Establishing ecologically-relevant nutrient thresholds: A tool-kit with guidance on its use

One component of a strategy to manage eutrophication is the ability to set realistic targets for nutrient reduction and thresholds above which healthy ecosystems no longer function. This paper introduces a toolkit of statistical approaches with which such thresholds can be set, and explains their rationale and the situations under which each is most likely to be effective. All methods assume a causal relationship between nutrients and biota but also recognise that nutrients rarely act in isolation. Many of the simpler methods have limited applicability in situations where other stressors are present. Where relationships between nutrients and biota are strong (e.g. phytoplankton v phosphorus in deep lakes) then type II regression is recommended (recognising uncertainty in both axes). Regression relationships can be extended to include additional stressors (e.g. nitrogen) or variables responsible for natural variation between water bodies (e.g. alkalinity, altitude). However, in situations where the relationship between nutrients and biota is weaker, then various categorical approaches are recommended. Of these, binomial regression and a new method based on classification mismatch are likely to be most useful although both are likely to underestimate threshold concentrations if a second stressor is also present. Whilst multivariate approaches such as TITAN are unlikely to be particularly useful for meeting the specific needs of EU legislation, other multivariate approaches such as decision trees may have a role to play. If other stressors are present, then it may be impossible to set precautionary nutrient thresholds. In such situations, approaches such as quantile regression allow thresholds to be established which set limits above which nutrients are likely to influence the biota, irrespective of other pressures. These may be useful as part of a management strategy, but more sophisticated approaches, often generating thresholds appropriate to individual water bodies rather than to broadly defined “types”, are likely to be necessary too. The importance of understanding underlying ecological processes as well as correct selection and application of statistical methods is emphasised, along with the need to consider local regulatory and decision-making systems, and the ease with which statistical methods can be communicated to non-technical audiences.

KELLY Martyn;  PHILLIPS Geoff;  TEIXEIRA Heliana;  VARBIRO Gabor;  SALAS HERRERO Maria Fuensanta;  WILLBY Nigel;  POIKANE Sandra; 

2022-04-01

ELSEVIER

JRC125997

0048-9697 (online),   

https://www.sciencedirect.com/science/article/pii/S0048969721060551,    https://publications.jrc.ec.europa.eu/repository/handle/JRC125997,   

10.1016/j.scitotenv.2021.150977 (online),