The responses of diatoms to the influx of tephras into lacustrine environments

Abstract

The aim of this research is to determine the response of diatoms to the deposition of tephras into lake environments. This is achieved by conducting high-resolution sampling of cores extracted from Lake Pupuke and Pukaki Crater; two maar lakes located in the Auckland Volcanic Field, and by analysing the diatom floras directly above and below selected tephra beds. Tephras were selected for differing composition and thickness. Nine tephras were selected: four TVZ rhyolitic tephras, two Taranaki Volcano andesitic tephras, and three locally derived A VF basaltic tephras, ranging from 2 mm to 20 cm in thickness. Eight of the sections record changes across the tephra bed; however, the mechanisms of response are not consistent between tephras of different composition or thickness. The most common diatom recorded at both sites is Cyclotella stelligera, and in a majority of the sections, tephra input increases the dominance of this taxon. Tephra input has no impact on many environmental variables, e.g. the pH, salinity, nitrogen availability and oxygen saturation levels of the lake waters. Trophic status is most readily altered by the influx of tephra, generally recording a reduction in nutrient levels above the tephra. The most common mechanism of response, recorded in three sections, is a reduction in phosphorus after tephra input probably due to the sealing off of underlying sediments by the tephra layer, thereby blocking the recycling of P from pre-tephra deposits. Other responses include effects associated with the physical properties of tephra particles interacting with diatom habitats, i.e. smothering of epiphytic habitats, and decreased turbulence in planktonic environments. Tephra also alters the relationship between Synedra ulna and a teratological variety of Synedra ulna v spathulifera. Suprisingly, thickness and chemical composition do not influence the response of the diatom assemblage. Five of the nine sections also indicate that the tephras sank through the underling sediment, coming to rest up to 1.75 cm below their original stratigraphic position. This is recorded as an offset between the tephra depth and the depth where changes in diatom assemblage are recorded. The most likely mechanism appears to be grain-by grain dissemination of tephra particles through the unconsolidated sediment at the sediment/water interface. Thickness and chemical composition of tephra do not appear to influence these events. The identification of displaced tephras has major implications in all paleoenvironmental reconstructions which utilise tephrachronology and tephrastratigraphy as dating and correlating tools. In particular, tephras may not constitute absolute time markers.