Shrinking Shellfish? Risks of Acidic Water in the Indian River Lagoon

FAU researchers measured aragonite saturation - a key indicator of water's ability to support calcifying organisms like clams and oysters - throughout the Indian River Lagoon.

Florida's Indian River Lagoon (IRL), one of the state's most ecologically productive estuaries, is facing a growing but invisible threat that could reshape its marine ecosystems. Over the past decade, the lagoon has suffered severe degradation caused by nutrient pollution, excessive freshwater runoff, harmful algal blooms (HABs), and declining water quality. These changes have led to the loss of tens of thousands of acres of seagrass and have negatively impacted shellfish, fish, dolphins, manatees and other key species.

A new study from Florida Atlantic University 's Harbor Branch Oceanographic Institute now reveals that these pressures are also contributing to coastal acidification, a chemical shift in the water that threatens the ability of shell-building marine organisms to grow and thrive.

Many marine animals, including oysters and clams, rely on a mineral called aragonite to build shells and skeletons. Scientists measure the water's ability to support aragonite using aragonite saturation (Ωarag).

To understand these changes, FAU Harbor Branch researchers studied the IRL from 2016 to 2017, measuring Ωarag and other water chemistry factors. They examined how nutrients, freshwater inputs, and other environmental conditions affect the lagoon's ability to support shell-building marine life.

The study used two approaches. First, researchers conducted a broad survey across the lagoon, from nutrient-rich northern areas to southern regions affected by freshwater inflows. Second, they did weekly sampling at three central sites with different salinity and land-use conditions: an urban-influenced canal, a river mouth affected by urban and agricultural runoff, and a relatively natural reference site with strong ocean exchange.

Results of the study, published in the journal Marine Pollution Bulletin, revealed clear patterns. Northern sites with high nutrient concentrations and frequent HABs had lower aragonite saturation. Southern sites, influenced by freshwater discharges, also had lower Ωarag, primarily due to reduced salinity and dilution of aragonite. In the weekly surveys, Ωarag was positively correlated with salinity and negatively correlated with nutrient levels, confirming that both freshwater input and nutrient pollution play a role in controlling water chemistry.

"For shell-building organisms, the consequences are clear," said Rachel Brewton, Ph.D., co-author and an assistant research professor, FAU Harbor Branch. "When aragonite saturation drops, growth slows and shells become weaker, leaving animals more vulnerable to predators, disease and stress. Over time, this can disrupt the food web, affecting fish, dolphins, and the human communities that rely on these species. Shrinking shellfish are more than a curiosity - they're a warning for the entire ecosystem."

This research provides the first comprehensive documentation of aragonite saturation throughout the entire IRL, filling a critical gap in our understanding of coastal acidification in shallow estuaries. Prior studies focused on nutrient pollution, algal blooms, or freshwater inflows, but none had examined how these factors interact to impact the water's chemistry and the health of shell-building organisms.

"Coastal acidification occurs when carbon dioxide, CO₂, from the atmosphere or from biological processes, such as microbial activity associated with decaying algae, dissolves in seawater. This CO₂ reacts with water to form carbonic acid, which lowers the water's pH and reduces the amount of carbonate ions available for shell-building," said Brian Lapointe, Ph.D., senior author and a research professor, FAU Harbor Branch. "In addition to atmospheric CO₂, nutrient pollution from urban runoff, agricultural sources, and wastewater can fuel algae growth. When these algae die and decompose, the process produces more CO₂, further acidifying the water."

Other factors, such as freshwater inflows from rivers and canals, also influence aragonite saturation by diluting the water and lowering salinity and mineral concentrations. In shallow estuaries like the IRL, where water circulation is slower than in the open ocean, these effects are amplified, creating localized hotspots where shell-building organisms are especially at risk.

The results have broader implications. Estuaries worldwide are experiencing similar pressures from population growth, land-use changes, stormwater runoff, and nutrient pollution.

"By identifying the environmental conditions that lower aragonite saturation, we can start to develop strategies to mitigate coastal acidification," said Megan Conkling, Ph.D., first author and research scientist at FAU Harbor Branch. "Managing nutrient inputs and freshwater flows more carefully could help protect oysters, clams, seagrass, and other critical species. Our work provides a roadmap for designing restoration and mitigation efforts, not just in Florida, but in estuaries around the world."

The study also emphasizes the importance of ongoing monitoring. FAU Harbor Branch's Indian River Lagoon Observatory Network of Environmental Sensors (IRLON) has been upgraded to track pH and CO₂ levels, which allows scientists to calculate aragonite saturation in near real-time. This IRLON data can help forecast future changes, identify vulnerable species and habitats, and guide targeted management actions.

"Protecting the Indian River Lagoon requires understanding not just what we can see on the surface, like algae blooms or seagrass loss, but also the invisible chemical changes affecting marine life," said Conkling. "This study provides essential insight into one of the less visible but critical threats facing estuaries today."

Study co-authors are Bret R. Kaiser; and Kristen S. Davis, IRLON manager, both with FAU Harbor Branch; and Mingshun Jiang, Ph.D., associate research professor, FAU Harbor Branch.

This research was supported by the Harbor Branch Oceanographic Institute Foundation through the Saves Our Seas Specialty License Plate Program awarded to Lapointe.

-FAU-