The Ecology of the Avon Banks at Bristol

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Abstract
The banks exhibit the typical zoning of the salt marsh, but the steep gradient of the bank and the shelter afforded by the cliffs of the gorge distinguish it from the open type of salt marsh. Atriplex hastata and Cochlearia anglica are characteristic plants and the unusual height of the Aster plants is definitely related to the sheltered position. The distribution of plants in zones I and II is similar to that obtaining in other salt marshes and the vegetation is entirely halophytic. The zonation is due to varying salinity which is determined by the frequency and duration of submergence by the tide. The slope of the bank, scour of the tide and the wave action and currents produced during high tide are responsible for the erosion of the bank and the production of different levels. The distribution of Glyceria maritima is limited by the slope of the bank. During favorable seasons the distribution of Euglena and diatoms in zone III is limited by the water content of the mud, Euglena showing its maximum development in a water content of 88 to 99% and the diatoms one of 11 to 114%. The mechanical effect of flood may often become the limiting factor in the occurrence of the organisms, since when the river rises they are carried away together with the surface mud. Nutrient material in the mud rarely becomes the limiting factor, but after periods of prolonged flood the organic material and nutrient salts are washed out of the mud. The temp. range for active growth of Euglena is 0-25[degree] C. and for diatoms 3-25[degree] C. Therefore during low temp. diatoms are more affected than the Euglenae. In both cases 15[degree] C. and above favors most active growth and reproduction. Euglena can withstand longer periods of freezing than can the diatoms, though in the latter small numbers appear to survive. Light intensity affects Euglena more than the diatoms. Using the daily values described in the text, Euglena requires a total daily light intensity of 30, and will not respond to the stimulus of light at an intensity of less than 1/3. Moreover, Euglena disappears when kept more than 10 days in the dark or 10 days at intensities lower than 30. For active photosynthesis and reproduction, intensities from 100-150 appear to be required, and such indicate days with several hours of sunshine. A period of light of less than 2 hours'' duration appears to be of little use to the plant. These features, studied by means of experiment in the field and laboratory, show a close relationship between the behavior of the Euglena and the daily, fortnightly and seasonal changes in light intensity. Diatoms grew and reproduced in a light intensity as low as 20. Both Euglena and diatoms appear to exist in the vegetative state all the year round, although showing considerable variation in numbers. This may be correlated with the absence of climatic extremes in the habitat.

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