On my home beaches and on beaches around the world, sand accumulates seasonally during calm weather and erodes during periods of more active weather and bigger surf. The ups and downs of sand accumulation can go unnoticed, unless you have a landmark. In this post I trace cycles of sand accumulation and loss on some of my favorite beach landmarks.
Burial, Loss, and Renewal
March 3, 2012
Winter storms eroded sand levels around this isolated rock, exposing about as much of it as I’ve ever seen. It’s a great landmark on an otherwise featureless reach of sand. At times, as in this photo, this rock is home to a flourishing community of anemones, Anthopleura xanthogrammica, mussels, Mytilus californianus, barnacles, Semibalanus cariosus and a green alga, probably Ulva. The size of the mussels on the upper third of the rock indicates that it’s been exposed a while; M. californianus take a few seasons to get that that large. Notice that below the large mussels the community is much less mature, indicating recent burial under an accumulation of sand.
June 3, 2012
After a mild spring, sand accumulation around the rock has reduced it’s above sand extent by about half. The anemone, mussel, barnacle, green algae community is still present, but for how long?
November 10, 2012
After a summer buried under accumulating beach sand, the first storms of fall have eroded enough sand to expose a now barren upper portion of the rock. The anemones, mussels, barnacles, and green algae that inhabited this rock don’t have what it takes to withstand prolonged sand burial; few rocky intertidal creatures do.
April 11, 2015 – Renewal
I visited the rock again last spring and found the exposed portion heavily encrusted with a different and less diverse community than I saw in the spring of 2012. The exposed surface was densely packed with a different barnacle, the acorn barnacle, Balanus glandula. It’s an early colonizer, one of the first species we expect to take over bare surfaces. There was also a growth of the familiar green alga, Ulva; probably the same one we saw in the spring of 2012.
A Summer Dilemma
Early in the morning on June 17, I came upon a moving sight in the sand. Over 50 ribbed limpets, Lottia digitalis, were crowded onto the tip of this sand-buried rock. Lottia don’t mind a little crowding. In fact they tend to lean in that direction, but this was something different. Burial was nigh, and the scene evoked a sense of urgency. I can’t say what the limpets felt, but leave or die can’t be far from it. The tide was running out and the fresh early morning sand showed tracks radiating out, and even a few fearless limpets and periwinkles, Littorina, heading overland in search of a new homes, perhaps ones less vulnerable to seasonal burial. Watching those tiny mollusks trekking across the sandy surface I was struck by how risky their journey was. But holding tight risky too. It was only mid-June, and the summer season of calm weather ahead promised only additional sand accumulations and deep burial. Of those 50 or 60 limpets and who knows how many periwinkles, how may immigrated and how many stayed put? Did the choices lead to different consequences? I would love to know. Many times since that early morning in June I have caught myself pulling for the fearless immigrants and agonizing over the fate of those that stayed behind.
These Pisaster ochracheus live on a large reef with no risk of sand burial. Yet, cycles of sand accumulation and erosion create an ever-changing lower limit to their movements in the lower intertidal. I frequently see Pisaster crowded together at the sand-rock interface. This cycling is important because Pisaster is potent predator and organizing force for lower intertidal communities. Anything that affects its movements affects the entire community.
The Concentration and Access Effect
If like me, you like to make casual counts, you’ll want to be aware that high sand levels create low tide crowding, as I have described for Lottia and Pisaster above, and opportunities for human access to sites that remain flooded during periods of low sand. If you’re counting mobile organisms, this can lead to an impression of greater low tide abundance during high sand compared to what you’ll observe during periods of low sand levels and more difficult low tide access.
Capitalizing on Periodic Burial
Some intertidal algae can tolerate limited periods of sand burial. Some even seem to flourish only where there is plenty of sand-scouring and periodic burial. This red alga, from June 2015, is one of them, It looks perky even though it is partially buried. Wherever space is at a premium, as it usually is in the lower rocky intertidal, one beneficial strategy might be to occupy substrates that present problems for other species. As I showed earlier, many rocky intertidal inhabitants cannot survive sand burial. Species like this red alga have found a solution to the space problem. To see some others that have come up with a similar solution, check out Seaweeds in the Sand.
Here One Year, Gone the Next
This old growth stump serves as a gauge for sand accumulation on a long featureless beach. If not for this lone stump I would have a tough time tracking sand cycles. I’ll regret it when, someday, it washes away. These photos show four years worth of net accumulation (though I can’t discount the possibility of some settling). On my home beaches, recent winter storms have lacked punch and there has been an overall sand accumulation in the upper intertidal. Sand cycles, like most oceanographic cycles, are more than just seasonal occurrences.