When Waves Reach Pebblecup Pool

Field Report: Pebblecup Pool

On a rocky beach, a class watches a tide pool they call Pebblecup Pool. Seawater fills a bowl-shaped hollow in the rock. A raised rock rim surrounds most of the pool, with a low notch on the ocean side. During the visit, wind pushes across the sea surface and helps make waves. Some waves reach the notch and spill into the pool. Later, the ocean level falls with the tide, and waves no longer reach the notch. Water remains below the rim.

Green sea lettuce, a kind of seaweed, is attached to rock inside the pool. Small snails graze on its surface. As water enters through the notch, the sea lettuce bends with the flow. After the tide falls, some sea lettuce near the upper edge is exposed to air, while sea lettuce lower down remains underwater. The students leave the organisms and rocks undisturbed. They notice that the rock's shape determines where water collects and which surfaces remain covered.

Even without a wave reaching the pool, water can leave its surface by evaporation. During evaporation, liquid water becomes water vapor and mixes with the surrounding air. Water vapor is water in its gas form. The class wonders whether moving air changes how much water leaves a surface. To investigate safely, students use clean water indoors rather than changing the tide pool. Their simple model represents a water surface exposed to air.

For each trial, students put 200 grams of water in each of two identical shallow dishes. They place both dishes on the same shaded bench. A small fan moves air across one dish; the other sits away from the fan in still air. The dishes begin with equal water depths and exposed surface areas. Students check that both stay at the same temperature. After four hours, they weigh the water remaining in each dish and calculate the mass lost. Nothing spills, and nothing is added. They repeat the comparison with fresh water. The graph shows their results.

The dishes do not include seawater, rock walls, waves, or organisms. A fuller coastal model needs these parts too. In that model, land includes the rock bowl and rim, water includes seawater and water vapor, air is the surrounding atmosphere, and living things include sea lettuce and snails. Labeled arrows should explain interactions, not just point between names.