The advantages and limitations of biophysical modelling as a tool for informing limit setting in New Zealand’s barrier-enclosed estuaries

Estuaries are the receiving environment for catchment-derived contaminants, the fate of which depends on the interplay between the estuarine geomorphology and hydrodynamics. In large estuaries, biophysical processes are spatially and temporally-diverse, which makes understanding and managing the impact of human activities challenging. Here we use two common modelling approaches to explore the advantages and limitations of biophysical modelling as a tool for limit setting in a large barrier-enclosed estuary in New Zealand. The model shows the large spatial variation in water quality associated with low upper harbour flushing. Variations can also be attributed to spatial variation in processes (such as denitrification). Although the non-linear interactions between processes within these models can limit the value of using specific detail of outputs for decision making, the general patterns and sensitivities can be used to define areas, explore connectivity, and provide some information when monitoring data is lacking. Even in a deterministic modelling environment, it can very difficult to attribute water quality variations output at one location to the loading that caused these variations. While biophysical modelling will likely remain a core tool for informing management, any future development of limit setting methods for estuaries should recognise the inherent constraints we describe here.

河口是流域衍生污染物的受纳环境，其归宿取决于河口地貌与水动力过程之间的相互作用。大型河口的生物物理过程具有显著的空间与时间异质性，这使得理解和管控人类活动的影响颇具挑战。本研究采用两种常用建模方法，探究生物物理模型（biophysical modelling）作为新西兰大型障壁封闭河口水质限值设定工具的优势与局限。该模型揭示了与上游港池换水效率低下相关的水质大幅空间差异。这种差异还可归因于各类过程（如反硝化作用(denitrification)）的空间异质性。尽管此类模型内部的非线性相互作用可能限制了直接采用模型输出细节进行决策的价值，但仍可借助其通用模式与敏感性分析结果，划定管控区域、探究连通性，并在监测数据缺失时提供参考信息。即便在确定性模型（deterministic modelling）框架下，也很难将某一地点的水质变化输出归因于引发该变化的负荷输入。尽管生物物理模型仍将是支撑河口管理的核心工具之一，但未来开发河口限值设定方法时，应充分认识本研究揭示的固有局限性。