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Climate Change

Water temperatures drive phytoplankton blooms in coastal zones

Journal source: Trombetta T, Vidussi F, Mas S, Parin D, Simier M, Mostajir B (2019) Water temperature drives phytoplankton blooms in coastal waters. PLoS ONE 14(4): e0214933. https://doi.org/10.1371/journal.pone.0214933

Phytoplankton: The cornerstone of marine food webs

Let’s talk about phytoplankton: these tiny wonders are the foundation of marine food webs because they use sunlight to transform nutrients that can sustain organisms higher up in the food chain.

A snapshot of the different types of phytoplankton found in our ocean ecosystems. Featured in this photo are circular and elongate forms of diatoms. Photo credit: Wikimedia Commons.

Although an individual phytoplankton is invisible to the naked eye, there are times of the year when phytoplankton communities grow rapidly, producing blooms in colors so vibrant that they can be seen from space!







A satellite image of a phytoplankton bloom, with swirling green hues off the coast of New Jersey. Photo credit: earthobseratory.nasa.gov via Wikimedia Commons.

These blooms are especially prominent in coastal ecosystems, such as estuaries, coral reefs, seagrass beds, and continental shelves. Blooms are highly productive periods that usually occur in the spring and fall, and provide a pulse of food to support high numbers of grazing zooplankton, which in turn are food for fish.

While blooms are critical events that help sustain coastal marine food webs, their underlying causes are not fully understood. Many scientists agree that factors such as nutrients and weather events contribute to bloom occurrence in coastal areas. Knowing the major driver of phytoplankton blooms is important because scientists are concerned that climate change will impact their timing and productivity. Such changes would undoubtedly affect coastal marine food webs and potentially impact the survival and hatching time of commercially important species, such as anchovies and tuna.


Investigating the drivers of blooms

Trombetta and colleagues of the University of Montpellier in France studied phytoplankton blooms at Thau Lagoon, a coastal site that hosts abundant oyster farms on the French coast in the northwest Mediterranean Sea.

Map of the study location, off the coast of France on the Mediterranean Sea. Photo credit: Figure 1 from Trombetta et al. 2019 under Creative Commons License.

From 2015 to 2016, the researchers set up water sensors that recorded daily water temperatures, salinity, winds, dissolved oxygen, and nutrient concentrations. They also measured chlorophyll-a concentrations in the water – that is, a pigment present in many phytoplankton that allows them to capture sunlight for energy, often used as an approximation for how many phytoplankton are in the water. They also took weekly water samples and looked at them under a microscope to track changes to the composition of phytoplankton species. By measuring such an extensive suite of variables, the researchers hoped to tease apart the main drivers of phytoplankton blooms.

How do you define a bloom based on the data collected from this study? Using the chlorophyll-a data, the researchers identified bloom periods based on consecutive days of positive growth, or increases in chlorophyll-a values from one day to the next. When chlorophyll values decreased, that signaled the end of a bloom.

What did they find?

Out of all of the variables examined in this study, water temperatures played the most significant role in determining when blooms occurred. The authors found that blooms in 2015 and 2016 occurred under a wide range of temperatures, which suggests that there is not a target temperature that triggers blooms; rather, they are initiated by a general trend of increasing temperatures.

Wind conditions and salinity were also associated with bloom events, but not as strongly as water temperature. Nutrients, which are required for phytoplankton growth, were not strongly linked with the onset of phytoplankton blooms. This could be due to frequent winds stirring up sediments that contain nutrients, generating a fairly constant nutrient supply to the shallow waters. Salinity was positively associated with blooms, and the authors hypothesized that seasonal inflows of saltier water from the open ocean brought in nutrients that enhanced phytoplankton growth.

Temperature is key

This study demonstrated that temperature has the most direct and significant impact on phytoplankton growth dynamics. When there is plenty of light available, higher temperatures increase the rate at which phytoplankton convert light energy into carbon, the building block of life. Furthermore, when nutrients are readily available, higher temperatures allow phytoplankton to speed up nutrient uptake, which allows them to grow and reproduce.

The authors also found that the winter of 2015/2016 was the warmest recorded in France, leading to abnormally high winter water temperatures. Cold winter temperatures are characteristic of Thau Lagoon, and these were recorded in the winter of 2014/2015. As a result of the differences in winter temperatures between the two years, phytoplankton abundances and bloom dynamics in spring 2016 were completely different from those in 2015. The bloom of 2016 was smaller and occurred later in the spring than the 2015 bloom.

Time series graph of the chlorophyll a levels measured during the study. The top panels show the blooms in 2015 and 2016. The graph shows how the 2016 bloom took longer to occur than in 2015. The bottom panel shows phytoplankton growth rates throughout the study. Photo credit: Figure 2 from Trombetta et al. 2019 under Creative Commons license.

In other marine systems, larger spring blooms usually occur after cold winters, while smaller blooms occur after mild winters. This suggests that water temperatures during the winter months are an important component influencing the dynamics of phytoplankton spring blooms in shallow coastal regions. Colder water temperatures tend to reduce feeding activities of grazing and filter-feeding organisms, which allows phytoplankton growth to outpace losses by grazing and increase their productivity. With climate change, mild winters such as the one in 2016 could become more frequent, and may reduce phytoplankton abundance and further delay spring blooms. This could lead to changes in other components of the food web. These changes become especially important in shallow coastal zones where blooms may affect fish and shellfish production.

While the authors found very different bloom dynamics in their study site, the findings reported here are based on just two years of data. The authors advise that long-term studies are needed to determine if the same trends hold true in other coastal ecosystems.

The bigger picture: What do warmer conditions mean for phytoplankton and coastal food webs?

Warmer water temperatures will lead to changes in the phytoplankton community, which will directly impact the whole food web and ecosystem functioning. In addition to the slower accumulation of phytoplankton to produce spring blooms, warmer waters may induce shifts in the species comprising these blooms. The authors noted that smaller species of Cyanobacteria (blue-green algae) dominated the bloom in 2016, while larger species, many of them diatoms, dominated in 2015. In particular, Cyanobacteria tend to increase in abundance with warmer temperatures. The warmer temperatures sustained during the 2015/2016 winter were likely an important factor explaining the dominance of small cyanobacteria. The potential for Cyanobacteria to become more prominent with warming temperatures may be worrisome because many of these species can produce toxins and can be a human health hazard.

This study makes it clear that phytoplankton dynamics are primarily driven by water temperatures. While warmer waters can be beneficial for phytoplankton growth, warming during winter months could lead to serious consequences for coastal food webs.





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