Saltpans or pools are characteristic of most salt marshes and many can be seen on Stanpit Marsh. The theories regarding their origin remain controversial. They include the unequal growth of plant life during marsh development, the damming of creeks and the death of vegetation in depressions due to the accumulation of detritus.
Because of their location and small size, saltpans are subject to many environmental hazards including flooding, freezing and drought. They are often completely full of water after very high tides or heavy rainfall. Conversely, they may dry out completely, the cracks caused by shrinking mud forming intriguing polygonal patterns. In winter they sometimes freeze over.
Plants and Animals in the Saltpans
Plants and animals living in these conditions face major problems of rapidly fluctuating salt content, oxygen depletion, dehydration and freezing. Few species tolerate such a hostile and rapidly changing environment but those that do are highly successful. Shore crabs [Carcinus maenas] forage amongst the detritus on the bottom while ragworms [Nereis diversicolor] emerge from the mud to capture unsuspecting prey. The shrimp [Palaemonetes varians] and brackish water Gammarus species may also flourish. However, the two commonest animals visible once the spring is here are the crustaceans, Sphaeroma and Corophium. Sphaeroma, a whitish, woodlouse like animal, swims freely on its back and can survive by burrowing in the mud. Corophium or Mud Shrimp, also whitish and closely related to sandhoppers leaves its U-shaped burrow in the muddy bottom to swim with its very large antennae outstretched. A few brackish water fish and certain microscopic organisms such as diatoms, protozoa and nematode worms are also able to maintain themselves in these fluctuating conditions.
Survival in the Saltpans
Survival depends on the ability of these animals to avoid, tolerate or physiologically maintain the integrity of their body fluids within this rapidly changing environment. Ragworms use an avoidance strategy by burrowing in the mud. Gammarids tolerate salinity changes in their body fluids, while shore crabs control their salt/water balance by a process known as osmo-regulation. Some species die off under extreme conditions, leaving resistant eggs to survive the drought or ice.
Saltpans are an important part of the Stanpit Marsh scene. Unpredictable, and demanding habitats, they provide a home for versatile species which in their turn are food for marsh birds and other animals.
The Question about the Brilliant White Spider on the Marsh Mallow
Well, without seeing the spider, I think it most probably was a Crab Spider, so called because they resemble the shape of a crab, and actually walk sideways like a crab. They vary in colour, and often tend to be the same colour as the flower they are lurking in.
They do not have a web, but stay perfectly still on the plant waiting for their prey to pass by. I have a picture of the Crab Spider