Extreme conditions and ecosystem disruptions of the relationships between phytoplankton, seagrass and macroalgal communities in the Indian River Lagoon, Florida, USA, 1997-2024
This study documents how extreme climatic events in the Indian River Lagoon, specifically the 2010 cold snap and the 2017-18 hurricane period, triggered cascading ecosystem disruptions that fundamentally altered the balance between phytoplankton, seagrass, and macroalgal communities over the 1997–2024 period.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
Coastal lagoons are shallow, sheltered bodies of water where the ocean meets the land, acting as nurseries for fish and filters for the environment. In these ecosystems, life depends on a delicate balance between three main types of plants: microscopic algae floating in the water, grasses rooted in the sand, and larger seaweeds that drift or grow on the bottom. These groups compete for the same resources, primarily sunlight and nutrients like nitrogen and phosphorus. When the water is clear and nutrient levels are moderate, the grasses and seaweeds usually thrive, forming the foundation of the food web. However, when too many nutrients enter the water, the floating microscopic algae can multiply rapidly, turning the water green or brown and blocking the sunlight needed by the plants on the bottom. This shift can collapse the entire ecosystem, but scientists have long wondered if sudden, extreme weather events could trigger these changes just as quickly as slow pollution buildup.
For twenty-eight years, researchers have been watching the Indian River Lagoon in Florida, a long, narrow stretch of water running along the state's east coast. This system is unique because it is very shallow and has few connections to the open ocean, meaning the water stays in place for months at a time. This slow movement allows nutrients to accumulate and algae to grow if conditions are right. A team of scientists, led by researchers at the University of Florida and the St. Johns River Water Management District, spent nearly three decades tracking the health of this lagoon. They measured the temperature, salinity, and clarity of the water, counted the different types of algae, and mapped the coverage of seagrass and seaweed at eight specific locations. Their goal was to understand how the balance between these different plant communities changed over time and what caused those changes.
The study revealed that the lagoon did not change gradually. Instead, it experienced two dramatic shifts in its biological makeup, each triggered by an extreme weather event. The first major turning point occurred around 2010 and 2011. During those winters, the region suffered from unusually cold temperatures, with water dropping to near freezing for extended periods. This cold snap killed off a significant amount of marine life, including fish and plants, both in the water and in the surrounding mangrove forests. As this dead organic matter decomposed, it released a massive pulse of nutrients back into the water. This sudden influx of food, combined with the slow-moving water, fueled a decade-long explosion of microscopic algae.
For ten years, from 2011 to 2020, the lagoon was dominated by these tiny floating plants. The water became so thick with algae that sunlight could barely penetrate the surface. The researchers found that the water clarity dropped so low that seagrass, which needs bright light to survive, could not grow. The seagrass beds, which had been the dominant feature of the lagoon for decades, withered and disappeared from large areas. The floating algae were mostly very small types, including a brown tide species that thrived in these nutrient-rich, low-light conditions. This period represented a complete reversal of the ecosystem: the bottom-dwelling plants were replaced by a floating carpet of microscopic life.
The second major shift began around 2021, driven by a different kind of extreme weather: a series of powerful hurricanes. These storms brought heavy rains and high winds, which washed away the remaining thick layers of algae and stirred up the water. As the water cleared, the seagrass began to make a slow return in some areas. However, the lagoon did not simply return to its old state. In many places, a different type of seaweed, a benthic macroalga known as Caulerpa, took over the empty space on the bottom. This seaweed is very aggressive and can grow quickly in the open, sandy habitats left behind by the dying seagrass. While the seagrass started to recover in the southern parts of the lagoon, the northern sections saw a massive surge in this Caulerpa seaweed, which now covers the bottom in dense mats.
The researchers used advanced computer models to confirm that these changes were not random. They found that the cold temperatures of 2010 were the primary driver that kicked off the decade of algae blooms, while the hurricanes of 2017 and 2022 helped clear the water and allowed the seaweeds to colonize the newly available space. The study suggests that the ecosystem is now in a new state, where the balance between floating algae, seagrass, and seaweed is different from what it was before 2010. The recovery of the seagrass is not guaranteed; in some areas, the fast-growing seaweed is outcompeting the grasses for space.
This long-term record shows that coastal ecosystems are not just slowly drifting toward a new normal due to pollution or climate change. They can be jolted into a completely different state by single, extreme events. The cold winters and the hurricanes acted as reset buttons, but they did not reset the system to how it was before. Instead, they pushed the lagoon into a new configuration where the relationships between the plants have fundamentally changed. The findings highlight that while reducing pollution is essential, the increasing frequency of extreme weather events poses a new and unpredictable threat to the stability of these vital coastal habitats. The future of the Indian River Lagoon will depend on how these competing plant communities respond to the next extreme event, whether it be a cold snap, a storm, or a period of intense heat.
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