Macroalgal Bioindicators of Recovery from Eutrophication in a Tidal Lagoon Following Wastewater Diversion and Earthquake Disturbance

Neill Barr,John Zeldis,David Schiel,Kristin Scheuer

doi:10.1007/s12237-019-00654-7

Abstract

Together, macroalgal tissue biochemical nitrogen indices (N-indices) and macroalgal abundance can be used as bioindicators of N-enrichment in estuaries. In this study, we examine the extent and rates of response of Ulva bioindicators during rapid N-enrichment perturbations in the eutrophic Avon-Heathcote Estuary (AHE) (Christchurch, New Zealand). With the diversion of the city’s wastewater discharge away from the estuary in March 2010, a ~ 90% reduction in the estuary’s N-concentration was expected. In turn, this was expected to reduce macroalgal biomass and improve the overall trophic condition of the estuary. We surveyed Ulva bioindicators over a five-year period spanning the diversion. There was a rapid (within one year) transition away from eutrophic condition reflected in N-indices (tissue-chlorophyll, -free amino acids, -N and -δ15N) following wastewater diversion, towards values corresponding with ‘cleaner’ water quality. This was accompanied by large reductions in Ulva percent cover, based on seasonal surveys conducted from 2001 to 2014. However, two large earthquakes in February and June 2011 caused a breakdown of the city’s wastewater infrastructure, resulting in overflows of untreated sewage into the estuary between February and November 2011. The re-enrichment of N and changes in N-sources (treated versus untreated sewage) were rapidly reflected in Ulva bioindicators, notably δ15N. Following repair of infrastructure, Ulva bioindicators again reverted towards a less eutrophic state. Overall, bioindicators were sensitive to changes in N-availability and N-source, and useful for identifying the position of algal populations on a eutrophic-to-oligotrophic gradient. These attributes demonstrated their utility as adjuncts to water quality monitoring and algal biomass surveys.

Highlights

Urbanisation, deforestation, agricultural intensification and fertiliser application (Fowler et al 2013) have resulted in Communicated by Lijun HouIn temperate estuaries, nitrogen (N) is generally the primary limiting nutrient during peak seasonal growth for macroalgae (Hanisak 1983), such as Ulva and Gracilaria (Rosenberg and Ramus 1982)
It is worth noting that while N-indices mentioned above showed clear post-diversion reductions, they did not fall to average minimum values observed in Ulva populations with low water column N loading from around New Zealand (Barr 2007), indicating Ulva in the Avon-Heathcote Estuary (AHE) still had a moderate N-status and N-availability from the water column
Because NO3-N loading to the estuary is largely riverine, and the rivers have not been remediated, our estimate of DIN reduction showed a less pronounced response to the diversion than expected from the estuary-wide budget. The shift in both Ulva biochemical indicators and Ulva abundance since the diversion of wastewater from the AHE clearly reflects a reduction in N loading over time, and at two separate locations in one estuary

Summary

Introduction

Urbanisation, deforestation, agricultural intensification and fertiliser application (Fowler et al 2013) have resulted in Communicated by Lijun HouIn temperate estuaries, nitrogen (N) is generally the primary limiting nutrient during peak seasonal growth for macroalgae (Hanisak 1983), such as Ulva and Gracilaria (Rosenberg and Ramus 1982). Sampled nutrient concentrations in shallow estuaries can vary significantly in space and time, making monitoring of concentrations problematic for assessing trophic condition (as a function of N loading) This can be further exacerbated by the capacity of fast-growing macroalgae to rapidly take up N and other nutrients, when in high biomass, decoupling observed concentrations from algal productivity and biomass (Björnsäter and Wheeler 1990; Thybo-Christesen et al 1993; Valiela et al 1997; Fong et al 1998; Flindt et al 1999). These factors underlie the often observed lack of correlation in estuaries between water column N (and nutrient) concentrations, and either productivity or growth of the primary producers that drive trophic condition (Fong and Zedler 1993; Fong et al 1998)

Methods

Results

Discussion

Conclusion