Tiny green pigment provides insights into how successive typhoons drive cumulative water, ecosystem changes

A microscopic green pigment can provide major insights into how severe tropical cyclones called typhoons impact water flow and ecosystems. Called chlorophyll a, the pigment is responsible for absorbing light and initiating the photosynthesis process for algae, other plants and some bacteria. The amount of chlorophyll a in a body of water acts as a proxy measurement for the organisms that feed on it, with sharp increases or decreases indicating a disrupted ecosystem.

Typhoons, which produce sustained hurricane-force winds, whip up waterways, often resulting in significant chlorophyll a changes. Now, for the first time, a team of researchers based in China have found that successive typhoons can have cumulative impacts on chlorophyll a density. Their work, which also includes a model that accurately simulated the effects of two real-world typhoons in 2021, was published on Feb. 27 in Ocean-Land-Atmosphere Research.

“Estuarine ecosystem responses to single typhoon events have been well studied, but the impacts of successive typhoons on three-dimensional chlorophyll a — referring to how the pigment appears throughout a vertical water column and not just at the surface — remain understudied, despite successive typhoons being able to drive cumulative hydrological and ecological changes,” said first author Shaojing Guo, a doctoral candidate in the School of Marine Sciences, Sun Yat-sen University, and Southern Marine Science and Engineering Guangdong Laboratory. “We aimed to investigate whether the hydrological and ecological background prior to typhoon passage can lead to different ecological responses, and explore how 3D chlorophyll a responds to successive typhoons.”

The researchers assessed the Pearl River, which is the second-largest river in China and stretches across nearly half a million square kilometers with an annual discharge — the volume of water that moves — of 10,524 cubic meters per second. The Pearl River estuary, where the river flows into the South China Sea, contains three sub-estuaries and eight inlets. More than half of the river discharges into Lingdingyang, the main sub-estuary and the focus area of the study.

“The huge volume of fresh water discharged into this marine environment forms a river plume, and substantial nutrient loading supports a high primary productivity ecosystem,” Guo said. “During the summer of 2021, however, runoff discharge was 46% of the climatological average, the lowest on record in the 2001-22 period.”

According to Guo, the study enhances understanding of how chlorophyll a responds to typhoons in and near the estuary under varying discharge conditions — a significant step toward forecasting variations in the estuarine ecosystem during typhoon events.

“Under ongoing ocean warming and increasing extreme climate conditions, the frequency of successive typhoons and extreme droughts or floods may increase,” Guo said. “Through their cumulative impacts on physical and ecological processes, successive typhoons cause sustained strong disturbances to ecosystems in the estuary. Thus, our next step should pay more attention on the impacts of these events on the estuarine ecosystems in the future, which need to advance the simulating accuracy of our high-resolution physical-ecological model.”

Other contributors include Feng Pan and corresponding author Xueming Zhu, School of Marine Sciences, Sun Yat-sen University & Southern Marine Science and Engineering Guangdong Laboratory; Zuanling Ji, National Marine Environmental Forecasting Center, Key Laboratory of Marine Hazards Forecasting, Ministry of Natural Resources; and Jihua Liu, Institute of Marine Science and Technology, Shandong University.

The Southern Marine Science and Engineering Guangdong Laboratory, the National Natural Science Foundation of China and the Ocean Negative Carbon Emissions Program supported this work.

This low discharge enabled the researchers to assess how chlorophyll a responded to successive typhoons with high sensitivity, as well as to study how varied ecological and hydrological states influenced the pigment. Typhoon Cempaka and Lupit passed through the Pearl River estuary on July 19, 2021, and Aug. 3, 2021, respectively. The researchers collected data from the surrounding waters and found that chlorophyll a significantly decreased as each typhoon passed through, but subsequently recovered in Lingdingyang. It also bloomed in the upper layer of the water, but it decreased in the subsurface layer after Cempaka and further disappeared after Lupit, Guo said.

Using direct observations and data collection from the surrounding waters, a large discharge experiment and a computational model that couples ecological and hydrological conditions, the researchers found that, under low discharge, chlorophyll a varied more in Lingdingyang and at the bottom layer than at other locations or in the upper layer of water.  

“The results show that our model effectively simulates the changes in chlorophyll a during successive typhoons, in which chlorophyll a considerably decreased during the passage of typhoons but subsequently recovered in Lingdingyang and the upper nearshore region,” Guo said. “Furthermore, chlorophyll a budget analyses demonstrate that chlorophyll a changes were controlled by different physical processes in different areas and typhoon periods.”