During the months of March and April 2021, thousands of salmon died as a result of a microalgae bloom that occurred in the Comau Fjord, in the Los Lagos region, affecting ten salmon farms and causing losses of more than 6,000 tons. The appearance of the raphidophyte Heterosigma akashiwo took the aquaculture industry by surprise.

 

This microalgae creates a brown coloration in the waters where it proliferates, due to the large biomass of cells that it manages to accumulate. During the night it migrates to the bottom of the water column in search of nutrients and during the day it rises to the surface to photosynthesize. It generates a decrease in dissolved oxygen, which strongly affects fish in captivity. In addition, it is capable of producing ichthyotoxins, which affect the gill of the fish; however, the type of toxin and the factors that alter the intensity and frequency of production are still unknown.

 

Although what happened in the Comau Fjord is not a high-frequency event, there were records of a massive fish mortality in nearby areas in 1988, when salmon farming began to proliferate in Chile. After this episode, phytoplankton monitoring work began in our country. According to the research, the concentrations of microalgae at that time were similar to what happened recently. 

 

"Microalgae blooms continue to increase in frequency, abundance and diversity,” says Dr. Jorge Mardones, a researcher at the Instituto de Fomento Pesquero (IFOP, "Fisheries Development Institute" in English) and the Centro de Investigación Dinámica de los Ecosistemas Marinos de Altas Latitudes (IDEAL, "Research Center Dynamics of High Latitude Marine Ecosystems" in English) of the Universidad Austral de Chile (UACh), lead author of the study.

The marine biologist explains that "there are many physicochemical and biological variables that influence the appearance of these microalgae blooms and that converge at the exact moment for them to be generated. However, it is true that these microorganisms are appearing more frequently."

 

According to this study, the effects of climate change continue to be a determining factor in elucidating what happened during those months. 2021 was the second driest year in seven decades and there is currently a precipitation deficit at this site. This strongly alters the inflow of freshwater into the fjords, which generates disturbances in the water column, mainly stimulating the growth of microalgae that produce mostly phycotoxins. 

 

The work addresses questions about nutrient loading in northern Patagonian fjords. An increased eutrophication of these systems has been hypothesized, i.e., more of these organisms in the water column generated by human activity, generally associated with the aquaculture industry. However, the work ruled out an excess in the fjords studied, which opens a new avenue for research.

 

"If there is no excess of nutrients to feed this bloom, how was the biomass of toxic microalgae maintained for so long? We have the hypothesis, which also opens up new research, about the role of mixotrophy, a form of mixed feeding where the amount and type of bacteria could play a very important role in this type of event," said Dr. Mardones.

 

The study published in the scientific journal Progress in Oceanography highlights the need to constantly study the response of Harmful Algal Blooms (HABs) to new environmental variables, considering the expected increase in droughts in the central-southern part of the country and the continuity of human activities.

 

Read the study here.