Mapping Coastal Nutrient Plumes
Alternative methodology to map coastal nutrient plumes
A new SA Water project is employing a macroalgal bioassay to map the extent of the impact of wastewater plumes in Adelaide coastal waters. Macroalgae is being used as active samplers of nitrogen in the water column, with macroalgal thalli deployed for 4 days at 200 sites along the coast. At the end of the deployment period, the isotopic composition of nitrogen accumulating in macroalgal tissues indicates the source of nitrogen reaching coastal waters. Areas impacted by wastewater outfalls are identified by an increase in the proportion of the heavy and rare isotope of nitrogen (15N) in comparison to pristine oceanic waters or regions affected by diffuse fertilizer inputs. This isotopic enrichment is used to map wastewater plumes and their dispersion with distance from outfalls.
There are two main advantages in using macroalgae as nitrogen bio-samplers:
- macroalgae can be deployed in situ over several days, and therefore integrate inputs over time,
- macroalgae only assimilate the fraction of nitrogen in the water column that is bioavailable, and therefore likely to induce ecological responses.
The technique thus avoids high-frequency natural variability typically associated with more traditional monitoring techniques (e.g. spot sampling of water) by providing a 'composite sample' of bioavailable nitrogen.
Map of wastewater distribution along the Adelaide coast based on the stable isotopic signature of nitrogen, produced during trials of the macroalgal bioassay in the summer of 2009.