Broads Uncovered: Life in the Dykes

Published: 23rd October, 2020
Little Whirlpool Ramshorn Snail (Anisus vorticulus)

Dyke Dipping in the Broads National Park is one activity that never grows dull however many times you do it in a day, week or month. There’s always something new to see, the animal assemblage changes over the months and the excitement is infectious as you realise the water, which has “nothing in it”, is actually teeming with tiny and not so tiny animals going about their business.

The Broads Specialities. The interaction of a number of  physical factors unique to the Broads National Park (the water coming from soils rich in carbonates, the strong saltwater influence from the sea and the ability of the shallow manmade broads and dykes to respond to air temperature changes) has resulted in an assemblage of aquatic life (plants, fish and invertebrates) like none other in the U.K. Many of these species are incredibly rare and have been identified as Broads Specialities.

Broads Specialities are species that the National Park holds either:

1. The entire U.K. population, like the Stonewort (Chara intermedia) and and Lesser Water Boatman (Sigara longipalis) or;

2. >80% of the U.K. population, like the Little Whirlpool Ramshorn Snail (Anisus vorticulus) and and Norfolk Hawker Dragonfly (Aeshna isosceles) or;

3. >50% of the U.K. population, like the Fen Raft Spider (Dolomedes plantarius) and Shining Ramshorn (Segmentina nitida).

Fen Raft Spider
Norfolk Hawker Dragonfly
Family of Segmentina nitida

 

 

 

 

 

 

 

 

 

Dykes cutting through the marsh.

Importance of the Dyke System. The dykes of the Broads National Park are an extremely important habitat. The changes, since the 1900’s, in how we use and manage the waterways in the Park has altered the conditions in many of the broads and rivers causing a loss of suitable habitat for those plants and animals that require clear, unpolluted water and an abundant variety of aquatic vegetation. Many of the dykes, especially those not directly connected to the rivers and unaffected by runoff from agricultural land, have become refuges for these animals and plants.

The salt intolerant Water Soldier.

It is not only human activities that threaten aquatic communities in the Park, but also the ever-present threat of salty sea water. Animals living in water have to maintain a balance between the salinity of their internal body fluids with the salinity of the surrounding water. The majority of freshwater invertebrates cannot control the movement of salt through their skin so any increase in salinity to the water around them is lethal – think of what happens to the snails and slugs in your garden when sprinkled with salt…. This is also the case for many aquatic plants like the Water Soldier. 63% of the Broads Specialities need to live in fully freshwater conditions so, in a tidal system like that found in the Broads National Park, the refuges provided by unpolluted dykes are vital for the continued survival of these salt intolerant plants and invertebrates.

Floating Water Lilies
Carnivorous Bladderwort

Plant Adaptations to Survive. The plants and animals that are found living in water have evolved many fascinating adaptations to cope with their environment. Plants can either float freely in or on the water, such as Duckweed, Frogbit, Water Soldier and Bladderwort, or are attached to the bottom or sides of the water bodies like Water Lilies. The free floating plants are reliant on the water for all their nutrients which can cause problems if the water is not nutrient rich. Bladderwort has solved this problem by becoming carnivorous! The aquatic plants attached to the bottom by simple roots either grow totally submerged, such as Canadian pond weed, or like Water Lilies send up long thin stems to the surface where their large flat leaves float.

Surveying the dykes.

Extracting oxygen from water is difficult so having leaves on or above the water surface is one way plants overcome this problem. For the fully submerged plants such as Water Violet and Canadian Pond Weed they have thin hair-like or spiky leaves that increase the surface area available for absorption of oxygen and nutrients. Water depth also influences the types of plants that are able to grow. Only in the relatively shallow broads or areas of exceptionally still water do most aquatic plants have sufficient light and firm enough anchorage to grow. Again this makes the shallow dykes with their relatively still waters ideal habitats for a range of aquatic vegetation to flourish.

Waterflea (Daphnia)

Animal Adaptations to Survive. The invertebrates living in water can be divided into those that need to breathe air and those like fish that are able to obtain oxygen from the water. The crustaceans like the freshwater shrimp and Daphnia (water flea) are able to extract oxygen from the water as do some of the mollusca like the bivalve pea mussels. Some snails like the common pond snail need to breath air and hang upside down from the surface to do so. Diving beetles, spiders, water scorpions, back swimmers, pond skaters all need to breathe air but hunt on the surface and or underwater and have evolved ways to not only walk on water but also swim and maximise the amount of time they can stay submerged.

Silver Diving Beetle Larva
Norfolk Hawker Dragonfly on the wing.

Many insects such as Dragonflies, diving beetles, Damselflies and Mayflies have a fully aquatic larval stage to their lifecycle. Their larvae are able to extract oxygen from the water and only just before they become adults does this change and they need to breathe air through their spiracles (breathing holes). Mosquito larvae on the other hand cannot obtain oxygen from water and have to breathe air. They hang upside down suspended by breathing tubes from the water surface leaving their heads free to grab passing food in the water below them. The benefit of living in water as larvae is the plentiful supply of food in the water and conditions (temperature) are relatively stable with little chance of drying out. However, it’s beneficial to move out of the water for the adult stage because it is possible to move greater distances by air to find mates and good habitats with few predators and plenty of food in which to lay eggs.

Worthy of Protection. The dykes of the Broads National Park provide the conditions for aquatic vegetation to thrive which in turn creates the habitat needed to support a diverse array of invertebrates – plentiful oxygen in the water, food for the herbivores and shelter for young and adults from predators. No wonder, with such a special community of plants and animals living in them, so many dykes in the park are listed and protected as Sites of Special Scientific Interest.

Have a look at the Prezi Presentation below to learn about some of the plants and animals that live in the dykes of the Broads National Park.