Using macroalgal bioindicators to map nutrient plumes from fish farms and other sources at a bay-wide scale

Human activities can elevate coastal levels of dissolved inorganic nitrogen (DIN). As macroalgae readily absorb and accumulate DIN, the elemental (total N and C:N ratio) composition of their tissues is less affected by temporal fluctuations compared to more direct measures of DIN concentration. Additionally, their isotopic (δ15N) composition can reflect that of the source, which could potentially be used to identify between multiple effluent sources. To investigate whether macroalgal ‘bioindicators’ could map and distinguish between multiple effluents, 2 species of macroalgae (Chondrus crispus and Palmaria palmata) were deployed in a bay containing a salmon farm and sewage treatment facility. Both species exhibited high total N and low C:N ratio near the salmon farm and sewage facility. However, the elemental composition of C. crispus was influenced over a greater distance than that of P. palmata. Differences were also observed between their isotopic composition, as C. crispus indicated that the salmon farm and sewage facility had distinct δ15N signatures, whereas values of δ15N in P. palmata had not changed after 10 d incubation in the field. Interestingly, the distinct isotopic signals observed in C. crispus were likely a result of higher DIN concentrations at the salmon farm, which likely caused macroalgae to fractionate and form biomass lighter in δ15N. Overall, this study suggests that macroalgal bioindicators can monitor and identify between multiple effluent sources, which could provide a useful tool for coastal management. However, some species of macroalgae may make more effective bioindicators than others, and the mechanisms underlying their fractionation require further investigation.

Keywords: Aquaculture, Isotope, Macroalgae, Nitrogen, Salmon, Sewage, Wastewater