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Life in Freshwater

Seasonal Change

All habitats show a seasonal change but ponds have a quite dramatic variation over the year. If water is constantly available then plant growth can progress unhindered. The tallest grasses are water plants and these can grow from stumps below the water to tall emergent vegetation in a matter of months. A pond with a wide open area of water may become choked with water lilies and water soldiers which appear to erupt out of the water. So dense are they that many land invertebrates can easily crawl across the water, even small birds. By late summer the great productivity is at its height. As autumn approaches the plants begin to decline until mid winter the soft emergent vegetation has collapsed beneath the surface. Water levels also begin to rise to cover most of the vegetation that remains.

In More Detail:

A Pond in Winter
1. Pond in Winter

During the winter the water level rises and the dead material from the previous years plant growth collapses into the pond. Animals like the water hoglouse begin to consume the dead material to start the recycling process. If the surface freezes the heavier water is at the bottom unfrozen and keeping alive the invertebrate life. Many of the insects have stopped growing and have entered a phase called diapause. This means that the may still be active (unlike hibernation) but not developing.

A Pond in Spring
2. Pond in Spring

The Reedmace is growing well but is still only half the final height. Water level is variable. The larval stages of insects will be at a late development as diapause ended a month or so back. Over the next month or two the first aerial insects will emerge. For example, the Large Red Damselfly (Pyrhosoma nymphula) will be one of the first, emerging in late April-May. The blue ones may not start emerging until June.

A Pond in Late-Summer
3. Pond in Late-Summer; no water available but it is still boggy and will keep many of the plants alive

Small water bodies inevitably have problems during long hot summers. Not every year, but occasionally a pond will completely dry out during the summer months unless there is a steady inflow of water via a stream or river. Animals that can fly like the various adult water bugs and beetles will take to the air and move to a new water body. Those unable to move are likely to die. The insect larval may already have emerged and departed although some like the larger dragonflies may take two years to develop. Some like the hydra will carry out sexual reproduction prior to this and leave behind eggs able to survive the drought. These will hatch when water again fills the pond.

 

 


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