According to Carol Thornber, associate professor of biological sciences, an abundance of seaweed shows up on the state’s beaches just about every year. It can be especially problematic on those beaches located in small coves where the shape of the coastline retains seaweed once it arrives rather than allowing it to get flushed out of the area. Easton’s Beach in Newport and the beaches in Greenwich Bay are particularly susceptible.
“Depending on where you are, you are likely to find very different species of seaweed,” Thornber said. “At Easton’s Beach, it’s mostly a species of small red algae that look like puff balls in the water. It most likely starts growing attached to some hard surface, but it can frequently get dispersed, or it gets fragmented in the water by a storm because it is susceptible to breaking into pieces. If it stays at the surface, it drifts in to nearshore waters and continues to grow.”
In Greenwich Bay the culprits are a wide variety of green algae that usually develop blooms as a result of nutrients running into the Bay. Sometimes when those blooms die, they sink to the bottom and deplete the oxygen in the water, which may have a negative effect on fish populations.
“It’s mostly just a nuisance,” said Thornber. “When it washes up on shore and decays, it may smell and discourage people from going to the beach. If we reduce the quantity of nutrients going in the water, that could reduce the propensity of blooms.”
Often when Rhode Islanders see red seaweed in the water, they think it’s a dangerous red tide, but they needn’t worry. The red tides that cause paralytic shellfish poisoning (PSP) in Maine and Massachusetts waters during some years have not affected Rhode Island waters in several decades.
Red tides are caused by single-celled algae that form dense aggregations and may appear red, but the algae are so small that you can’t pick them up from the water. While there are many types of red tides, the dangerous variety produces a toxin that can accumulate in shellfish.
“Those toxins do not seem to affect the shellfish, but they do affect humans who eat contaminated shellfish,” said Lucie Maranda, associate marine research scientist at the Graduate School of Oceanography. “For example, the microalgae responsible for PSP produces a family of toxins that can cause a variety of symptoms, including tingling in the fingers and lips, and, if you’ve eaten a lot of it, you’ll have trouble breathing.”
Maranda emphasizes that while the species of algae that cause red tides are often present in Rhode Island waters, so far they have not been present in large enough quantities to cause concern. The closest thing to a red tide to have affected Rhode Island recently was a “brown tide” that affected area waters in the mid-1980s. That occurrence wasn’t toxic to people or shellfish, but it was so abundant that it made it difficult for shellfish to feed and led to their starvation. It also caused a depletion of oxygen in the water.