Estuaries: Gateways to the sea

Until I moved to Southampton for my PhD, I had no idea how interesting estuaries are. Here, I want to give you a better idea of the chemistry going on in these fascinating gateways to the sea.

Estuaries deliver sediments from rivers into the ocean, but they actually receive even more sediment back from the ocean. This means that all estuaries are slowly being filled in with sediment. That’s why economically important estuaries have to be regularly dredged to prevent ships from running aground.

Sediments are important because they contain a wealth of nutrients which support life within estuaries. But how do the nutrients get there? This is an important question for chemists and biologists alike. One source is the river water itself, which picks up nutrients from soils and weathered rocks. However, a much more important source is from estuarine vegetation like kelp, seagrasses, mangroves and marsh grasses. This vegetation contains a large amount of cellulose which makes it unpalatable for most herbivores. Instead of being consumed by them, most of it is broken down by detrital feeders like bacteria and fungi, which leads to a large amount of organic matter and nutrients being stored in estuarine sediments.

The meeting of fresh water and sea water also plays a role. Water circulation is special in estuaries due to the presence of these two water types, which do not completely mix due to differences in density. Nutrients are entrained from the dense underlying sea water into the fresher surface waters, and this allows an abundance of phytoplankton to flourish. When this phytoplankton dies, some will sink into the more saline layers and be carried back towards land where the organic matter will be recycled. This means that estuaries tend to retain the nutrients they contain.

Filter feeders like bivalves can be important in regulating the amount of sediments in estuarine waters. These animals capture sediments from the water column as they feed and deposit it in larger packets on the estuary bed. This process clarifies the water, allowing sunlight to penetrate and phytoplankton and vegetation to flourish. At the same time excess nutrients are also removed which could otherwise cause eutrophication, where oxygen is removed from the water by bacteria causing ecosystems to die off.

Estuaries are highly important in terms of inorganic (non-biological) chemical reactions as well. In fresh waters, organic and sedimentary particles carried by rivers don’t stick together because they have a net negative charge which makes them repel one another. When these particles meet seawater however, the cations from dissolved salts neutralise the negative charge and weak intermolecular forces take over, causing particles to become attracted to one another (these are collectively known as van der Waal’s forces – you can read more about them here). This aggregation of particles is known as ‘flocculation’, and is another important way in which the sediment load within estuaries is regulated.

Because the flow of fresh and sea waters oppose each other, the current where the waters meet is slower and sediment particles (both flocculated and otherwise) tend to accumulate here before falling to the estuary bed. This removal of particles from the water performs another crucial function which is of interest to trace metal scientists like myself. Many pollutant metals such as lead, chromium and cadmium like to ‘stick’ to these particles and often undergo reactions on their surfaces. This means that instead of travelling further out to sea, a large proportion of these metals are stored in estuarine sediments. Without these natural removal processes, the problem of metal pollution would spread further out to sea and threaten our oceans.

Estuaries face serious threats from anthropogenic pollution by several routes. Heavy metals and other poisonous industrial chemicals can work their way up the food chain and kill animals, whilst an over-abundance of nutrients due to fertiliser run off and sewage input can cause waters to be starved of oxygen. Dredging and land reclamation also threaten habitats above and below the water. A delicate balance must be struck in order to allow humans to utilise estuaries as transport routes and as a source of food and pleasure for years to come, whilst also protecting these unique and fascinating habitats. I hope this article has given you a new appreciation for estuaries!

 

References:

C.M. Lalli, T. R. Parsons, Biological Oceanography: An Introduction, 2nd edition (1997), Butterworth-Heinemann.

The Open University, Waves, Tides and Shallow Water Processes, 2nd edition (2000), Butterworth-Heinemann.