Researchers in Palm Beach County, Florida, have documented how much beach nourishment projects can disrupt nearshore waters – and they’re worried it could be a problem for both sharks and swimmers.
Beach nourishment is when sand is added to a beach to protect it from erosion and widen recreational areas so people have more space to enjoy the shoreline. This happens in many regions around the world, and in northern Palm Beach County, it takes place every year.
Although this can have positive impacts, such as preventing erosion and storm damage, a new study in Journal of Coastal Research highlights concerns about how this activity might be impacting sharks.
“Beach nourishment is a critical tool for combating erosion and protecting infrastructure, but in some cases it may come with trade-offs that we’re only beginning to fully quantify,” says co-author Tiffany Roberts Briggs, associate professor of geosciences at Florida Atlantic University’s Charles E. Schmidt College of Science.
When sand is added to the shore through beach nourishment works, it can stir up the seabed and cause murky waters (turbidity).
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For two years, researchers monitored turbidity events in northern Palm Beach County through monthly aerial surveys while underwater cameras recorded the presence of sharks and other marine life.
Through more than 10,000 photos, the researchers documented 24 sediment plumes over their research period (2020 and 2021). Some reduced the water quality across huge stretches of water: running almost 15km along the shoreline and out for more than 250m.
The “plumes [stretched] for kilometres and persisted across the exact time and space where marine species are most active,” says Briggs. “That tells us we need to take a closer look at how the entire process of beach nourishment may be influencing marine habitat.”
In particular, they’re worried about sharks. Blacktip sharks (Carcharhinus limbatus) migrate to these nearshore waters looking for food every winter, congregating in their thousands in the same areas made murky by the beach nourishment.
“What’s striking is how closely the distribution of blacktip sharks overlaps with areas impacted by turbidity events,” says co-author Stephen Kajiura, professor of biological sciences in FAU’s Charles E. Schmidt College of Science.
This could be problematic for both sharks and humans. “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,” says Kajiura. “It also has implications for people, since these sharks are present in large numbers right where we swim.”
The scientists want to see better monitoring so we can understand the true impact of these beach replenishment initiatives.
“Reduced water clarity can limit our ability to track sharks, but more importantly, it may alter their behaviour,” adds Kajiura. “Because these sharks play a key ecological role along the US East Coast, understanding how changing conditions – especially reduced visibility – affect their feeding, aggregation and migration is critical.”
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