About our expert
His research interests include species rich meadows, long-term experiments, plant life-history and phylogeny. He is also a keen naturalist: "Nearly all biologists nowadays only study one organism or even just a few molecules, but I have a plea that people should be aware of their whole environment."
The river runs dry
Ocean or dumping ground?
We use the sea as a handy dumping ground but how long can we continue trashing the waves?
Britain's estuaries have an international importance for wildlife. Those apparently empty bleak windswept places are actually larders packed with food used by overwintering birds. Dig into the mud and you'll find loads of worms and shellfish which only give away their presence by a small hole or cast on the surface.
Ask any birdwatcher where the best place is to be on a cold winter morning and they'll not say curled by a cosy fire but instead they'd much rather be out with a telescope on one of our many estuaries.
We'll come back to those exciting winter months later, first take a look at your local estuary in summer. Don't just zoom past it on the way to the sandy beach, stop and have a look at the plants and animals living there and ask how they can survive in a place that often gets covered in salty water.
Why have the plants got thick fleshy leaves like desert succulents instead of thin flimsy leaves that you might expect to see on plants from other damp places. Well there is plenty of water around but it generally contains a high concentration of salt. Water flows from areas with a low concentration of salts (the plant cells) to the more concentrated sea water so 'normal' plants in these circumstances would actually loose too much water from their cells and die.
One way specialised saltmarsh plants overcome this problem is by building up the levels of salts in their roots, once the concentration inside the roots is higher than in the surrounding mud then water can flow into the plant. The leaves are tough and waxy to reduce water loss and prevent physical damage by wave action. Some of the plants also excrete salt through glands on their leaves and you can sometimes see - and taste - a fine crusting of salt all over them.
The other problem saltmarsh plants face is a lack of oxygen, plants need oxygen to 'breathe' (respire) just as much as humans do but down in the thick black mud of the estuary there is often very little oxygen so the roots are in danger of dying. To counteract this lack of air many of the plants have hollow tubes going down from the leaves into the roots.
Light green Glasswort plants with barnacles on a rock in the background.
If you look across the estuary you'll immediately see that the vegetation is arranged in zones. Lowest down towards the waterline there is Glasswort - its soft fleshy bright green round stems looking like miniature cacti. It is a good edible vegetable which can be prepared in the same way as asparagus and has a very long tradition of being enjoyed as a good nutritious food.
If you are thinking of eating anything from estuaries - be it plants, shellfish or fish - then you do need to bear in mind that many of Britain's estuaries have been polluted in the past and may still contain toxic chemicals or harmful micro organisms.
Another very common plant on the marsh and one that is good at holding the mud together is cord-grass.
Moving up the shore where the next vegetation zone may look rather grey and is not necessarily inundated with every tide, it often contains large stands of sea purslane.
Further up again the shore can be ablaze with purple sea lavender and late in the year sea aster comes into flower looking rather like a garden plant sticking up above the grey surrounding vegetation.
The plants are living in a very rich mud, nutrients having been brought down by the river and deposited as flow rate slows before entering the sea. This muddy soup is also ideal for a wide range of filter feeding shellfish such as oysters, mussels and cockles, various worms such as ragworms, lugworms and fish such as juvenile sea bass, dabs and flounder.
Water flow patterns through the estuary are often rather complex and can vary with different states of the tide and at different times of year. The speed that the water is moving can be very important in determining the size of particles that are deposited, lighter sediments only fall out of suspension when the water is still. If the pattern of flow is disrupted - for example by building a new marina - then the distribution of sediments will change and this may have adverse effects such as depositing large amounts of material on an oyster-bed and so destroying it.
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