By Dr Jason van de Merwe
Estuaries are critical coastal habitats that support ecologically and economically important fish, prawn and crab species. However, estuaries are often impacted by coastal urbanisation, which can affect estuarine health through changes to nutrient dynamics. We have devised a new approach to monitoring the health of urbanised estuaries.
Based on “you are what you eat” principles, we can trace nutrients into and through estuarine systems using stable isotopes. Distinct stable isotope ratios occur in different sources of natural and human-derived nutrients. The stable isotope ratios in nutrients are then passed onto their consumers.
For example a prawn eating algae that was fertilized by spill-over of nutrients from aquaculture will retain the isotope signal from the aquaculture nutrients. By measuring the stable isotopes in animals and their food sources we can therefore trace nutrients through estuarine food webs.
Different organisms within an estuary assimilate nutrients at different rates and occur in different places throughout the estuary. A range of organisms must therefore be monitored to get a full understanding of the impacts of urbanisation on estuarine nutrient dynamics and health.
In this study, we collected aquatic organisms from the subtropical, urbanised Logan-Albert estuary over wet and dry seasons, and analysed the carbon and nitrogen stable isotopes of phytoplankton (very fast turnover rates and ubiquitous distribution), prawns (moderate turnover rates and localised home ranges) and fish (slow turnover rates and ubiquitous distribution).
We found that nutrients from wastewater treatment plants and aquaculture facilities were rapidly assimilated into the estuarine food web. However, this impact was diluted during the wet season. In addition, our results indicate that urbanisation, through the input of anthropogenic nutrients and changes to catchment vegetation, can more generally impact the nutrient dynamics of these critical habitats. These impacts may have flow on effects to economically and ecologically important fish, prawn and crab species.
This study illustrates the impacts of urbanisation on estuarine health over a range of spatial and temporal scales. This research also highlights the application of stable isotopes in assessing estuarine nutrient dynamics, and provides direction on the types of organisms that should be used to assess different spatial and temporal trends. This is important information for managers of these critically important coastal habitats.
This work was published in Ecological Indicators (van de Merwe et al. 2016)