ASPECTS OF THE ECOLOGY OF THE COMMON REED, PHRAGMITES AUSTRALIS IN THE NORFOLK BROADLAND (ENGLAND).

摘要

Available from UMI in association with The British Library. Requires signed TDF.; Reed (Phragmites australis) is an important part of the Broadland ecosystem in Norfolk, U.K. but large areas of reedswamp have disappeared in recent decades. This thesis is concerned with the growth and regression of Phragmites in Broadland and with its role in nitrogen transformations.; Reed in Broadland grows rooted in sediment and as floating mats. Regression has occurred since the 1940s in only 27% of sites where reed is rooted in sediment but in 73% of sites where it formed floating mats.; Recent changes in the chemistry of Broads' water predispose floating reed to mechanical break-up. This hypothesis was supported by the results of controlled growth experiments which showed that the biomass of shoots is increased more than that of rhizomes at loadings of nitrate-nitrogen giving concentrations greater than about 6 mg per litre. Sampling of field populations showed that floating reed has a lower biomass of horizontal rhizomes (340 {dollar}pm{dollar} 180 g per m{dollar}sp2{dollar}) than reed rooted in the sediment (1280 {dollar}pm{dollar} 320 g per m{dollar}sp2{dollar}). In both growth forms the development of sclerenchymatous tissue was inversely related to the ratio of nitrogen to potassium content of rhizomes. These give mechanical support to the swamp as well as functioning in the uptake and storage of nitrogen.; Reed rhizomes contained around 20 g nitrogen per m {dollar}sp2{dollar} in Hickling Broad. This represented the major part of the nitrogen stock in the lake, which has 120 ha of open water and 18 ha of reedswamp. Internal recycling of nitrogen maintained this stock over five years. Annual losses of nitrogen from senescing shoots varied between 3.0 and 8.0 g per m{dollar}sp2{dollar}. These losses were sufficient to account for increases in concentration of nitrogen in open water during autumn, though these were small, and not likely to increase significantly the effects of eutrophication (which is a general problem in Broadland) because of dilution and subsequent flushing.; Uptake of nitrogen by established reed plants is not likely to ameliorate the effects of nutrient enrichment of water. A potential for loss of nitrogen from the Broads' system lies in the decomposition of the annual input of litter from shoots, which was 230 {dollar}pm{dollar} 50 g per m{dollar}sp2{dollar} in 1981. Numbers of denitrifying bacteria associated with this litter were highest in October (1.1 {dollar}times{dollar} 10{dollar}sp{lcub}11{rcub}{dollar} per g dry weight of litter). Macroinvertebrates did not play a significant part in the decomposition of litter.; Restoration of reedswamps in Broadland is the cheapest means of stabilising rapidly eroding banks. Successful techniques have been used, though the ultimate solution to reed regression appears to lie in a reduction in the loading of nitrogen to the waterway.