Estuaries

Estuaries are river mouths that reach the sea.  They are embayments that experience reduced salinities due to the influence of the flowing fresh river water into the area.

The area usually has extensive mudflats or sand flats at low tide, with a build up of silt from the river. This silt is as a result of the river dropping its load, land, rock and other materials it has eroded on its journey from the mountains. 

This fertile silt can extend over quite a significant area beyond the mudflats and sand flats.  Flooding can carry it onto neighbouring agricultural land, this area in close proximity to the river banks is a very fertile plain but subject to the constant threat of flooding.

Salt marshes are usually a characteristic of estuaries and infaunal species (which are aquatic animals that like to live beneath the sea or lake floor) like oligochaetes (worms), are found in large numbers. Birds like the Oystercatcher are found around estuaries too, they use their strong beaks to open molluscs like cockles & muscle shells. Shells and stones are common, green algae Enteromorpha sp. and Ulva sp., along with the brown algae Fucus ceranoides are generally found on the rocks and stones.  Other fucoids also appear in abundance too.

Estuary food-webs are initiated by the solar energy of the sun and photosynthesis (the conversion of sunlight into plant food) by plants and plankton in the estuary.  There are two main feeding routes.  One starts with marsh grass when it’s leaves and debris fall into the water and are broken down by bacteria & fungi to become detritus.  Detritus is then consumed by small animals like snails, worms & shell fish.  These creatures subsequently are consumed by large aquatic species like fish & birds.

The second pathway is initiated by microscopic phytoplankton which are very tiny plants.  They are eaten by zooplankton which in turn become food for snails, shell fish and other bottom feeders.

The food-web
The food-web processes of an estuary.

The effects of climate change on estuaries

Estuarine environments are subject to multiple stresses and physical processes like tidal, wave, surge, river discharge & sediment supply.

These physical processes along with the natural predation cycle, regulate nutrient fluctuations, pollutants, pathogens & viruses.  All of these things are effected by climate change either directly or indirectly.  The environmental health of the estuary and neighbouring coastline, along with it’s robustness are influenced greatly by human activity much of which is contributing to global warming and pollution.

Recent global trends and projections show modifications to climate drivers like the increase in the overall earth temperature and changes to ocean acidification & this in turn has changed and will further impact potential implications on estuarine bio-physical processes.

Sea surface temperature is increasing. The UKCP09 report has forecast temperature increases of 1.5-2.5 degrees celsius in the ocean over the next 100 years with larger rises of 2.5-4 degrees celsius in the Celtic, Irish & Southern North Sea. Also in the last century river water temperatures have increased between 1-3 degrees celsius since river water temperatures are heavily influenced; almost synchronised with air temperatures.

Sea level rising and other flood risks. Increased estuarine flood risk is mainly associated with sea level rise which is a result of the melting of the polar ice caps.

Estuarine flooding can also be caused by storm surges and extreme river flow due to man-made intervention.  For example the building of dams on a river or other building and bridge construction further upstream.

Storm surges are occurring more frequently. Research by UCC for the Irish Environmental Protection Agency investigating Irish storms shows that both 10 year return period storms (1940 – 1975, 1976 – 2005) and 30 year return period storms (1940 – 1975, 1976 – 2005) are increasing.  Looking at storm intensity-duration-frequency relationships indicates that storms are increasing in intensity and in duration along with also increasing in frequency.  These results were recorded at both Dublin Airport and Valentia.  Increased storm surges make estuarine flood risk and inundation a real threat and monitoring day to day increases is a great way to record small changes that maybe occurring to the rain fall.  It’s possible to measure these changes with a simple rain gauge.  This is something you can even make yourself and regularly monitor.  (I will be showing you how to make one in my next blog post!)

River flow and rain falls are increasing. Looking again at research carried out by UCC for the EPA here in Ireland shows that increased annual and seasonal rainfall (particularly in March & October) since 1975 has been recorded.  There has also been an increase in extreme rainfall events since the mid 1970’s. Looking at three Irish rivers the Suir, the Nore and the Fergus shows an increase in river flows after the change point years of around 1976.  These changes in river flow correlate with the increase in precipitation in the last part of the 20th century.

The effects of pollution

  • Plastic

Plastic can find its way into water sources from numerous different places.  It can end up being pushed down stream to an estuary river mouth and can be hidden underwater, unseen.  Plastic can trap animals causing injury and preventing them from feeding and animals can also inadvertently eat plastic pieces which can cause the toxic release of chemicals.  These chemicals are then transferred through the whole food-web causing damage to the Eco-system of the estuarine habitat and also effect the human food chain.

  • Runoff

Runoff from farms and industry has a negative effect on the water quality of rivers and estuaries.  It often contains hazardous substances, rubbish and untreated sewage.  These elements can block a rivers flow and destroy fish nurseries and feeding sites.

  • Eutrophication

Point sources and non-point sources flood the river with untreated waste which causes tuberity or cloudiness and increased algae growth as a result of the influx of nutrients like phosphorus and nitrates and an increase in water temperature due to the accelerated growth of algae and microbial activity and the resulting higher water surface area coverage of algae creating insulation.  All of this pollution results in fish mortalities and loss of habitat, along with the deterioration in the robustness of the estuarial eco-system to weather anomalies, like storms.

Siobhan.

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