Nitrogen cycling processes in Lake Illawarra, an intermittently closed/open estuary in south-east Australia

Abstract

Mass balance - stoichiometric budgets, such as those developed by the Land-Ocean Interactions in the Coastal Zone (LOICZ) Programme can indicate the biogeochemical functioning of an estuary, and thus help assist understanding the dominant natural processes within the system. This is the basis to developing cost-effective management strategies. In this study, nutrient budgets for Lake Illawarra, a typical intertidal coastal barrier lagoon in New South Wales, Australia, were firstly examined utilizing the LOICZ model. The LOICZ budget classified this lake as a heterotrophic ecosystem, producing carbon through net respiration. The budget results also showed the Lake to be a net denitrifying and nitrogen limited system. Benthic flux measurements of O2, TCO2, alkalinity, NH4+, NO2- +NO3- and N2 were then made at selected stations from January 2002 to January 2003 to compare the characteristics of benthic biogeochemical processes (benthic metabolism, nutrient fluxes and denitrification) for different primary producers (seagrass or microphytobenthos (MPB)) and/or sediment types (sand or mud), and thus to verify the reliability of LOICZ budget approach. The seagrass meadows exhibited significantly higher gross primary productivity than MPB in shallow sandy or deep muddy regions of the Lake. Net CO2 effluxes and O2 uptake were generally observed for all sites, indicating the Lake was an overall net-respiratory system, meaning more organic carbon was decomposed than produced. This supported the LOICZ modelling conclusion. In general, nutrient fluxes displayed typical diurnal variations, with an efflux in the dark and uptake or reduced efflux in the light. Dissolved inorganic nitrogen (DIN) net fluxes in the unvegetated sediments were directed from the sediments towards the water column and dominated by the NH4+ fluxes; seagrass beds displayed a net DIN uptake dominated by NO2-+NO3- fluxes, which may be due to the enhanced denitrification and/or assimilation activity by rooted plants. Finally, based on the measured benthic fluxes, N2 flux rates were estimated using carbon and nitrogen stoichiometry, which gave an average net denitrification rate of 0.09 mmol m-2 h-1. This was of similar magnitude to the rates measured using the N2/Ar technique (0.04 mmol m-2 h-1) or estimated using LOICZ modelling (0.03-0.05 mmol m-2 h-1).