A pollen and diatom study of the late-flandrian sediments of Lough Neagh, Northern Ireland

Abstract

The present study alms to t r ace the late-Flandrlan development of Lough Neagh from the microfossil I record of the lake sediment. Th e assumption is made that the history of the lake is intimately connected with the history of the drainage basin and that major changes in the character of the lake can be explained by reference to this relationship. The history of vegetational change in the catchment area is studied through pollen diagrams and where possible these are substantiated by reference to primary and secondary documentary historical sources. Changes In the history of the lake itself are inferred from the sedimentary diatom record. and radiocarbon, palaeomagnetic and pollen analytical evidence is used to construct a sediment chronology.

The study uses conventional percentage frequency methods for the microfossil I counts and for some of the diatom counts an absolute method was also used. A new method for the rapid and accurate estimation of diatom number s is presented, and the results of analyse s using this method are used with diatom cell volume determinations and rate of sediment deposition estimations to calculate the preserved diatom biomass cm - 2 yr -1 in the lake for the last 1400 years. A comparison of the figures for the topmost sediment with the estimated soluble silica usage of the planktonic diatom crops in 1970 suggests the possibility of considerable annual diatom dissolution.

After taking into account problems of diatom preservation and I imitations in the understanding of diatom ecology it Is suggested that the results of the analyses indicate four major stages in the late-Flandrian development of the lake. From a comparison of the diatom results with the evidence from pollen analyses and documentary sources it is considered that these stages represent a progressive increase In the influence of man on the take. The earliest stage (:>- 2000BC - 1100AD) which is characterised by very stable percentages of the fossil diatom taxa is thought to represent a pre- human influence period
although pollen evidence shows that prehistoric and Early Christian agricultural activities were widespread within the lake catchment area. The second and third stages (100AD - I 600AD; I 600AD - I 900AD resp.) are both thought to have been due to drainage basin disturbance by agricultural activity, but whilst the former appears to be related to acidification from the in wash of (upland?) peat and soil , the latter is considered to be related to the beginning of lake eutrophication consequent upon the clearance of forest and the leaching of lowland soils. The final stage (20th century) is thought to correspond to the development of the use of the lake for water extraction and waste disposal. It is suggested from evidence of the diatom paleo-productivity curve and the sediment chronology that the present lake productivity level is a consequence of post-war pollution

Date of Award	1973
Original language	English