Year

2004

Degree Name

Doctor of Philosophy

Department

School of Earth and Environmental Sciences - Faculty of Science

Abstract

Lake Illawarra is a typical shallow intertidal barrier lagoon in south-eastern Australia. The lake is considered as strongly nitrogen limited, and eutrophication is the key environmental problem as a consequence of an expansion of catchment development with associated increases in urban, rural and industrial pollution in recent decades. The aim of this study was to investigate the internal biogeochemical nitrogen cycling processes and the interactions with the carbon cycling processes in Lake Illawarra, in order to a improve the understanding of the system and to increase the overall knowledge base on nutrient biogeochemical cycling in coastal water bodies. Firstly, nutrient budgets for Lake Illawarra were updated using the LOICZ modeling approach based on the most recent and reliable water quality data available. This indicated the biogeochemical functioning of the estuary and assisted in understanding the dominant natural biogeochemical processes within the system. The LOICZ budget classified the lake as generally a heterotrophic environment, producing carbon through net respiration. The budget results also indicated that the lake was a net sink for nitrogen, a net denitrifying system and was nitrogen limited. Benthic flux measurements of O2, TCO2, alkalinity, NH4+, NO3 -+NO2 - and N2 were then made using the sediment-core incubation technique at selected stations to compare the characteristics of benthic biogeochemical processes (benthic metabolism, nutrient fluxes and denitrification) for different primary producers (seagrass, microphytobenthos (MPB) and macroalgae) and/or sediment types (sand or mud), and thus to verify the reliability of LOICZ budget approach. The sedimentary organic matter in the macrophyte beds was mainly from seagrasses (Ruppia or Zostera), which was supported by the relatively higher sediment C/N ratios at these sites compared with the adjacent shallow bare sand sites and the deep mud site. On the other hand, the organic matter pool in the unvegetated sites was generally dominated by MPB, which was supported by 1) the lower C/N ratios, 2) the detection of Chl-a, and 3) the microalgae (mainly diatoms) identified in the surface sediments. On an annual basis, seagrass beds exhibited the highest gross primary productivity (O2 or TCO2 fluxes), while the lowest rates occurred in the deep central basin of the lake. Seasonally, there was a general trend of highest production in spring or summer, and lowest production in winter or autumn. Organic carbon oxidation scenarios, evaluated by either calcium carbonate dissolution or sulfate reduction models, indicated that both models can explain organic matter mineralization. Trophic status was evaluated using different indices including BTSI, net O2 fluxes and P/R ratios for Lake Illawarra, which led to similar trophic classifications in general, and also the same trends in spatial and seasonal variation. Overall, these data indicated that the lake was heterotrophic on an annual basis, as the total community carbon respiration exceeded production, and this supported the LOICZ modelling conclusion. In general, nutrient fluxes displayed typical diel variations, with reduced effluxes or enhanced uptake by the sediment in the light, due to the photosynthetic activities of the plant-MPB-sediment community in Lake Illawarra. On an annual basis, unvegetated sediments displayed net DIN effluxes, while seagrass beds showed a net DIN uptake, which may be due to the enhanced denitrification and/or assimilation activity by rooted plants and macroalgae. The effect was most efficient during periods of net growth. Moreover, based on the measured benthic fluxes, N2 flux rates were estimated using C and N stoichiometry, suggesting the lake was a net denitrifying system. LOICZ budget modelling results and the direct denitrification measurements using the isotope pairing and N2/Ar techniques generally support this conclusion. It was found that the benthic metabolism and nitrogen transformation processes in Lake Illawarra were influenced or controlled by physico-chemical and biological variables and their complex interactions, which include light, temperature, water movement or mixing (stirring effect), sediment type (e.g., sand and mud), sedimentary organic matter quality, primary producers (e.g., seagrass, MPB and macroalgae) and infauna (bioturbation). Finally, recommendations for future work have been discussed.

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