Assessing the Eutrophic Susceptibility of New Zealand Estuaries
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Assessing the Eutrophic Susceptibility of New Zealand Estuaries David R. Plew 1 & John R. Zeldis 1 & Bruce D. Dudley 1 & Amy L. Whitehead 1 & Leigh M. Stevens 2 & Barry M. Robertson 3 & Ben P. Robertson 3 Received: 14 June 2019 / Revised: 6 December 2019 / Accepted: 11 March 2020 # The Author(s) 2020
Abstract We developed a method to predict the susceptibility of New Zealand estuaries to eutrophication. This method predicts macroalgae and phytoplankton responses to potential nutrient concentrations and flushing times, obtained nationally from simple dilution models, a GIS land-use model and physical estuary properties. Macroalgal response was based on an empirically derived relationship between potential nitrogen concentrations and an established macroalgal index (EQR) and phytoplankton response using an analytical growth model. Intertidal area was used to determine which primary producer was likely to lead to eutrophic conditions within estuaries. We calculated the eutrophication susceptibility of 399 New Zealand estuaries and assigned them to susceptibility bands A (lowest expected impact) to D (highest expected impact). Twenty-seven percent of New Zealand estuaries have high or very high eutrophication susceptibilities (band C or D), mostly (63% of band C and D) due to macroalgae. The physical properties of estuaries strongly influence susceptibility to macroalgae or phytoplankton blooms, and estuaries with similar physical properties cluster spatially around New Zealand’s coasts. As a result, regional patterns in susceptibility are apparent due to a combination of estuary types and land use patterns. The few areas in New Zealand with consistently low estuary eutrophication susceptibilities are either undeveloped or have estuaries with short flushing times, low intertidal area and/or minimal tidal influx. Estuaries with conditions favourable for macroalgae are most at risk. Our approach provides estuaryintegrated susceptibility scores likely to be of use as a regional or national screening tool to prioritise more in-depth estuary assessments, to evaluate likely responses to altered nutrient loading regimes and assist in developing management strategies for estuaries. Keywords Estuaries . Eutrophication . Susceptibility . Nutrients . Macroalgae . Phytoplankton . New Zealand
Introduction Increased input of nutrients to land has caused worldwide increases in coastal eutrophication, the process whereby a water body becomes enriched with nutrients that stimulate excessive primary production (Fowler et al. 2013; Howarth 2008; Vitousek et al. 1997). In estuaries, common responses are Communicated by Mark J. Brush Electronic supplementary material The online version of this article (https://doi.org/10.1007/s12237-020-00729-w) contains supplementary material, which is available to authorized users. * David R. Plew [email protected] 1
National Institute of Water and Atmospheric Research, Christchurch, New Zealand
2
Salt Ecology Limited, Nelson, New Zealand
3
Robertson Environmental Limited, Nelson, New Zealand
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