Clues in the temperature
“This is a plot of the temperature anomaly for the region compared to its average state from late 2022 to mid-2025. The positive and negative anomalies are of interest because the upwelling that fuels harmful blooms is not only associated with nutrients, it is also associated with a decrease in temperature, because the deep waters of the ocean are much cooler than the surface waters. Usually, the stronger the upwelling, the cooler the local waters will be.
Sea surface temperature (SST) anomalies from late 2022 to mid-2025 for the area of the Californian coastline in which the bloom of Pseudo-nitzschia occurred confirms a period of colder-than-average waters before and during the bloom
Credit: Dr Ben Loveday, using data from the Copernicus Marine Service, funded by the European Commission and implemented by Mercator Ocean International
“As you would expect, there is a natural cycle in sea surface temperature with warmer temperatures at the end of summer and cooler at the end of winter. There are also cycles in upwelling intensity with more intense upwelling in the spring and summer.
“Sea surface temperature anomalies in this region can be induced by higher or lower than normal upwelling with stronger upwelling making waters colder than expected for a given time of year. If a negative anomaly occurs during spring, which is typically a strong upwelling period of the year, then it could indicate very strong upwelling.
“That is what happened in this case. From February to April 2025, we see a really strong negative sea surface temperature anomaly just before we had this bloom period, which makes a lot of sense when we think about it mechanistically. When there is a period of more intense upwelling that pumps cold, nutrient-rich water into the surface, the algae begin to make use of those nutrients. That leads to a period when the numbers of algae will grow, but are not yet at hazardous or problematic levels. It takes some time for the algae to use up all the nutrients.
“But that means that period of very cold water, which is detectable from the temperature anomalies, could be considered as an indicator of bloom risk. This is not a guarantee that there absolutely will be an algal bloom or that such a bloom will be toxic, but it does mean there is a flow of nutrients from the deep waters that could cause excessive algal growth. Also, the cold period persisted through the bloom. The nutrients associated with the cold temperatures are probably what sustained the bloom’s relatively high intensity over a long time.
“Basically, any time that you’ve got this negative blue on the bottom of a chart like this, and there is more upwelling, the amount of nutrients in the surface waters increase, which sets the stage for a bloom. It is like putting fertilizer on your lawn and then a week later, your grass grows like crazy.
“The main takeaway of this chart is to consider the implications of temperature anomalies. Although there is a natural cycle of cold and warm periods, there are not always temperature anomalies. Anomalies can be indicators of a change from the ‘normal’ state of the system. For example, there was a cold anomaly in January 2023 and a cold anomaly in January 2025, but we do not see one in 2024.”