Intertidal Zones and Habitat Functions of Estuarine Marsh Areas

An estuary is, essentially, a place where rivers meet the sea. It's a place where fresh river water mixes with salty ocean water, and water levels have a tidal cycle. River detritus and sediments bring in a lot of nutrients, and at the ocean end there are waves.

The lower two thirds of an estuary's intertidal zone is mud flats. The upper third - up to the highest reaches of saltwater during spring high tides, where the treeline begins - is occupied by vascular plants that can tolerate salt and having their roots submerged part of the time. Mangrove forests dominate this area in the tropics, but give way to salt marsh in the temperate zone where they can't survive winter.

Salt marsh plants include a wide variety of tall reedy plants, like Spartina, Phragmites, and Juncus. Spartina in particular has a habitat function of raising the overall marsh elevation, both with below-ground growth of rhizomes which add bulk, and by functioning as water baffles during ebb tides, which causes soil to build up along the marsh edges to the same level as the highest high tide. The marsh edges creep out along the mud flats until they stop being wide, shallow areas, and instead are narrow, deep channels - which are called tidal creeks (more about tidal creeks on page 2).

 Although tall reeds are the most obvious feature of a salt marsh, most of the primary production (the turning of sunlight into food) comes from benthic microalgae - mainly diatoms, also blue-green algae, dinoflagellates, and euglenids - which live on bottoms and other surfaces. Benthic microalgae are an excellent protein source compared to other plant sources of protein, about on par with animal protein sources, which makes them an excellent food source for grazing herbivores that are short-lived, or need to grow quickly. Phytoplankton, which float in the water column, provide high quality protein as well, but there is much less of it than benthic microalgae.

In addition to the primary producers, well-established estuarine marsh areas contain highly diverse communities of bacteria. Along with fungi, these work together to decompose dead plant matter, detritus, and other organic waste brought in by the rivers, functioning as Nature's septic system for everywhere upstream.

Many of the bacteria are obligate anaerobes, which means they cannot survive in an environment that contains oxygen. Many are ancient, having been in existence since life began. At that time, there was no oxygen in the atmosphere and they lived everywhere, excreting oxygen as the toxic waste product that it was until the world ran out of rocks to rust and it filled the atmosphere in the world's first, most catastrophic, and most far-reaching pollution event.

Nowadays they have mostly been relegated to anoxic marsh muds. Although ancient, they are just as efficient and effective at decomposing things as the aerobes. Bacteria are an important food source for marsh detritivores (worms, shrimp). Many of the detritivores are actually bacteriovores that pass the detritus the bacteria were on through their systems without actually digesting it.