Potential implications of canal estates for nutrient budgets in coastal systems

Abstract

Coastal ecosystems continue to decline globally due to a wide range of anthropogenic pressures. Among these pressures, the development of canal estates for housing development has led to the significant alteration of some estuarine ecosystems with uncertain outcomes for their ecology and biogeochemical function. Both the construction of residential canal estates and the consensus over their long-term impacts on adjacent waterways have concomitantly risen in recent decades. It is recognised that one of the main functional changes is the reduction of water exchange in canal systems, which may lead to poor water and sediment quality, low oxygen levels and consequently, potential changes in macroinfauna and ichthyofauna communities. Accordingly, canal estates may represent a significantly different set of fate pathways for both natural and anthropogenic materials such that they stand to potentially alter the overall performance of the wider ecosystem within which they sit. South-East Queensland has the highest number of artificial waterways in Australia. Unfortunately, there is very little research describing their biogeochemical function and behaviour; especially with regard to the wider role they may play as sources or sinks for nutrients and materials within the wider estuarine setting. In this context, two residential canal estates have been studied to address this gap in our understanding of materials cycling within canal ecosystems and to assess the main implications that canal design and function may have for wider coastal ecosystem performance and management. This research has identified spatial and seasonal variations in water column structure and nutrient fluxes. Notably, the canals tend to behave like enriched estuaries showing a release of ammonium and phosphorus from the sediment to the water column. Additionally, there is a great potential for some locations to behave as a sink of N due to high denitrification rates. More importantly, the nitrogen budget shows that both artificial waterways are importing N from the adjacent estuaries and that they are a net sink for N through denitrification and/or through the recycling of N by primary producers. Similarly, the phosphate budget shows that during the dry season canal estate acted like other Australian sub-tropical estuaries; they are net autotrophic and act as a sink for P. However, during the wet season this changes and the canal estate became a net source of P, exporting P to adjacent estuaries. Accordingly, management of canal estates and the materials they receive via anthropogenic and sources (dissolved and particulate) needs to be considered at the level of N and P they deliver.