Restoration of estuary hydrological state with freshwater inflows: Non-linear processes, values-led decisions and multiple world views

Abstract

The quantity, quality and timing of freshwater inflow into estuaries is critical for ecosystem health. Coinciding with the United Nations Decade of Restoration (2021–2030), there is great interest in re-creating functional estuarine ecosystems by modifying the physio-chemical characteristics, with the premise that a functional ecosystem will follow (ecoengineering). To restore estuarine ecology, the physical processes of the system must be conductive to the re-establishment and sustenance of biota. These physical processes are generally under-monitored and often not used as a measure of restoration success. We explore ecoengineering to restore freshwater inflows to estuaries, focused on hydrological state. We use a Pressure—State—Response (PSR) framework where Pressure refers to anthropogenic pressures on freshwater inflows into estuaries such as dams and dredging. Pressure affects State — the physical estuarine condition (hydrological state), such as salinity structure, flushing time, water level and energy. A degraded state may result in information flow, such as from monitoring, and lead to a societal Response – a decision or action that attempts to prevent or reduce these pressures. Such responses may include dam removal, river rediversion, reconnection of tidal channels, dam release combined with mechanical mouth breaching and estuarine dredging. Judging restoration success remains difficult and is often a quality judgement with inherent observer bias. Most targets for improved state focus on ecological expectations and metrics, rather than hydrological state. Moreover, often the reference condition for estuaries is poorly defined, compared to rivers and lakes.   As an example of values-led decision making, we focus on Aotearoa New Zealand, where Māori are the Indigenous people. Here, it is becoming more common to identify values to set environmental outcomes, including based on Te Ao Māori (Māori world view) that are holistic, and inherently include a wealth of Indigenous knowledge. Te Ao Māori is underpinned by core values engrained in an intergenerational perspective, from whakapapa (connections, genealogy between humans, ecosystems, and all flora and fauna), a theme shared by many other Indigenous Peoples, through to the concept of mauri, the life force or internal energy in living and non-living things that can be damaged when plants, water, soils and ecosystems are degraded. Mauri has been used to guide efforts to maintain/ restore mahinga kai (traditional areas for gathering kaimoana/ sea food). For example, in restoration works in Te Awa o Ngātoroirangi (the Maketū Estuary), one of the major drivers is to restore the mauri of the river and estuary. There are many other examples from around the world where Indigenous and Local Knowledge (ILK) and world views have been applied in environmental restoration, such as to provide baseline ecosystem information to inform restoration targets and give motivation for restoration. As values are increasingly used as a tool to frame management protocols, a more holistic approach is gradually gaining momentum. However, connecting the less quantifiable values to attributes which can be engineered is an ongoing challenge.