Use of the d15N signatures of different functional forms of macroalgae and filter-feeders to reveal temporal and spatial patterns in sewage distribution

We examined whether δ15N levels of marine biota with different nutrient uptake characteristics can be used to trace the dispersal of sewage effluent in highly mixed, nitrogen-limited waters, and whether they can reveal the dispersal of sewage over different timescales. We hypothesised that macroalgal species with fast uptake rates would display a spatial pattern in δ15N levels reflecting recent sewage dispersal while those with slower rates would provide a signal integrated over a longer time period. Filterfeeding sponges and ascidians were also sampled to see if they reflected patterns in the dispersal of sewage particulate organic matter (POM). A laboratory experiment was performed to test whether the δ15N level of 3 macroalgal species (Ulva australis, Vidalia sp. and Ecklonia radiata) and 2 filter-feeding species (Clathria sp. and Pyura australis) was altered after cultivation in sewage nitrogen. We then sampled each organism along transects radiating away from the outlet of a wastewater treatment plant north of Perth, Western Australia, to determine spatial patterns in δ15N. U. australis and Vidalia sp. developed higher isotopic signatures when exposed to low concentrations of sewage nitrogen (1:500 dilution in seawater) for 7 d in the laboratory. U. australis and Vidalia sp. showed an increase of 1.7 and 1.4‰ in treatments respectively. In the field, macroalgae sampled north and south of the sewage outlet generally had higher δ15N levels than those sampled west of the outlet and at the reference site, and algae within 500 m of the outfall tended to have lower values than at 1000 m or more from the outfall. These trends are consistent with our current knowledge of plume dynamics: a predominantly northerly drift of effluent as a buoyant plume that tends not to be fully mixed in the water column for the first 500 m. The results confirmed that the δ15N signature of macroalgae could be used to trace sewage disposed in well-mixed waters. The strength of the spatial trends varied between algae, with E. radiata, the species with the lowest nutrient uptake rates and affinity, having the least spatial variability. We interpret this as reflecting a wider regional dispersal of sewage in the longer time frame, but a strong northerly drift in the short term, which was reflected in the δ15N values of the species with the fastest nitrogen uptake rates. The results were consistent with our hypothesis and are suggestive of a relationship between algal functional form and isotopic signatures that can be applied to determine the dispersal of sewage over different timescales. The δ15N values of benthic filter feeders did not provide strong evidence to suggest that they can be used to represent the dispersal of sewage POM, but trends found in the field experiment for Clathria sp. warrant further investigation