In the shimmering coastal waters of Florida’s Indian River Lagoon (IRL), a silent yet transformative ecological drama is unfolding. Over the past decade and a half, this once-thriving marine ecosystem has witnessed a catastrophic decline in seagrass coverage, with far-reaching consequences for its biodiversity and ecological resilience. Seagrasses, renowned for their critical role in providing habitat, stabilizing sediments, and buffering coastal shorelines against erosive wave action, have been decimated chiefly due to a persistent series of intense algal blooms starting in 2011. These blooms, driven by elevated nutrient pollution from wastewater discharge and agricultural runoff, have fundamentally altered the substrate of the lagoon, allowing opportunistic macroalgae to colonize vast expanses that were historically dominated by native seagrasses.
One macroalgal species, Caulerpa prolifera, a prolific green seaweed, has aggressively filled the ecological niche once held by seagrasses such as Halodule wrightii. This transition represents a profound shift in benthic habitat composition and function, given that the IRL historically supported seven distinct seagrass species covering much of the sandy lagoon floor. The dominance of Caulerpa prolifera signals a potential reorganization of the ecosystem’s foundational structure, raising pressing questions about the capacity of this green macroalgae to support marine faunal communities in ways comparable to the extinct seagrass meadows.
Recent research conducted by marine ecologists at Florida Atlantic University’s Harbor Branch Oceanographic Institute provides invaluable insights into this unfolding ecological transformation. Between 2020 and 2021, researchers meticulously surveyed microbial and meso-faunal assemblages within and surrounding Caulerpa prolifera meadows at four distinct lagoon sites, where seagrass abundance had precipitously declined. Their field observations, coupled with quantitative analyses, reveal that the faunal communities inhabiting Caulerpa prolifera beds retain compositional similarities to historic seagrass-associated fauna but exhibit significantly reduced abundances. These findings underscore a critical degradation of habitat quality, with potential repercussions for the broader estuarine food web and ecosystem services such as fisheries productivity.
Published in the journal Marine Biology, this observational study marks a pivotal contribution to our understanding of macroalgal colonization dynamics following seagrass loss. The researchers emphasize that while Caulerpa prolifera provides a habitat refuge during seagrass scarcity, it is an imperfect substitute. The reduction in small, resident animal populations—organisms integral to nutrient cycling, prey availability for higher trophic level species, and overall ecological interactions—highlights a loss of biodiversity and ecosystem functionality. This diminished faunal density is a red flag for resource managers aiming to restore the IRL’s ecological integrity.
Compounding concerns regarding the ecological role of Caulerpa prolifera is its biochemical composition. Unlike seagrasses, Caulerpa species produce caulerpenyne, a terpene toxin that has deleterious effects on certain animals, including sea urchins and mosquito fish. Although many species avoid grazing on this toxic macroalgae, its proliferation has indirect yet significant impacts. Notably, manatees in the lagoon have shifted their diets from seagrass to macroalgae following seagrass die-offs, resulting in malnutrition and increased susceptibility to fatal infections. Similarly, bottlenose dolphin populations, reliant on fish species linked to healthy seagrass habitats, have exhibited stress correlating with seagrass decline, reflecting cascading trophic disturbances.
An intriguing biological agent now playing a role in the modulation of Caulerpa prolifera meadows is the sap-sucking sea slug, Elysia subornata. Long implicated in the historical collapse of Caulerpa blooms in the late 1980s, these small, green gastropods have surged in numbers within the lagoon. Observations from recent studies reveal that Elysia subornata is actively consuming and decimating Caulerpa prolifera stands, with documented eradication at sites like Turkey Creek. Researchers are closely monitoring the gastropod’s expansion to elucidate its feeding rates, dispersal patterns, and ecological consequences.
The resurgence of Elysia subornata presents a paradoxical scenario for the Indian River Lagoon ecosystem. On one hand, these grazing sea slugs may facilitate the restoration of seagrass beds by clearing macroalgal dominance. On the other hand, a swift and extensive reduction of Caulerpa prolifera could destabilize the transient habitat now relied upon by a range of marine species during seagrass scarcity. The delicate balance between macroalgal control and fostering seagrass recovery remains uncertain, necessitating further experimental and longitudinal research to predict outcomes for local biodiversity and fisheries.
Moreover, this situation exemplifies broader challenges facing coastal ecosystems worldwide, where anthropogenic nutrient enrichment triggers harmful algal blooms that disrupt native vegetation and associated faunal assemblages. The IRL case study offers critical lessons in managing nutrient inputs via improved wastewater treatment and stormwater control to mitigate eutrophication and its cascading ecological effects. Tailored restoration strategies must integrate knowledge of species interactions, biochemical pathways, and habitat dynamics to holistically address the compounded crises of seagrass loss and macroalgal proliferation.
The importance of this research extends beyond regional environmental management. It challenges marine ecologists to reconsider the functional roles of macroalgal habitats as potential surrogates in altered coastal ecosystems globally. While Caulerpa prolifera and red drift algae may afford some refuge to estuarine fauna, the long-term implications for water quality, species interactions, and ecosystem resilience remain underexplored. The nuanced balance between supporting biodiversity and controlling toxic macroalgal proliferation must inform conservation priorities moving forward.
In summary, the Indian River Lagoon stands at an ecological crossroad shaped by the interplay of pollution-driven habitat loss, opportunistic macroalgal growth, and biological control agents like Elysia subornata. The pathway of this complex transition will influence the future of biodiversity, fisheries, and ecosystem services in this vital estuarine environment. As research continues, targeted efforts to reduce nutrient pollution, monitor invasive species dynamics, and promote seagrass recovery will be essential to safeguard the IRL’s ecological heritage and ensure the resilience of its marine communities for generations to come.
Subject of Research: Animals
Article Title: Macroalgae filling the habitat void following catastrophic losses of seagrass in the Indian River Lagoon, FL
News Publication Date: 7-May-2025
Web References:
- https://link.springer.com/article/10.1007/s00227-025-04642-3
- https://www.fau.edu/
- https://www.fau.edu/hboi/
References:
Brewton, R. et al. (2025). Macroalgae filling the habitat void following catastrophic losses of seagrass in the Indian River Lagoon, FL. Marine Biology. DOI: 10.1007/s00227-025-04642-3.
Image Credits: FAU Harbor Branch
Keywords: Environmental impact assessments, Conservation biology, Conservation ecology, Ecological restoration, Ecosystem management, Marine conservation, Wildlife management, Wildlife refuges, Marine resources, Wastewater, Water quality, Sewage, Environmental sciences, Environmental chemistry, Pollution, Nitrogen deposition, Water pollution
