An investigation of Holocene environmental change in the Lough Neagh basin diatoms

Abstract

The sedimentary record provides evidence of environmental changes in a lake ( e.g. trophic status, pH, conductivity) and its catchment (e.g. vegetation development, hydrological change, human impact). In the Lough Neagh basin (Northern Ireland), there has been particular interest in Holocene water levels. Though there is general agreement that water levels varied throughout the Post-Glacial, there is some debate as to the timing, magnitude, direction and significance of these fluctuations. The aim of this study was to use a number of indicator parameters from a range of sites to identify Holocene events in the basin, including water level changes. This study describes and interprets stratigraphical (physical properties) and ecological (pollen, diatoms) characteristics of Holocene sediments obtained from a number of deep water, shallow water and terrestrial diatomite cores.

An 8m Kullenberg core from a 17 .Sm site on Lough Neagh spans the period from the Younger Dryas to the Sub-Atlantic and is the most complete Holocene record yet obtained from the lake. Canonical correspondence analysis demonstrated that pollen tax.a were significant predictors of diatom community change, indicating that catchment processes are reflected in the lake biota. The Younger Dryas/Pre-Boreal transition was marked by a dramatic change in several physical and biological parameters. The high Hydrogen Index between c.9600 and c.8600 BP suggested that relatively high algal productivity overlapped a period of low lake levels at c.9200 BP, indicated by peat outcrops on the shoreline. However, maximal diatom productivity and abundance of the 'eutrophic' tax.on Stephanodiscus neoastraea occurred much later (between c.8600 and 7000 BP), indicating that diatoms were not the source of the earlier productivity. At the Boreal-Atlantic transition (the 'alder rise' at 7000 BP), Aulacoseira subarctica increased dramatically to form over 40% of the planktonic flora. Dominance of A. subarctica indicates high winter silica levels and mixing of the water column. The 'elm decline' at 5100 BP coincides with the decline of 'oligotrophic' taxa Cyclotella aff. gordonensis and C. aff. comensis, and the appearance of the small centric Stephanodiscus 'B', a probable eutrophic indicator.

A 2.2m sequence obtained from Lough Beg, a shallow lake 1km north of Lough Neagh, also contained an uninterrupted Holocene record. Changes in its planktonic flora largely matched those of the Lough Neagh core. These observations do not support an earlier hypothesis of very low lake-levels between 8000 and 6000 BP causing a sedimantary hiatus in Lough Beg. Spatial studies of cores suggested that Lough Beg has a relatively small, dynamic sedimentary basin.

Diatom communities of cores obtained from nearby diatomaceous earth deposits at Toomebridge and Newferry were compared with those from Lough Neagh. Dominance of A. subarctica in the diatomite, the only diatom that actively grows in Lough Neagh during the winter, suggested a major role of seasonal flooding. Detrended correspondence analysis showed similarities between Toomebridge and Lough Neagh planktonic diatoms of broadly similar age. However, a preliminary constrained ordination identified significant differences in matched planktonic diatom samples related both to sample age and location. The results are discussed in relation to published accounts of landscape and climatic change during the Holocene.

Date of Award	Dec 1999
Original language	English