Anthropogenic impacts on the ecosystems of coastal lagoons: modelling fundamental biogeochemical processes and management implications

Abstract

This paper presents a biogeochemical model of a coastal lagoon intended to be representative of lagoons occurring along the south-east and south-west coasts of Australia. Many of these lagoons are threatened by increased nutrient loads because of land use change, by alterations to their freshwater inflows and by modification to their tidal flushing regimens. The model simulates the biogeochemical response of the lagoon to nutrient (nitrogen) loading and includes nutrient transformation processes in the sediments, as well as in the water column. The paper focuses on the response of primary producers to increasing and decreasing nutrient loads and how the response is altered by changes in the flushing rate of the lagoon with the sea. In common with lakes, the modelled lagoon exhibits alternative stable states representing macrophyte or phytoplankton dominance depending on nutrient loading and history. A third state representing severe degradation occurs when denitrification shuts down. A characteristic of Australian coastal lagoon systems is that, due to highly sporadic rainfall patterns, nutrient inflows are dominated by intermittent extreme events. The modelling demonstrates that such a loading regimen may be expected to generally increase the vulnerability of the lagoon to increasing nutrient loads. The results of the analysis presented are pertinent to several questions raised by coastal managers, such as what are the expected benefits of improving flushing by dredging and what are the consequences of altering the timing and magnitudes of the loads reaching the lagoons?