Paleoclimates in Southwestern Tasmania during the Last 13,000 Years

Plant-sociological and climatic classification of the Australian Nothofagus cunninghamii rain forest provides the basis for a new, semiquantitative approach to interpretations of late-Quaternary paleoclimates from four pollen sequences in southwestern Tasmania. Varying proportions of rain-forest pollen types in the records were related to different modern rain-forest alliances and their specifc climatic regimes, such as Eastern Rain Forest, Leatherwood Rain Forest, and sclerophyllous, Subalpine Rain Forest. According to this interpretation, early Holocene climates were characterized by 1,600 mm annual precipitation and 10°C annual temperature, conditions substantially warmer and drier than previously thought. Maximum precipitation levels of 2,500 mm annually were not reached until 8,000 years B.P. A short-term cooling episode between 6,000 and 5,000 years B.P. led to the establishment of modern rain-forest distribution in western Tasmania, characterized either by a precipitation gradient steeper than before, or by greater climatic variability. To interpret paleoclimates from before 12,000 years B. P., when non-arboreal environments dominated in western Tasmanian bollen records, various modern treeless environments were studied in search for analogs. Contrary to earlier interpretations, late-glacial environments were not alpine tundra with a treeline at modern sea level, but steppe, with marshes or shallow lakes instead of the modern lakes. Climate was characterized by 50% less precipitation than today, resulting in substantial summer droughts. To explain such drastic precipitation decrease, the westerlies that dominate Tasmanian climate today must have been shifted polewards. This suggestion is supported by climate models that take Milankovitch-type insolation differences into account as well as sea-surface temperatures. Paleolimnological information based on diatom analyses support the general paleoclimatic reassessment.

Late Pleistocene and Holocene Paleolimnology of Two Mountain Lakes in Western Tasmania

The analysis of diatoms from two lake-sediment cores from southwestern Tasmania that span the Pleistocene-Holocene boundary provides insight about paleolimnological and paleoclimatic change in this region. Both Lake Vera (550 m elevation), in west-central Tasmania, and Eagle Tarn (1,033 m elevation), in south-central Tasmania, have lacustrine records that begin about 12,000 years ago. Despite significant differences in location, elevation, and geologic terrane, both lakes have, had similar, as well as synchronous, limnological histories. Each appears to have been larger and more alkaline 12,000 years ago than at present, and both became shallower through time. Fossil diatom assemblages about 11,500 years old indicate shallow-water environments that fluctuated in pH between acidic and alkaline, and between dilute and possibly slightly saline hydrochemical conditions ( The synchroneity and similar character of the paleolimnological changes at these separate and distinctive sites suggests a regional paleoclimatic cause rather than local environmental effects. Latest Pleistocene climates were apparently more continental and drier than Holocene climates in southwestern Tasmania.

Melting Alpine Glaciers Enrich High-Elevation Lakes with Reactive Nitrogen

Alpine glaciers have receded substantially over the last century in many regions of the world. Resulting changes in glacial runoff not only affect the hydrological cycle, but can also alter the physical (i.e., turbidity from glacial flour) and biogeochemical properties of downstream ecosystems. Here we compare nutrient concentrations, transparency gradients, algal biomass, and fossil diatom species richness in two sets of high-elevation lakes: those fed by snowpack melt alone (SF lakes) and those fed by both glacial and snowpack meltwaters (GSF lakes). We found that nitrate (NO3-) concentrations in the GSF lakes were 1-2 orders of magnitude higher than in SF lakes. Although nitrogen (N) limitation is common in alpine lakes, algal biomass was lower in highly N-enriched GSF lakes than in the N-poor SF lakes. Contrary to expectations, GSF lakes were more transparent than SF lakes to ultraviolet and equally transparent to photosynthetically active radiation.Sediment diatom assemblages had lower taxonomic richness in the GSF lakes, a feature that has persisted over the last century. Our results demonstrate that the presence of glaciers on alpine watersheds more strongly influences NO3- concentrations in high-elevation lake ecosystems than any other geomorphic or biogeographic characteristic.

Book review of Paleolimnology of EuropeanMaar Lakes by J. F.W. Negendank & B. Zolitschka (eds).

This book represents a collection of papers presented at a symposium in May, 1991 on the formation and paleolimnology of European maar lakes. Paleolimnological discussions embrace maar lake deposits of late Quaternary to early Tertiary age in Germany, France and Italy. The goal of the symposium and its outgrowth, this book, is to develop a funding initiative to study, core and analyze the deposits of these remarkable depositional systems.

Phytoplankton Dynamics in Three Rocky Mountain Lakes, Colorado, U.S.A.

In 1984 and 1985 seasonal changes in phytoplankton were studied in a system of three lakes in Loch Vale, Rocky Mountain National Park, Colorado. Three periods were evident: (1) A spring bloom, during snowmelt, of the planktonic diatom Asterionella Formosa, (2) a mid- summer period of minimal algal abundance, and (3) a fall bloom of the blue-green alga Oscillatoria limnetica. Seasonal phytoplankton dynamics in these lakes are controlled partially by the rapid flushing rate during snowmelt and the transport of phytoplankton from the highest lake to the lower lakes by the stream, Icy Brook. During snowmelt, the A. formosa population in the most downstream lake has a net rate of increase of 0.34 d-1, which is calculated from the flushing rate and from the A. formosa abundance in the inflow from the upstream lake and in the downstream lake. Measurement of photosynthetic rates at different depths during the three periods confirmed the rapid growth of A. formosa during the spring. The decline in A. formosa after snowmelt may be related to grazing by developing zooplankton populations. The possible importance of the seasonal variations in nitrate concentrations were evaluated in situ enrichment experiments. For A. formosa and O. limnetica populations, growth stimulation resulted from 8- or 16-micromolar amendments of calcium nitrate and sulfuric acid, but the reason for this stimulation could not be determined from these experiments.