Rocky topography in a submerged kelp forest could trap cold, dense seawater from a retreating internal wave and create underwater ‘tidepools’.

Hidden tidepools of the kelp forest

Along the rocky coastline of California, tidepools are a common sight. On a sunny weekend in Monterey, you can often see children, families, and curious tourists scrambling over seaweed to catch a glimpse of the diverse critters hiding in and around the many tidepools nestled among the rocks.

Tidepools are special because they exist at the boundary of two worlds: air and seawater. As the ocean waves come rolling in with the tide, they flush the entire rocky shore with seawater. But when the tide and waves retreat, seawater swirls and trickles around the topography of the rocky shore. Some of it gets trapped in the depressions between the rocks, leaving behind little pockets of ocean surrounded by air: tidepools.

But the ocean’s surface isn’t the only place where waves exist. Within the ocean itself, there is another boundary between two worlds: world of warmer, surface water, and the world of cold, deep water. Just as hot air rises above colder air (allowing hot air balloons to work!), warmer water is less dense than cold water, and it floats on top of the cold water to form two distinct water layers in the ocean. This is called stratification.

Stratification in the ocean. Waves move on the surface and at the boundary between the warmer, surface seawater layer and the colder, deeper seawater layer. The waves on the seawater boundary are slower and larger than the ones on the surface.

And just like the waves that move on the surface between air and seawater, there are waves that move on the boundary between the warm water layer and the cold water layer. Oceanographers call them ‘internal waves.’ And although these internal waves are almost invisible from the ocean’s surface, their effects are certainly not invisible to submarines, oil rigs, and offshore wind farms. You can visualize how internal waves work using a tank or a bottle of mineral oil and water, like in the following video:

So what happens when an internal wave runs into a bunch of underwater rocks? The complex topography of California’s rocky shore doesn’t end at the tide line – it continues down into deeper water, where giant kelp forests form habitat for all kinds of animals like fish, crabs, sea stars, and sea urchins. As the giant internal waves crash and retreat, do they also leave behind ‘ internal tidepools’ of cold water from the deep? It makes sense, but no one had ever tried to look for these underwater tidepools before. Most scientists who study internal waves work further out at sea, not in nearshore waters where kelp forests grow.

Rocky topography in a submerged kelp forest could trap cold, dense seawater from a retreating internal wave and create underwater ‘tidepools’.

So in the summer of 2014, former Hopkins Marine Station graduate student Paul Leary and his collaborators set out to look for internal tidepools in the kelp forest right next to the Hopkins Marine Station. They searched the rocky floor of the kelp forest to find places where ‘internal tidepools’ were likely to form – bowl-shaped depressions in the rocky reef that looked like they could trap seawater. Then they set up small, coke can-sized sensors on both the top ridge and the bottom of these rocky depressions to detect incoming internal waves and the potential formation of internal tidepools.

Since the two layers of ocean water have really different properties, the scientists could use oceanographic sensors to tell them apart. Seawater from the deep layer of the ocean is colder, more acidic, and contains less dissolved oxygen, while seawater from the surface layer is warmer, less acidic, and contains more oxygen. The sensors recorded both the temperature and the oxygen content of seawater at their respective locations, so that the scientists could compare the two and see if they would reveal the existence of internal tidepools.

Formation of an internal tidepool in the kelp forest. Illustrations by N. Low, adapted from Leary et al. (2017).

And it worked! When Paul and his collaborators looked at the data from their sensors, they could see that when an internal wave came, it would first start to fill up the bottom of the depression with cold, low-oxygen water from the deep layer.

Then, the entire kelp forest would be flooded with deep-layer water, the same way the ocean floods the rocky shore during high tide. When the internal wave retreated, the rocky depressions in the kelp forest trapped pools of colder, lower-oxygen seawater. These internal tidepools could last for several hours, just like many tidepools that you can find on the shoreline.

Why do these underwater tidepools matter? Just as tidepools on the rocky shoreline create a patchwork of habitats, internal tidepools create a patchwork of environments for the creatures that live in the kelp forest. When an internal wave of cold, low-oxygen water comes through the kelp forest, a sea urchin living in an internal tidepool might have to hold its breath for much longer than its friend who lives right outside the tidepool. Fish may choose to swim away and avoid these tidepools of low-oxygen water, making the internal tidepools an ideal hiding place for their prey…if the prey can tolerate low oxygen! The dynamics of internal waves and internal tidepools make the rocky floor of the kelp forest just as complex and diverse as the rocky shores of the California coast.

So, the next time you watch the waves rolling into shore or explore a rocky tidepool, don’t forget about the internal waves and tidepools that lie hidden under the water’s surface. They, too, shape the lives and fortunes of ocean critters and ecosystems in our dynamic world.