Sewage-derived nitrogen dispersal and N-fixation in Port Phillip Bay in south-eastern Australia

Abstract

Context. Extreme values of nitrogen isotope ratios (δ15N) in biota identify areas influenced by sewage discharges and nitrogen fixation. Aims. To measure the ecological footprint of the Western (sewage) Treatment Plant (WTP) by using δ15N measurements and an NPZ model, and to identify regions where nitrogen is fixed in Port Phillip Bay, in south-eastern Australia. Methods. δ15N was measured in situ for four intertidal species at 27 sites around the perimeter of the bay and simultaneously in an alga and a mussel deployed at 42 sites throughout the bay. The intertidal species were also transplanted from an area of low 15N to areas of high 15N. Key results. δ15N was elevated for all species adjacent to the WTP and along the north-western shoreline of Port Phillip Bay and for ~10–15 km offshore. Transplanted algae adopted the δ15N of their new environment in 6–12 weeks, whereas the half-life of tissue turnover of 15N in intertidal filter-feeders was 90–234 days. Nitrogen fixation contributed to N available in two geographically isolated seagrass-dominated bays. Conclusions. Spatial patterns of δ15N of deployed alga and phytoplankton-consuming mussels were similar to the modelled distribution of dissolved inorganic nitrogen and the modelled distribution of flagellates, respectively. Implications. Preferred species for on-going N monitoring are identified.