Coastal managers may need to rethink beach replenishment
Pumping sand ashore to beef up beaches appears to have long-lasting effects on coastal ecosystems, according to UC San Diego biologists who studied the issue across eight different beaches in San Diego County from Oceanside south to Imperial Beach.
Moving sand from offshore caused a twofold reduction in the abundance of certain invertebrate species living in the intertidal zone. That means less food for shorebirds and small fish that live near the shoreline, the scientists said.“Such reductions may have far reaching consequences for sandy beach ecosystems,” the researchers warned in findings, published this week in the journal Estuarine, Coastal and Shelf Science.
The study was launched in 2012, when San Diego-area governments and the U.S. Army Corps of Engineers decided to 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 UC San Diego biology professor Joshua Kuhn. “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.”
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 Heather Henter, a biologist and academic coordinator of the Natural Reserve System. “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.”
“In San Diego there are multiple species of tiny worms called polychaetes,” Henter said. “Little bean clams, Donax gouldii, are sometimes on our beaches by the thousands and there are various crustaceans such as amphipods (sandhoppers) and mole crabs, Emerita analoga, that stick their feathery antennae up above the sand to filter food out of the waves in the swash zone.”
The study found big declines in nearly all the species they studied. Populations of sandhoppers and bean clams recovered within one year. On some beaches, populations of mole crabs bloomed four months after replenishment and were even more numerous for a short time than on control portions of beaches, but subsequently declined. Polychaete worms, the most common invertebrates at the beach, meanwhile, showed sharp declines at all of the beaches that continued to the end of the study.
“There’s a lot we don’t yet know about the effect of sand replenishment on the community of organisms that live in the sandy beach, and the animals that depend on them,” said Henter. “In our study, some species seemed to increase in abundance after replenishment, others decreased, but this was really variable.”
The researchers said their findings highlight the importance of continuing similar studies, considering that sandy beaches make up two-thirds of the world’s shorelines, and that many other beach communities around the nation and world employ costly replenishment efforts to combat erosion at economically important beaches.
“There are large gaps in our knowledge,” said researcher Tyler Wooldridge. “For instance, how long will the effects we observed last? What is the effect of reduced invertebrate abundance on bird and fish populations? Another key question is how frequent and widespread should efforts to replenish beaches be? Are there times of the year when it is more or less disruptive for the animals that live in the sandy beach? To answer those questions, we need more studies.”