Beach replenishment may have ‘far reaching’ impacts on ecosystems

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Sand replenishment at Imperial Beach was one of the eight San Diego beaches examined in the UC San Diego study. Credit: Heather Henter, UC San Diego

UC San Diego biologists who examined the biological impact of replenishing eroded beaches with offshore sand found that such beach replenishment efforts could have long-term negative impacts on coastal ecosystems.

The scientists, who studied the effects of beach replenishment efforts on the abundance of intertidal invertebrates at eight different beaches in San Diego County, discovered that the movement of sand onto those beaches resulted in a more than twofold reduction in the abundance of intertidal invertebrates after 15 months.

“We found rather long lasting declines in invertebrate abundances due to replenishment,” said Joshua Kohn, a professor of biology who headed the study, which was published this week in the journal Estuarine, Coastal and Shelf Science. “These invertebrates are what shorebirds eat when they poke their bills in the sand. They are also food for small inshore fish.”

“Such reductions may have far reaching consequences for sandy beach ecosystems,” the researchers warn in their paper, “as community declines can reduce prey availability for shorebirds and fish.”

While other researchers had previously looked at the impact of beach replenishment on certain taxa of invertebrates at specific beaches, the UC San Diego study is unusual in that it examined the impact on the total invertebrate community across eight different beaches in San Diego County from Oceanside south to Imperial Beach. The UC San Diego biologists were also able compare their results at each beach where sand was pumped onshore to an untreated section of the beach, which served as a control.

The opportunity to conduct the study came about in the fall of 2012 when the San Diego Association of Governments, or SANDAG, and the U.S. Army Corps of Engineers embarked on an ambitious project to replenish eight beaches with a total of 1.76 million cubic meters of sand. Only portions of each beach were replenished so that other sections of the beach could be used for foraging birds and fish.

“This provided the strongest experimental system yet to assess the effects of replenishment,” said Kohn. “With replenished and control sections of each beach, we could assess both the general effects of replenishment as well as variation among beaches in their invertebrate communities and responses to replenishment.”

To conduct their study, Kohn and the other co-authors of the study, Heather Henter, a biologist and academic coordinator of the Natural Reserve System, and Tyler Wooldridge, a graduate student, enlisted the help of undergraduates in UC San Diego’s Environmental Systems program to sample the replenished and control sections of each of the eight beaches starting in the fall of 2012. The students then resampled those sections four months later, 12 months later and 15 months after the initial replenishment.

The eight beaches sampled from north to south were South Oceanside Beach, North Carlsbad Beach, South Carlsbad Beach, Batiquitos Beach, Moonlight Beach, Cardiff State Beach, Fletcher Cove and Imperial Beach.

“When people look at the sandy beach it looks like nothing could possible live there,” said Henter. “It looks devoid of life. But when you actually dig down into the sand, there are a lot of creatures. This seems odd because there is no primary production on the beach. No plants grow there so there should be nothing to eat. But the sandy beach animals feed on seaweed and detritus cast ashore and plankton that washes in from the ocean.”

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