Quantifying the Extent of Anthropogenic Eutrophication of Lakes at a National Scale in New Zealand

Abstract

Quantifying environmental changes relative to ecosystem reference conditions (baseline or natural states) can inform assessment of anthropogenic impacts and the development of restoration objectives and targets. We developed statistical models to predict current and reference concentrations of total nitrogen (TN) and total phosphorus (TP) in surface waters for a nationally representative sample of ≥1033 New Zealand lakes. The lake-specific nutrient concentrations reflected variation in factors including anthropogenic nutrient loads, hydrology, geology, elevation, climate, and lake and catchment morphology. Changes between reference and current concentrations were expressed to quantify the magnitude of anthropogenic eutrophication. Overall, there was a clear increase in lake trophic status, with the most common trophic status being oligotrophic under a reference state and mesotrophic under current conditions. The magnitude of departure from reference state varied considerably within the sample; however, on average, the mean TN concentration approximately doubled between reference and current states, whereas the mean TP concentration increased approximately 4-fold. This study quantified the extent of water quality degradation across lake types at a national scale, thereby informing ecological restoration objectives and the potential to reduce anthropogenic nutrient loads, while also providing a modeling framework that can be applied to lakes elsewhere.

Reference and current Trophic Level Index of New Zealand lakes: benchmarks to inform lake management and assessment

“…Measurements from the NWQMN were collected from the surface or within the epilimnion in stratified lakes, with sampling typically undertaken monthly (Burns et al 2000;Davies-Colley et al 2012). The NWQMN database (1976 was first screened, primarily to remove measurements collected prior to 2000 and to identify lakes with ≥10 measurements of TN, TP and chlorophyll a (see Abell et al (2019) for details of additional screening criteria). This sample was used to develop models to predict chl a (n= 73; Table 1), and then screened further to identify lakes with ≥10 measurements of Secchi (not available for 30 of the lakes) to create a sample to develop models to predict Secchi (n= 43; Table 1).…”

“…We used the existing dataset of modelled TN and TP concentrations in NZ lake surface waters developed during a previous study by Abell et al (2019), which we briefly summarize in this section. The authors used nutrient mass loading models ('box models') to estimate annual average lake nutrient concentrations that corresponded to reference (REF_TNlake and REF_TPlake) and current states (TNlake and TPlake).…”

“…Confidence intervals were calculated for the chl a model estimates with a parametric bootstrap (n=1,000) that accounted for the propagation of errors in multiple models (note that, for unmonitored lakes, chl a was modelled based on nutrient concentrations that were in turn modelled based on other variables). On each iteration i of the Abell et al (2019) by recalculating TNlake and TPlake using lake inflow nutrient concentrations that were adjusted by adding a random error term based on the standard error of lake inflow nutrient concentrations (see Abell et al 2019). Each iteration of the bootstrap therefore accounted for uncertainty in the model (e.g., structural error) through the refitting of the model coefficients, as well as the uncertainty in the predicted lake inflow nutrient concentration (errors in variables) that were used to predict TN * lake and TP * lake during the previous step.…”

“…Due to the influences of catchment and within-lake factors, reference conditions for lake trophic status variables should ideally be developed for lakes on an individual basis, although reference conditions for similar lakes can be grouped to develop ranges of values for lake categories (e.g., Carvalho et al 2008, Poikāne et al 2010. Recently, Abell et al (2019) used a modelling approach to estimate annual mean TN and TP concentrations for the surface waters of ~1,030 NZ lakes, with concentrations estimated separately for reference and current states. Their approach built on work by McDowell et al (2013), who derived median reference concentrations for nutrients in categories of NZ streams and rivers that accounted for climate, topography and geology.…”

“…The objective of this study was to use the TN and TP concentrations estimated by Abell et al (2019), alongside other variables, to predict reference and current values of chl a and Secchi, which could then be used to estimate reference and current TLI values for a sample of >1,000 NZ lakes. We hypothesised that predicting reference and current lake trophic status for individual lake types could provide benchmarks to guide lake management and enhance understanding of the magnitude of anthropogenic changes to lake water quality in NZ.…”