain, estuarine) into the surf zone supply 13 % of phytoplankton requirements for primary production; (2) the 3 trophic assemblages together recycle 99% of the nitrogen required for total phytoplankton primary production, all 3 being important in this, and (3) the dissolved and particulate organic nitrogen recycled by the macrofauna can supply 24 % of the nitrogen requirements estimated for the microbial loop. The observation that fauna regenerate almost all the nitrogen required for phytoplankton primary production supports the concept of the surf zone as a self-sustaining, semi-closed ecosystem.