Each winter, thousands of blacktip sharks (Carcharhinus limbatus) migrate to the clear, shallow waters off South Florida, where they are easily spotted from the air – a movement that coincides with seasonal beach nourishment projects.
In northern Palm Beach County, beach nourishment – adding sand from nearby or offshore sources, such as Jupiter Inlet – is conducted nearly every year to widen beaches, protect shorelines and provide habitat and recreational space.
While these projects are beneficial, repeated nourishment can create prolonged turbidity – cloudy or murky water caused by suspended sediment that persists much longer in areas with silts and clays. Despite its potential impact, research on how the long-term ecological effects of extended turbidity can affect many nearshore species, especially sharks, is limited.
To investigate how large, long-lasting plumes of murky water coincide with the annual blacktip shark migration, Florida Atlantic University researchers tracked the size, duration and timing of turbidity events during the 2020 and 2021 seasons. The research took place along northern Palm Beach County's coastline, spanning both nourished (managed) and natural (unmanaged) beaches in a dynamic, wave-driven environment.
Over two years, researchers tracked how beach nourishment and sediment movement affected nearshore conditions and marine life. They combined monthly aerial surveys – flying low over the coast to capture more than 10,000 images – with underwater camera stations placed at varying distances from shore.
The aerial data were used to map water clarity and sediment plumes, while the underwater cameras recorded shark presence and fish diversity. This study provides one of the clearest looks yet at how coastal engineering projects intersect with marine life – and human safety – along South Florida's busy shoreline.
Results of the study, published in the Journal of Coastal Research , found a consistent pattern: thousands of blacktip sharks gather in nearshore waters each year, tightly hugging the coastline within about 50 meters of shore, where prey is concentrated. This strong preference for shallow, nearshore waters suggests that prolonged turbidity could interfere with the sharks' ability to hunt and may increase the likelihood of accidental encounters with humans.
"What's striking is how closely the distribution of blacktip sharks overlaps with areas impacted by turbidity events," said Stephen Kajiura , Ph.D., co-author and a professor of biological sciences in FAU's Charles E. Schmidt College of Science . "Because these sharks rely on clear water to visually hunt, sustained reductions in visibility can change where they go, how successfully they feed, and how they interact with their environment. It also has implications for people, since these sharks are present in large numbers right where we swim."
Originally designed to examine how blacktip sharks use nearshore habitats – and how beach nourishment might influence them – this study instead revealed a more immediate and disruptive factor: widespread, persistent turbidity. Over two years, researchers documented 24 sediment plumes, some stretching nearly 15 kilometers alongshore and extending more than 250 meters offshore, dramatically reducing water clarity.
"Beach nourishment is a critical tool for combating erosion and protecting infrastructure, but in some cases it may come with tradeoffs that we're only beginning to fully quantify," said Tiffany Roberts Briggs , Ph.D., co-author, chair and associate professor of geosciences in FAU's Charles E. Schmidt College of Science. "These projects involve dredging and placing large volumes of sediment onto the beach, and what we observed was turbidity in the adjacent nearshore on a scale that exceeded what's typically described in the literature – with plumes stretching for kilometers and persisting across the exact time and space where marine species are most active. That tells us we need to take a closer look at how the entire process of beach nourishment may be influencing marine habitat."
The research underscores a delicate balance: beach nourishment protects coastlines from erosion, but it may also disrupt key ecological processes and increase risks in ways that are not yet fully understood. The study calls for improved monitoring, refined sediment management practices, and a closer look at how these projects affect both marine ecosystems and coastal communities.
"The consequences are significant. Reduced water clarity can limit our ability to track sharks, but more importantly, it may alter their behavior," said Kajiura. "Because these sharks play a key ecological role along the U.S. East Coast, understanding how changing conditions – especially reduced visibility – affect their feeding, aggregation and migration is critical."
Blacktip sharks have a remarkably predictable migration, returning to the same nearshore waters each winter to feed before heading north. Their numbers peak in late winter – February to March – before they migrate north by late spring, a movement strongly driven by rising water temperatures.
Study co-authors are Nicholas Brown, Ph.D., a Ph.D. graduate of FAU's Department of Geosciences; and Mackenzie Smith, a doctoral student in FAU's College of Engineering and Computer Science .
- FAU -
About Florida Atlantic University:
Florida Atlantic University serves more than 32,000 undergraduate and graduate students across six campuses along Florida's Southeast coast. Recognized as one of only 13 institutions nationwide to achieve three Carnegie Foundation designations - R1: Very High Research Spending and Doctorate Production ," " Opportunity College and University ," and Carnegie Community Engagement Classification - FAU stands at the intersection of academic excellence and social mobility. Ranked among the Top 100 Public Universities by U.S. News & World Report, FAU is also nationally recognized as a Top 25 Best-In-Class College and cited by Washington Monthly as "one of the country's most effective engines of upward mobility." To learn more, visit www.fau.edu .
