Scientists highlight the need to study the types of feeding that toxic microalgae have to maintain a bloom for a long time, considering the new environmental variables generated by climate change.
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, Los Lagos Region, affecting a total of ten farms and causing losses of more than six thousand 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 accumulate. During the night it migrates to the bottom of the water column for nutrients and during the day it rises to the surface to carry out photosynthesis. It generates a decrease in dissolved oxygen, which strongly affects captive fish. In addition, it is capable of producing ichthyotoxins, which affect the gills of 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, which occurred in 1988, when salmon farming began to proliferate in Chile. After this episode, phytoplankton monitoring work began in our country. According to the investigation, the concentrations of microalgae at that time were similar to what happened recently.
“Microalgae blooms continue to increase in frequency, abundance, and diversity,” said Dr. Jorge Mardones, a researcher at the Fisheries Development Institute (IFOP) and the Center for Dynamic Research of High Latitude Marine Ecosystems (IDEAL) of the Austral University of Chile (UACh), lead author of the study.
The marine biologist explained that “there are many physicochemical and biological variables that affect 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 with greater frequency”.
According to the work, 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 rainfall deficit in place. This strongly alters the entry of fresh water into the fjords, which generates disturbances in the water column, mainly stimulating the growth of microalgae that mainly produce phycotoxins.
The work addresses questions about the nutrient load in the fjords of northern Patagonia. A greater eutrophication of these systems has been hypothesized, that is, a greater quantity 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, so a new edge to investigate is opened.
“If there are no excess 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 investigations, on 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 center-south zone of the country and the continuity of activities carried out by humans.
Read the study here.