Use of a multi-metric macroalgal index to document severe eutrophication in a New Zealand estuary

ABSTRACT

Environmental indicators that reliably describe estuary trophic response to nutrient loading are required for the effective management of New Zealand estuaries. A common estuary eutrophication response is frequent, extensive and persistent opportunistic macroalgae blooms. This study explores the utility of the Opportunistic Macroalgal Blooming Tool (OMBT) in the New River Estuary (∼4600 ha), a shallow intertidal-dominated system within a pastoral farming catchment in Southland, New Zealand. Total nitrogen (TN) loads increased from 3206 T y⁻¹ in 2000 to 5143 T y⁻¹ in 2020. Concomitantly, areas of opportunistic macroalgae, measured using the OMBT, expanded across the available intertidal habitat (AIH) from 34.9 ha or 1.4% of the AIH to 1383 ha or 47.6% of the AIH, and the average wet weight biomass increased from 19.3 g m⁻² to a peak of 1326 g m⁻² in 2019. We hypothesise that opportunistic macroalgae expanded rapidly in direct response to temporal increases in TN load, with increases disproportionately contributed by expansion and intensification of dairy farming in the catchment. This study confirmed that the OMBT predictably responded to changes in TN concentration and may therefore be useful in defining estuary-specific TN concentration thresholds to inform likely load reductions required to achieve improved environmental outcomes.

KEYWORDS:

• Macroalgae
• OMBT
• EQR
• catchment land use
• eutrophication
• dairy
• TN
• nitrogen
• New River Estuary
• indicator

Acknowledgements

The authors thank Environment Southland for supplying data collected in their State of Environment estuary monitoring programme and acknowledge the work of their data providers; Cawthron Institute (2001), Wriggle Coastal Management (2006-2016) and Salt Ecology (2018-2021). Megan Southwick (Salt Ecology) developed the GIS scripting tools used for quality assurance of the GIS data. The authors also thank the two anonymous reviewers and the associate editor Dr Andrew Lohrer for their thoughtful and constructive review of the manuscript .

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The data analysis and preparation of the manuscript were internally funded by Salt Ecology Partnership. Other parts of this work (nutrient load calculations) were funded by New Zealand’s Ministry of Business Innovation and Employment via the NIWA Strategic Science Investment Fund.