Abstract
Estuaries host unique biodiversity and deliver a range of ecosystem services at the interface between catchment and the ocean. They are also among the most degraded ecosystems on Earth. Freshwater flow regimes drive ecological processes contributing to their biodiversity and economic value, but have been modified extensively in many systems by upstream water use. Knowledge of freshwater flow requirements for estuaries (environmental flows or E-flows) lags behind that of rivers and their floodplains. Generalising estuarine E-flows is further complicated by responses that appear to be specific to each system. Here we critically review the E-flow requirements of estuaries to 1) identify the key ecosystem processes (hydrodynamics, salinity regulation, sediment dynamics, nutrient cycling and trophic transfer, and connectivity) modulated by freshwater flow regimes, 2) identify key drivers (rainfall, runoff, temperature, sea level rise and direct anthropogenic) that generate changes to the magnitude, quality and timing of flows, and 3) propose mitigation strategies (e.g., modification of dam operations and habitat restoration) to buffer against the risks of altered freshwater flows and build resilience to direct and indirect anthropogenic disturbances. These strategies support re-establishment of the natural characteristics of freshwater flow regimes which are foundational to healthy estuarine ecosystems.
Highlights
In many aquatic ecosystems, the freshwater flow regime, defined as the quality, quantity and timing of flows (Kotzé, 2016), is regarded as the key variable shaping ecosystem processes (Bunn and Arthington, 2002; Poff and Zimmerman, 2010)
The flow regime is critically linked to key estuarine ecosystem processes (Figure 5) that are potentially amenable to restoration strategies involving changes in flow dynamics
Knowledge of freshwater flow regimes to support fully functional, healthy estuaries is still lacking compared to river and floodplain ecosystems
Summary
The freshwater flow regime, defined as the quality, quantity and timing of flows (Kotzé, 2016), is regarded as the key variable shaping ecosystem processes (Bunn and Arthington, 2002; Poff and Zimmerman, 2010). We highlight the degradation estuaries have experienced as a consequence of modifications to freshwater flow regimes (Arthington, 2012; Kiwango et al, 2015; Stein et al, 2021), resulting in a decline in estuarine habitat quality due to altered ecosystem processes (Pinckney et al, 2001; Mbandzi et al, 2018) This can induce problems, such as eutrophication (see Section 4), due to biological responses (e.g., algal blooms, seagrass dieback) associated with modified ecosystem structure and function (Cottingham et al, 2018; Scharler et al, 2020). Our synthesis can help guide estuarine catchment management to define appropriate freshwater flow strategies and limits of acceptable change in the face of current and future climate and direct anthropogenic pressures including climate change
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