Patagonian droughts linked to harmful algal blooms

Patagonia’s fjords are home to spectacular biodiversity, with many endemic and endangered aquatic species. They are also an environment suitable for thriving fisheries that support food security and local economies, being the second largest producer of farmed salmon after Norway. However, climate change is impacting the ecosystem, in particular the increasing occurrence of droughts.

New research, published in Frontiers in Marine Science, has investigated the impact of droughts in the Reloncaví Fjord, Northern Chilean Patagonia.

Associate Professor Lauren Ross and Ph.D. researcher Elías Pinilla, of the University of Maine, used modeling to analyze the impacts of river discharges, tides and wind when comparing a normal year (2018) with an extreme drought year (2016). During the latter, 40,000 tons of salmon died as a result, leading to $800 million in economic losses.

The researchers identified a distinct shift in fjord salinity during times of drought, with reduced freshwater input. Total Exchange Flow (the transport of water, and therefore salinity, between ocean and estuary) was 14.3% lower when entering the fjord in 2016 compared to 2018 and 16.7% lower when exiting the estuary to flow back out to sea.

In 2018, river discharge accounted for 74% of Total Exchange Flow, while tides and wind contributed 17% and 9%, respectively. During the 2016 drought year, though, the influence of tides and wind on Total Exchange Flow increased to 21% and 16% respectively, which highlights the increasing significance of their role in destratification when freshwater availability via river discharge is low.

Consequently, this wind interference caused the fjord’s water to destratify, with increased mixing of the dense high-salinity water that usually sits below a thin layer of lower-salinity freshwater at the surface. This has significant implications for ecosystem dynamics and the scientists used this information to predict fjord stratification patterns linked to recorded incidences of harmful algal blooms over 40 years (1980–2021).

In so doing, they linked salinity levels with the presence of Pseudchatonella spp. and Alexandrium catenella algae. During periods of stratification, A. catenella algae bloomed in spring, but during summer drought destratification, Pseudchatonella algae proliferated.

The latter algal blooms can be harmful to the fjord ecosystem as their rapid growth at the surface can block sunlight from reaching organisms living below, as well as creating oxygen minimum zones when the decomposition of dead algae uses up the water’s dissolved oxygen. The subsequent hypoxic conditions can make it severely challenging for other organisms to survive.

This research is significant as drought episodes are predicted to double in frequency by 2060 as climate change progresses and El Niño events become more extreme, causing a reduction in precipitation and therefore a 10% decrease in annual freshwater input to the Reloncaví Fjord.

Thus, the worsening proliferation of algal blooms is likely to have a detrimental impact on Patagonia’s fjord biodiversity and aquaculture industry in years to come.

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