How do lagoon floors differ between coastal lagoons and coral lagoons?

The floors of coastal and coral lagoons differ primarily in their composition and topography due to the distinct processes that form them.

Coastal Lagoons

Coastal lagoons, which are separated from the sea by barriers like sandbars or barrier islands, are typically zones of fine sedimentation. Their floors are generally flat or gently undulating and are composed of fine sediments, such as mud, transported by rivers from inland areas. While the overall profile is often flat, deeper channels may exist near tidal inlets where currents are strong enough to prevent the accumulation of fine material.123

Coral Lagoons

In contrast, coral lagoons are defined by their organic origins. Because they are formed by coral reefs, their sediments contain significant amounts of calcareous material. While these floors can be smooth and flat due to this sedimentation, they are frequently characterized by the presence of knolls—mounds of growing or dead coral—that rise from the bottom. Unlike coastal lagoons, which rely on river-borne sediment, the floor structure of a coral lagoon is intrinsically linked to the upward growth of the reef.124