Habitat Function of Salt Marsh Tidal Creeks
Aside from functioning as Nature's septic system, the tidal creeks of estuarine marsh areas provide nursery habitat for a large number of marine fish and shellfish species. Some of these are residents in the estuaries throughout their life cycles (such as oysters, clams, mussels), and some live offshore as adults (lots of fish and shrimp species). Blue crabs actually have the opposite pattern of living in the estuary as adults while sending their larvae offshore.
Tidal creeks are small, blind channels that drain the marsh during low tide. They are areas where conditions are constantly changing, often unpredictably. Shallow water over black bottoms can become very hot. Marsh plants take up water and dump salt, making the water hypersaline. Then it can suddenly become cold and fresh in a thunderstorm. Oxygen level can be very high during the day due to photosynthesis, then plummet at night as everything uses it up. Nutrients, waste products, and pollutants can build up during low tide and dilute out over the marsh when it floods at high tide. Water can be anywhere from stagnant at the tops and bottoms of tides, to very fast-moving during ebb flow.
Despite the challenges, tidal creeks offer many benefits to the larval and juvenile stages of many marine animals. The higher heat lets them grow faster. There is lots of high protein food, both from the benthic microalgae and from bacteria. Large marine predators don't go into tidal creeks because there isn't enough oxygen to support them, and birds don't eat things quite that small.
As the juveniles grow, they gradually move out of the tidal creeks and into deeper areas of the estuary. Those that are offshore as adults tend to leave all together in a particular egress season.