Green seaweed replaces seagrass, but slugs pose new threats

Seagrasses are critical to coastal ecosystems – offering habitat, stabilizing the seafloor and buffering wave energy – but globally they’re increasingly under threat. Beginning in 2011, a series of intense algal blooms – fueled by pollution and nutrient overload – wiped out vast stretches of vital seagrass beds in Florida’s Indian River Lagoon (IRL).

In the wake of this collapse, the fast-growing green seaweed Caulerpa prolifera has spread across areas once dominated by native seagrass species like Halodule wrightii, filling in the empty seabed. The IRL is home to seven seagrass species, which historically covered most of its sandy bottom, so seaweed becoming the dominant seafloor cover represents a major ecological shift.

As this pollution-tolerant macroalgae replaces seagrass, it may be reshaping marine life and reducing biodiversity. Although scientists studied C. prolifera’s habitat value in the 1980s, its current role remains unclear. Could this opportunistic green algae now provide a lifeline for marine species in a changing ecosystem?

Researchers from Florida Atlantic University’s Harbor Branch Oceanographic Institute surveyed the small animals living in and around this green seaweed in the lagoon between 2020 and 2021 during a period of extreme seagrass scarcity. They studied four sites, measuring both seaweed cover and the diversity of marine life. Their goal: to assess how effectively C. prolifera supports marine communities in the absence of seagrass.

Published in Marine Biology, results reveal that C. prolifera coverage on the lagoon floor varied over time. The small animals living within these beds were similar to those historically found in seagrass beds, but their abundance was notably lower – an important insight for resource managers focused on habitat quality and ecosystem recovery in the IRL.

“Results from our study suggest that there has been a noticeable decline in the number of marine animals typically found in seagrass habitats,” said Rachel Brewton, Ph.D., senior author and an assistant research professor, FAU Harbor Branch. “Our research zeroed in on the smaller, resident species that are critical to the ecosystem. This shift in focus reveals just how much these vital species have been affected.”

Researchers also discovered that red drift algae – collected incidentally – supported many of the same species as C. prolifera, suggesting it may also provide valuable habitat. In the absence of seagrass, both types of macroalgae could serve as temporary refuges for estuarine life, though with fewer animals than in the past. These findings may help guide restoration efforts in other regions affected by seagrass loss and rising macroalgal blooms.

“One concern for resource managers is that Caulerpa prolifera might pose risks to animals that rely on it for food or shelter,” said Brian Lapointe, Ph.D., co-author and a research professor at FAU Harbor Branch. “This algae contains a toxin, caulerpenyne, which can harm certain species like sea urchins and mosquito fish. While many animals avoid eating it, there’s evidence that manatees, whose diet shifted from seagrass to macroalgae after seagrass loss, suffered from malnutrition and lethal infections. Similarly, the decline in seagrass was linked to stress in bottlenose dolphins, as it affected the availability of their preferred prey.”

Following this study of C. prolifera dominance in the IRL about five years ago, the researchers recently noticed something very interesting – sea slugs called Elysia subornata.   

“These small, green sap sucking sea slugs have become very numerous in the lagoon,” said Brewton. “The complete decimation of a C. prolifera bloom in the lagoon during the late 1980s was previously attributed to these slugs, and now, we have seen them eliminate the cover at one of our study sites at Turkey Creek. We’re currently monitoring their expansion at another site from this latest study in the journal, Marine Biology, to document how they spread and how quickly they can consume a C. prolifera meadow.”

Emory Tudor, an FAU Harbor Branch Link Foundation summer intern, became interested in how these tiny slugs might facilitate major ecosystem shifts and will be studying their feeding rates and observing their development in the lab over the next month.

“If these slugs eliminate C. prolifera beds again, the consequences for local fisheries could be dire, although this decline might also create opportunities for seagrass recolonization,” said Tudor. “Understanding how these slugs move and how quickly they consume C. prolifera will be critical in the coming months if their spread continues.”

The researchers say that given the global decline in seagrass, further research on macroalgal habitats and their role in supporting fish populations is crucial, especially in places like the IRL. In addition, improving wastewater treatment and stormwater management to reduce nutrient pollution should continue to be a key focus for protecting the lagoon’s health.

“While Caulerpa prolifera has offered some temporary hope as a habitat substitute in the absence of seagrass, there is still much we don’t know,” said Brewton. “We need to better understand the long-term impacts of these algae blooms, including their effects on water quality, faunal populations and the potential for seagrass recovery. As these blooms continue to play a role in the ecosystem, more research is needed to fully grasp how they affect the lagoon and its wildlife. For example, these changes may be negatively impacting sportfish populations.”

This work was supported by the Harbor Branch Oceanographic Institute Foundation through the Florida Center for Coastal and Human Health and the Indian River Lagoon Graduate Research Fellowship program (2019-22).

- FAU -

About Harbor Branch Oceanographic Institute:
Founded in 1971, Harbor Branch Oceanographic Institute at Florida Atlantic University is a research community of marine scientists, engineers, educators, and other professionals focused on Ocean Science for a Better World. The institute drives innovation in ocean engineering, at-sea operations, drug discovery and biotechnology from the oceans, coastal ecology and conservation, marine mammal research and conservation, aquaculture, ocean observing systems and marine education. For more information, visit www.fau.edu/hboi.

About Florida Atlantic University:
Florida Atlantic University, established in 1961, officially opened its doors in 1964 as the fifth public university in Florida. Today, Florida Atlantic serves more than 30,000 undergraduate and graduate students across six campuses located along the Southeast Florida coast. In recent years, the University has doubled its research expenditures and outpaced its peers in student achievement rates. Through the coexistence of access and excellence, Florida Atlantic embodies an innovative model where traditional achievement gaps vanish. Florida Atlantic is designated as a Hispanic-serving institution, ranked as a top public university by U.S. News & World Report, and holds the designation of “R1: Very High Research Spending and Doctorate Production” by the Carnegie Classification of Institutions of Higher Education. Florida Atlantic shares this status with less than 5% of the nearly 4,000 universities in the United States. For more information, visit www.fau.edu.