Climate change overtakes coastal engineering as the dominant driver of hydrological change in a large shallow lagoon

Abstract

Abstract. Ecosystems in shallow micro-tidal lagoons are particularly sensitive to hydrologic changes. Lagoons are complex transitional ecosystems between land and sea, and the signals of direct human disturbance can be confounded by variability of the climate system, but from an effective estuary management perspective, the effects of climate versus direct human engineering interventions need to be identified separately. This study developed a 3D finite-volume hydrodynamic model to assess changes in hydrodynamics of the Peel–Harvey Estuary, a large shallow lagoon with restricted connection with ocean; this was done by considering how attributes such as water retention time, salinity and stratification have responded to a range of factors, focusing on the drying climate trend and the opening of a large artificial channel over the period from 1970 to 2016, and how they will evolve under current climate projections. The results show that the introduction of the artificial channel has fundamentally modified the flushing and mixing within the lagoon, and the drying climate has changed the hydrology by comparable magnitudes to that of the opening of the artificial channel. The results also highlight the complexity of their interacting impacts. Firstly, the artificial channel successfully improved the estuary flushing by reducing average water ages by 20–110 d, while in contrast the reduced precipitation and catchment inflow had a gradual opposite effect on the water ages; during the wet season this has almost counteracted the reduction brought about by the channel. Secondly, the drying climate caused an increase in the salinity of the lagoon by 10–30 PSU (Practical Salinity Unit); whilst the artificial channel increased the salinity during the wet season, it has reduced the likelihood of hypersalinity (>40 PSU) during the dry season in some areas. The opening of the artificial channel was also shown to increase the seawater fluxes and salinity stratification, while the drying climate acted to reduce the salinity stratification in the main body of the estuary. The impacts also varied spatially in this large lagoon. The southern estuary, which has the least connection with the ocean through the natural channel, is the most sensitive to climate change and the opening of the artificial channel. The projected future drying climate is shown to slightly increase the retention time and salinity in the lagoon and increase the hypersalinity risk in the rivers. The significance of these changes for nutrient retention and estuary ecology are discussed, highlighting the importance of these factors when setting up monitoring programmes, environmental flow strategies and nutrient load reduction targets.

Human impacts and their interactions in the Baltic Sea region

“…Climate-related changes in the Baltic Sea like warmer temperatures, changed stratification patterns, altered ecosystems and biogeochemical pathways may change the fate of nutrients in the sea (Arheimer et al, 2012;Meier et al, 2012). Sea level rise may have an impact on the internal loading of phosphate, through potentially increased saltwater inflows in the future with rising sea levels, affecting ecosystems in micro-tidal lagoons (Huang et al, 2020) and increasing the hypoxic areas in the deep water and the associated phosphate release (Meier et al, 2017).…”

Spatiotemporal Variability in Environmental Conditions Influences the Performance and Behavior of Juvenile Steelhead in a Coastal California Lagoon

“…These habitats often provide unique ecological services and are particularly important as nursery areas for estuarine-resident and freshwater fish species. Unfortunately, the productivity and resiliency of many estuaries and lagoons have been reduced by intensive habitat alteration and management (Heady et al 2015), and these habitats may be especially vulnerable to climate change (Huang et al 2020). In coastal California, there is growing evidence that extreme climatic events are becoming increasingly commonplace, altering the annual timing and duration of lagoon formation (i.e., sandbar open/close dynamics) and diminishing the quality of lagoon rearing habitat for imperiled juvenile salmonids (Osterback et al 2018;Huber and Carlson 2020).…”

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

“…The residence time of water in an estuary (inversely related to the flushing rate, #1 Figure 1) is strongly influenced by freshwater flow (Wolanski et al, 2006;Huang et al, 2020). It affects the distribution of salinity, dissolved oxygen and resident organisms, sedimentation rates of particulates, processing times of nutrients, contaminants (e.g., toxins, heavy metals) and pathogens, and contaminant exposure risk to resident organisms (Cottingham et al, 2018;Clark and O'Connor, 2019;Fonseca et al, 2020).…”

“…However, at the catchment scale, variability in the direction of changes has been observed between flow indices (Douglas et al, 2000) and the same flow indicator at different parts of the catchment (Fleming et al, 2020). Recent declines in freshwater flows have been observed in southern Australia (Zhang et al, 2016;Huang et al, 2020), the Mediterranean (Haddeland et al, 2014;Greve et al, 2018), southern Africa (Haddeland et al, 2014) and southern Asia (Mondal and Mujumdar, 2012). Conversely, increases have been observed for rivers flowing to the Arctic Ocean (Durocher et al, 2019), northern Europe (Haddeland et al, 2014;Greve et al, 2018) northern Asia (Tananaev et al, 2016) and northern North America (Durocher et al, 2019).…”