Coastal lagoons like salty soup – how climate change and human activities are affecting them

In May 2025, a team of researchers from the University of Adelaide, led by Dr. Chris Keneally, published a paper entitled Microbial ecology in hypersaline coastal lagoons: A model for climate-induced coastal salinisation and eutrophication. The paper summarizes research findings on the causes and effects of increased salinity in coastal lagoons around the world and their microbial and ecosystem consequences.

Coastal lagoons are important ecosystems with multiple functions: they provide habitat for various groups of organisms, provide resources for fishing activities, play a role in carbon sequestration and nutrient cycling. Their functioning in biological and ecosystem terms depends on complex assemblages of microorganisms responsible for, among other things, the processes of nitrification, denitrification and mineralization of organic matter. Disruptions in the structure of these assemblies can lead to the degradation of the entire ecosystem.

Salinity of lagoons – scale and causes

The phenomenon of increasing water salinity, exceeding seawater salinity (hypersalinity), in coastal lagoons is global in scope and is particularly common in arid and semi-arid zones, including Australia, the Mediterranean and the Persian Gulf. In Australia, where the study was conducted, the rate of salinity increase has accelerated over the past four decades.

The main reasons for the occurrence of lagoons with elevated salinity include:

• Increasing temperatures (which increases evaporation);
• A decrease in rainfall and a reduction in freshwater inflows;
• Sea level rise;
• Human activities, including: river regulation, urban and infrastructure development, groundwater extraction.

A single extreme weather event, such as a very hot and dry summer, can instantly change the nature of an entire lagoon ecosystem, according to research. Dr. Keneally describes this as a shift from a balanced environment to a “salty, green soup” – with a dominance of salt-tolerant but less diverse microorganisms.

Ecological consequences of the phenomenon

Increased salinity in coastal lagoons disrupts basic microbial processes:

• reducing nitrification and denitrification results in nitrogen accumulation;
• Changes in the decomposition of organic matter lead to an increase in greenhouse gas emissions;
• The quality of habitat for fish, shellfish and migratory birds is deteriorating;
• algal blooms (sometimes toxic) are increasing;
• The proportion of seagrass community coverage is decreasing, weakening coastal protection against flooding.

Of particular concern are observations of the potential transformation of high-salinity lagoons from carbon sinks to carbon emitters – as a result of biogeochemical imbalances. Studies suggest that degraded lagoons could become a source of potent greenhouse gas emissions, amplifying climate change.

Health and economic impact

The degradation of coastal lagoons with increased salinity also translates into losses for local communities and economies:

• Massive fish die-offs hit fisheries,
• toxic algal blooms can pose a health risk (e.g., by emitting aerosols that irritate the respiratory system),
• The loss of natural barriers (like seagrass meadows) increases the vulnerability of coasts to the effects of storms and flooding.

Suggestions for corrective actions

A team of researchers from the University of Adelaide presents a list of specific measures to mitigate the effects of rising salinity in coastal lagoons:

1. restoring environmental flows – restoring freshwater flow to lagoons (e.g., by controlling river outflows ) helps maintain circulation and reduce salt concentration;
2. Reducing nutrient inputs – upgrading wastewater treatment plants, restoring wetlands and reducing fertilization in agriculture can reduce eutrophication;
3. Managing the hydrological structure – opening closed straits and channels can improve water exchange and reduce stagnation;
4. Supporting salt-tolerant vegetation – improving vegetation cover can reduce evaporation and water loss, and promote sediment stability.

The authors note that lagoons with elevated salinity can recover relatively quickly – if appropriate action is taken. An example is Coorong Lagoon in South Australia, whose microbiome improved significantly in just a few months after the 2022 Murray-Darling Basin flood, thanks to increased freshwater inflows.

A few words of summary

Dr. Chris Keneally’s team’s research is an important contribution to understanding the impact of climate change and human activities on coastal ecosystems. The presented model of hypersalinization and eutrophication of coastal lagoons shows the threats not only to biodiversity and ecosystem services, but also to the quality of life and human safety. At the same time, it emphasizes that corrective measures are possible and can yield relatively quick results – provided that water is properly managed and habitats are protected.

Source: https://www.sciencedirect.com/science/article/pii/S0012825225001114?via%3Dihub