Carolina Environmental Diversity Explorations
Cape Fear estuaries · By Steve Keith
Port of Wilmington
Moving to the south end of Wilmington we come to the ports. Ports and marinas are hard to avoid in estuaries, but large ports are worth a look. The ships visiting Wilmington’s port are oceangoing cargo ships that need deep water to navigate.
What does this mean for the Cape Fear Estuary? The river bottom must be kept deep enough for the large ships to enter, and that means that the river bottom must be dredged every few years. This requires some attention because a vital part of the estuarine food web can be found in the sediments. The sediment dwelling or benthic organisms include several type of worms, amphipods, molluscs, and insect larvae. These organisms serve as food for crabs and many juvenile or larval fish.
The benthic inhabitants change with the salinity, and also with the coarseness of the sediment. Sediment deposition is governed by chemistry and water velocity. In high-speed flows, such as in fast-moving parts of the river or near the inlet, fine grained particles are swept along, leaving only coarser material behind. Finer particles fall out as the water speed slows down.
The middle reaches of an estuary typically have the slowest flows and thus the finest sediments. Water chemistry also plays a role. Riverborne particles are repelled from one another by negative charges resulting from clay minerals and from organic matter. The ions present in seawater “short out” these repulsive charges, and as a result particles aggregate (flocculate) and, suddenly heavy, fall out of the water column. This flocculation happens to the greatest extent when the salinity begins to increase at the upper end of the estuary (near where the previous photo was taken).
The upshot of all of this is that sediments are heavily influenced by the water movement and chemistry above them. What does this have to do with big ships? Dredging removes the sediment from its “natural” position, exposing sediments that may have been deposited when the conditions in the estuary were much different. As well, the benthic organisms removed are displaced, moving a food supply to wherever the dredge spoils are dumped. In the Cape Fear River, several small islands have been formed by the dumping of dredge spoils, and these provide important nesting sites for birds.