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Mahoney Lake is a small, meromictic saline lake in south-central British Columbia noted for its unique layer of purple sulfur bacteria. First examined in 1969, this lake has undergone physical, chemical, biological, and pre-historical research to generate an understanding of how the lake and its biota function have developed through time. Advances in understanding the sulfur transformations and bacterial nutrient cycling over the last fifty years have been prolific, resulting in the description of several new taxa. Mahoney Lake is exceptional in its limnological characteristics and is an ideal site for training future limnologists.

Pollen and chironomid analyses and radiocarbon dating at Pye Lake on the eastern shore of Nova Scotia are used to outline the vegetation and climatic history of the area. The coast was deglaciated prior to ~12200 [sup.14]C BP (14300 cal BP), and herbaceous tundra vegetation invaded the area. Midge-inferred maximum summer surface-water temperatures in the lake ranged between 9 and 11°C. Subsequent gradual warming to ~18°C by 10 800 [sup.14]C BP (12 725 cal BP) favoured the migration of a variety of herbaceous and shrub taxa into the region. Rapid cooling to ~10°C saw vegetation revert to herbaceous tundra communities. This interval, related to the Younger Dryas cold interval of the North Atlantic and Europe, lasted until ~10000 [sup.14]C BP (11630 cal BP). The climate then warmed again to conditions similar to those that prevailed immediately before onset of Younger Dryas cooling. Further warming saw successive tree species migrate into the area until, by the mid-Holocene, the forests contained most of the taxa prevalent today. Since ~3500 years ago, cooling of the climate has favoured conifer species over broad-leaved taxa. Agriculture and logging practices in the last 150 years have altered the forest composition, but pollen analysis of the most recent sediments cannot resolve these changes adequately. Des analyses polliniques et des chironomides ainsi que des datations radiochronologiques ont ete effectuees sur les sediments d'un lac sur la cote est de la Nouvelle-Ecosse (Pye Lake) afro de reconstituer l'histoire de la vegetation et du climat de cette region depuis la deglaciation. La cote fut liberee des glaces il y a environ 12 200 ans [sup.14]C BP (14 300 ans etal. BP). A cette epoque, la region immediate du lac fut caracterisee par une toundra arbustive. La temperature de l'eau de surface du lac en ete deduite des assemblages des chironomides se situait entre 9 et 11[grados]C. Un rechauffement du climat vers 10800 ans [sup.14]C BP (12725 ans etal. BP) a permis l'arrivee de nombreuses especes herbacees et arbustives. La temperature de l'eau de surface a alors atteint 18[grados]C. Un refroidissement d'environ 10[grados]C est ensuite survenu, engendrant un retour vers un couvert vegetal domine par des especes herbacees. Cet episode froid associe au Dryas recent a persiste jusque vers 10000 ans [sup.14]C BP (11630 ans etal. BP) lorsque les conditions climatiques sont redevenues similaires a celles qui prevalaient avant le debut du Dryas recent. Cette nouvelle hausse des temperatures a permis la migration dans la region des especes arborescentes. Toutes les especes arborescentes presentes aujourd'hui dans la region du site a l'etude etaient presentes des 'Holocene moyen. Un leger refroidissement du climat amorce il y a environ 3500 ans a entraine une augmentation de la presentation de certaines especes coniferiennes au detriment d'especes decidues. Au tours des 150 dernieres annees, es coupes forestieres et les activites agricoles ont modifie la composition des forets, mais la resolution stratigraphique des analyses polliniques des sediments correspondent a cette periode n'a pas permis de retracer ces changements avec precision., Introduction Paleoecological investigations at Pye Lake, located along the eastern shore of Nova Scotia between Halifax and Chedabucto Bay (Fig. 1), provide a unique insight into postglacial conditions for a [...]

A comprehensive review of Holocene paleoenvironmental data has been prepared, providing the basis for evaluating natural variability in climate and ecosystem dynamics in the northern Columbia River basin. The paleoenvironmental record reveals four major climatic shifts and a number of less well-defined climatic changes throughout the Holocene. The major climate changes are (1) a cool or cold, late-glacial climate at the end of the last glaciation (>11500 cal. year BP), (2) an interval of drought and maximum summer warmth during the early Holocene (ca. 10500 to 8000 cal. year BP), (3) a mid-Holocene trend towards a cooler, more moist climate (ca. 8000 to ca. 4000 cal. year BP), and (4) a relatively stable climate persisting from ca. 4000 cal. year BP until the arrival of European settlers. Air temperatures have warmed by about 1[degrees]C over the past century. Minor climate events that are emerging as global in nature include Little Ice Age (1200-150 BP) conditions, and a late Holocene cool wet period (3500-2500 BP). These are observed in some study sites examined in this paper and may be related to large-scale cycles of 2800-2000 and 1500 years. A discussion of changing atmospheric circulation patterns, and the causes of long-term climatic change are included, together with a discussion of the implications of the paleoenvironmental record for understanding future climate, and the likely response of communities and ecosystems. Key words: climate change, paleoclimate, paleoecology, British Columbia, Washington. Les auteurs ont prepare une revue d'ensemble des donnees paleo environnementales de l'Holocene, constituant ainsi une base pour l'evaluation de la variability naturelle de la dynamique du climat et des ecosystemes, dans le bassin de la riviere Columbia. Les donnees paleo environnementales revelent quatre changements climatiques majeurs et un nombre de modifications du climat moins bien definies, tout an long de l'Holocene. Les changements climatiques majeurs sont (1) un climat de fin de glaciation vers la fin de la derniere glaciation (>11,500 AP) frais on froid); (2) un intervalle de secheresse et de temperatures estivales maximales an cours du debut de l'Holocene (ca. 10,500 a 8,000 AP); (3) une tendance vers un climat plus humide et plus frais vers le milieu de l'Holocene (ca. 8,000 a ca. 4,000 AP); et (4) un climat relativement stable a partir de ca. 3,000 AP jusqu'a l'arrivee des colons europeens. Les temperatures de fair ont rechauffe d'environ 1[degre]C an cours du siecle dernier. Les changements climatiques mineurs de nature globale incluent le Petit age glaciaire (1,200-150 AP) et une periode humide et fraiche en fm de l'Holocene (3,500-2,500 AP). On observe ces phenomenes sur certains sites d'etude mentionnes dans ce travail, et on pent les relier a des cycles a large echelle de 2,800- 2,000 et 1,500 ans. Les auteurs presentent une discussion sur les patrons changeant de la circulation atmospherique, incluant les causes du changement climatique a long terme, ainsi qu'une discussion des implications des donnees paleo environnementales pour comprendre le climat futur, ainsi que les reactions probables des communautes et des ecosystemes. Mots-cles : changement climatique, paleoclimat, paleoecologie, Colombie canadienne, Washington. [Traduit par la Redaction], Introduction Many paleoecological and paleoclimatological studies have been conducted in the Columbia River basin. These studies provide essential background information on the natural variability of climate and ecosystem dynamics, prior [...]

The existence of strong summer thermal gradients in North America is investigated using palaeoecological methods. These gradients are believed to have been formed by the combination of high summer insolation and the presence of the Laurentide ice sheet during the period when the last glacial climate was shifting to the present interglacial climate. Reconstructions of temperature gradients for the late-glacial period in the study region are found to be much stronger than those of the modern period.