Science1 publisher3 min readPublished
Sand hoppers excavated up to 50 kg of sand per meter of California shoreline overnight
A UC Santa Barbara team pinned frames into the burrowing zone at Isla Vista Beach, waited out one summer night and weighed the mounds. The authors put the volume on par with longshore transport and some Southern California rivers.
The Scientist · Science desk
What happened
- Researchers sectioned off plots with metal frames in the sand hoppers' preferred burrowing zone at Isla Vista Beach, a bluff-backed shore next to the UC Santa Barbara campus, after a summer high tide.
- Overnight the centimeter-long amphipods excavated up to 50 kg of sand for every meter of shoreline, which the authors call among the highest sediment-moving rates in the animal world.
- Hubbard and his co-authors report in the Journal of Geophysical Research: Earth Surface that the volume moved puts the amphipods on par with longshore processes and some major Southern California rivers.
- Instead of reusing a burrow like most digging animals, the hoppers excavate a new one each day, hunting the sand moisture their tunnel walls need to stay intact.
Compiled by The ScientistSomething wrong?How this is made
Why it matters
- capability Measuring the animal contribution to a beach sediment budget now takes frames, flags, a scale and one tidal cycle, so other littoral cells can be checked without new instruments.
- constraint Kilograms excavated and kilograms delivered along the coast are different quantities, so this figure cannot enter a littoral-cell flux until someone follows where the mounds go.
- precedent If the rate holds beyond Isla Vista, coastal sediment models that already carry cliffs, dunes and river input will be expected to carry a biological term as well.
Fifty kilograms per meter of shoreline per day converts to 50 tonnes per kilometer of shoreline per day [15]. The figure it converts is an upper bound: the paper reports up to 50 kg, from plots weighed after one summer night at a single bluff-backed beach [1][5]. Whether the same rate holds in February, or on a beach with dunes behind it instead of a bluff, is a different measurement.
Moisture set the design. Hubbard said the animals "don't like saturated sand" because the burrows collapse immediately, and that they avoid "the powdery dry sand above the high tide line." "They like the Goldilocks sweet spot in between where the burrow will stay intact because the moisture in the sand is just right," he said [9]. Because they re-dig daily chasing that band, the sand they bring up is material the wind and waves have recently worked over [8].
"They have one of the highest rates of bioturbation of anything on the planet," said Hubbard, a marine scientist at UC Santa Barbara [2]. The overnight mounds buried the gear. "When we came back in the morning, we couldn't even see some of the frames," he said. "They were completely buried, and we were so glad we had put flags next to them just in case" [10]. The team then collected the excavated sand and weighed it [6].
Baxter, who did the work as a postdoc with UCSB geography professor Ian Walker and is now a research fellow at the University of Oxford, had not expected the volume [13]. "It was quite impressive and we weren't really expecting that," he said. "I think for me that was the biggest shock" [11].
Hubbard and his co-authors make the comparison to rivers and longshore processes on the strength of the volume moved [3]. A beach sits inside a littoral cell. Sediment enters from backing cliffs and dunes, from rivers and streams draining coastal watersheds, and from onshore processes that push sand up from the nearshore, and the beach holds it as a reservoir while it travels along the coast [12]. Rivers and drift move sand into and along that cell; the hoppers lift sand from 10 to 30 cm down to the surface of a beach it already sits on [7]. The phys.org account does not name the rivers or give the volumes on either side of the comparison. On par, here, describes kilograms mobilized per day.
That distinction matters for beach management. The hoppers feed on kelp wrack and shelter under it [7], so raking wrack off a beach plausibly changes how much digging happens there. How much, this experiment cannot say: it did not vary the wrack.
What to watch
- A winter repeat of the same plots would show whether 50 kg a meter is a summer peak or a year-round rate.
- Any tracer or repeat-survey work that follows excavated mounds into wind and swash transport.
- Whether beaches with different hopper abundances across California give the same per-meter figure.