Advancing the Science of Environmental Flow Management for Protection of Temporarily Closed Estuaries and Coastal Lagoons

Abstract

The science needed to inform management of environmental flows to temporarily closed estuaries and coastal lagoons is decades behind the state of knowledge for rivers and large embayments. These globally ubiquitous small systems, which are often seasonally closed to the ocean’s influence, are under particular threat associated with hydrologic alteration because of changes in atershed land use, water use practices, and climate change. Managing environmental flows in these systems is complicated by their tight coupling with watershed processes, variable states because of intermittently closing mouths, and reliance on regional scale sediment transport and littoral processes. Here we synthesize our current understanding of ecohydrology in temporarily closed estuaries (TCEs) and coastal lagoons and propose a prioritized research agenda aimed at advancing understanding of ecological responses to altered flow regimes in TCEs. Key research needs include agreeing on a consistent typology, improving models that couple watershed and ocean forcing at appropriate spatial and temporal scales, quantifying stress–response relationships associated with hydrologic alteration, improving tools to establish desired conditions that account for climate change and consider cultural/indigenous objectives, improving tools to measure ecosystem function and social/cultural values, and developing monitoring and adaptive management programs that can inform environmental flow management in consideration of other stressors and across different habitat types. Coordinated global efforts to address the identified research gaps can help guide management actions aimed at reducing or mitigating potential impacts of hydrologic alteration and climate change through informed management of freshwater inflows.

Environmental Flow Requirements of Estuaries: Providing Resilience to Current and Future Climate and Direct Anthropogenic Changes

“…The widespread degradation of estuaries is notably caused due to eutrophication (Pinckney et al, 2001;Davis and Koop, 2006;Maier et al, 2009;Howarth et al, 2011) and has been the focus of research for many decades (e.g., Barlow et al, 1963;Caperon et al, 1971;Livingston, 1996). However, we also 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).…”

“…Freshwater flow requirements to support fully functional, healthy estuaries are largely ignored compared with those of river and floodplain environments (Peñas et al, 2013;Adams, 2014;Kiwango et al, 2015;Van Niekerk et al, 2019b;Stein et al, 2021). This has been attributed to a lack of understanding of both the mechanisms which drive estuarine ecological functioning (Gippel et al, 2009;Peñas et al, 2013) and the responses to changes in flow (Gippel et al, 2009;Peñas et al, 2013;Adams, 2014;Van Niekerk et al, 2019b), both of which are attributable to a lack of long-term data (Peñas et al, 2013;Van Niekerk et al, 2019b).…”

Consequences of Mouth Closure and Hypoxia-Induced State Changes in Low-Inflow Estuaries: Benthic Community and Trait-Based Response

“…Despite the potential benefits of opening closed lagoons such as LPL, tidal inlets and their management are extremely complex, and mechanical opening of tidal inlets is one of the most contentious management issues associated with ICEs (Jacobs et al 2011;Clark and O'Connor 2019;Largier et al 2019;Stein et al 2021). It is clear that naturally closing systems provide a variety of unique functions and services (e.g., support for anadromous fish) that artificial opening can compromise, especially when intermittent systems are made to be permanently open (Jacobs et al 2011).…”

“…In general, mouth opening should be viewed in the context of trade-offs that inevitably arise with such actions and be informed by both detailed assessment of estuarine condition (including benthic fauna) and broader consideration of socio-ecological factors (Southern California Wetlands Recovery Project 2018). Stein et al (2021) highlighted a series of management recommendations for ICEs including the need to (1) develop tools that measure function and identify thresholds for "healthy" systems; (2) identify cumulative and interacting effects of management actions (such as increased flushing) on functions; (3) develop strategies for monitoring, assessment, and adaptive management of flows to inform ongoing management and improve performance of models; and (4) identify sensitive ecological indicators. Fundamental to this is the availability of long-term data to assess the consequences of management action (or inaction), including mouth management, changes to watershed inputs, or use of compensatory mitigation to offset impacts to ICE habitats and functions (Zedler and Callaway 2000;Desmond et al 2002).…”

Resilient rivers and connected marine systems: a review of mutual sustainability opportunities

“…Actionable recommendations from the Brisbane Declaration, across governance, management, and research bodies suggest the development of adaptive management frameworks that focus on balancing environmental flows for both human and ecological water requirements (Arthington et al, 2018). An environmental flows programme focused on river-estuary connections could forge new paths to identify key moments to implement mitigation measures (Stein et al, 2021). The timing and magnitude of river discharge predictions could, for example, provide insights on plastic dispersal ranges in relation to coast types (Harris et al, 2021), or could also be used by managers to know when to clean litter traps to avoid bypass or overflow during a flood event.…”