Trophic Indicators of Ecological Resilience in a Tidal Lagoon Estuary Following Wastewater Diversion and Earthquake Disturbance

Abstract

Estuary ecological resilience can be gauged by response of estuary trophic state to abatement of nutrient pollution. Changes in trophic indicators were studied in the Avon-Heathcote Estuary (AHE) in Christchurch, New Zealand, over 6 years, spanning diversion of city wastewater inputs to an offshore outfall in 2010, and to temporary enrichment caused by the 2011 Christchurch earthquake. It was hypothesised that the tidally well-flushed and sandy AHE would not harbour a ‘legacy’ of eutrophication and would rapidly gain improved ecological function following the diversion. AHE sediments were coarse (156 μm median grain size) with low organic matter (OM 1.2%, N 0.03%, C 0.3%), which changed little either with diversion or earthquake. Upon diversion, median water column and porewater ammonium (36, 185 μmol) decreased by 87% and 57%, respectively, benthic microalgae (269 mg chlorophyll-a m−2) fell by 58%, and enrichment-affiliated polychaetes (3700–8000 m−2) fell by 60–80% at sites with largest benthic microalgal reductions, all within < 1–2 years. Oxygen and ammonium fluxes were usually oligotrophic and changed little upon diversion, except near the historic wastewater discharge site. Denitrification became more important for N loss, increasing from 5 to 29% of estuary N load. Responses to earthquake-driven enrichment were transient. Despite decades of heavy N loading and eutrophic growths of benthic microalgae and macroalgae, the AHE did not store a eutrophic legacy in its sediments. It reacted rapidly to improved water quality allowed by the outfall, showing that this common estuary type (sandy, well-flushed tidal lagoon) was resilient to eutrophication upon stressor removal.