When you picture an estuary, you maybe think of a warm, clear water with small boats gently bobbing in the slow current, birds feeding along the foreshore and sheep grazing on the saltmarsh. What you may not think about is cold, deep, turbulent, murky brown water with huge ocean-going ships gliding past busy urban areas.
So why is the Mersey brown? High levels of fine and heavy sediments that make up the riverbed, coupled with the strong currents generated by the second highest tidal range in the UK gives the Mersey it’s recognisable “Chocolate milkshake” look. The soft riverbed and strong flows mean that the riverbed is highly mobile and mudbanks can disappear and reappear as the currents change throughout the year.
This ever-changing environment means that navigation in the estuary requires a very high level of skill and expertise. However, this murky environment is bursting with life and much of this is thanks to the thing that makes the river look so dirty. Mud.
Anyone who has spent time in the upper estuary will be familiar with the glue-like nature of the Mersey mud. In places it can be over waist deep and its sticky, glue like consistency can make it very dangerous (and difficult to clean off!)
This gloopy mud though plays a vital role in the river by providing excellent habitat for mud dwelling animals such as shrimps, snails and worms which in turn support the huge numbers of wading birds. In fact, the international and national designations for the Mersey are because of the large numbers of overwintering birds that use the mud for feeding and roosting. The Mersey supports Internationally and Nationally significant numbers of species like Dunlin, shelduck and Redshank. All of these species come to the Mersey because of the mud and the food that it provides.
Figure 1: Oystercatcher with a luworm on Wigg Island (Photograph taken By Emily Caldwell-Lay)
Figure 2: Ruddy shelduck on the mudflats at Spike Island (Photograph taken By Emily Caldwell-Lay)
The glorious Mersey mud is not just key in supporting bird populations. Many fish species can be found living in and around the mud including flounder, common gobies and even sole. Toward the estuary mouth, large numbers of thornback rays can be found, these feed on crabs and other animals that live in the muddy areas. These fish feed on the huge numbers of mysis, estuarine prawn and brown shrimps that live in and around the mud.
One of the more unusual mud dwellers are the peculiar looking pogge (also known as the hooknose). This peculiar looking fish can be found living in muddy areas and is covered in hard bony plates making it very difficult for its predators to eat. The muddy water also provides vital cover from predators for many other species of fish such as sand smelt, thin lipped mullet, sprats, whiting and European smelt all of which can be found in the estuary. Sea bass, porpoises and cod are the main predators of these species and are frequent visitors into the river, all thanks to the mud!
Figure 3: Estuarine prawn caught during our plankton surveys in Widnes.
So as you can see, the mud is an incredibly important part of the river and without it, the River Mersey would be a very different (and much less diverse) place. For me it will always be a source of fascination and mystery as we try to learn more about what lives below in the murky depths.
Andrew Wolfenden, Biodiversity Manager