A Whole-Basin Diatom Accumulation Rate for a Small Eutrophic Lake in Northern Ireland and its Palaeoecological Implications

Abstract

Twelve cores from a lake affected by creamery effluent disposal (Lough Augher, Co. Tyrone, N. Ireland) were analysed for diatom stratigraphy. Cores were dated by biostratigraphic correlation to a 210Pb-dated master core. Diatom accumulation rates were estimated for individual cores, and for the average of all cores. The plankton diatom succession caused by the creamery effluent disposal represents a simple eutrophication sequence, and agrees well with that hypothesized by culture studies of diatom responses to changing Si:P ratios. Relative frequency stratigraphies in deep-water cores are clearly repeatable. However, diatom accumulation rates for individual cores are variable, and no single core is strictly representative of the mean accumulation rate for the whole lake, because of a complex pattern of diatom deposition. A mean accumulation rate was calculated for the whole lake. The maximum, approximately 18 .times. 106 cells cm-2 year-1, occurs between 1940 and 1957, suggesting that this was the most productive diatom period. In contrast, maximum Stephanodiscus parvus percentages, a diatom commonly assumed to indicate lake eutrophy, occur between 1964 and 1974. This later period relates, perhaps, to maximum whole-lake productivity with greater contributions from non-siliceous algae. Percentage data and accumulation rates in individual cores under-estimate the importance of the periphyton; their mean accumulation rate c. 1900-80 for the whole lake (10-12 .times. 106 cells cm-2 year-1) is greater than that of plankton (1.5-6 .times. 106 cells cm-2 year-1). This has significant palaeoecological implications for the assessment of diatom responses to eutrophication. The contrasting palaeoecological interpretations of the different data types are discussed. The different conclusions are reconcilable in terms of the variable nutrient requirements and sources of the plankton and periphyton.