Roger McNeely

A high resolution proxy-climate record from an arctic lake with annually-laminated sediments on Devon Island, Nunavut, Canada

Sediments from a 3 ha lake (75 °34.34′N, 89 °18.55′W) from the coastal region of northern Devon Island, Nunavut, Canada, contain discrete laminations in the deepest part of the basin. The laminations are varves as indicated by the correspondence between counts and thickness measurements of the couplets and 210Pb dating. A 14 cm core representing 150 years of sedimentation contained laminated couplets consisting of a lighter inorganic layer with a higher percentage of calcium and magnesium, alternating with fine darker bands, typically more cohesive, and comprising higher proportions of silica and carbon. A reddish oxidation zone with higher iron and aluminum frequently separates the laminations. The dark layer represents a biogenic component deposited in summer and is made cohesive by bacterial filaments among the other particles. The light inorganic layer represents clastic deposition from allochthonous sources. Deposition rates were relatively consistent through the core with an increase in varve thickness in the 1950s. Diatom concentrations in the sediments increased by two orders of magnitude in this century, with major increases in the 1920s and 1950s. The increase in varve thickness and diatom abundance coincides with an increase in summer melt percentage in an ice core from the Devon Island Ice Cap (Koerner, 1977). The relatively high sedimentation rate (0.15 cm yr-1) coupled with the consistency of deposition makes this lake a significant indicator for recent climate changes of the Canadian Arctic Archipelago.

Cultural disturbances and trophic history of a small meromictic lake from central Canada

The recent history of Little Round Lake, a small meromictic lake in southeastern Ontario, is considered. Pollen analyses were used to identify past changes in terrestrial vegetation, whilst limnological conditions were interpreted on the basis of diatom and chrysophyte microfossils. Contemporaneous with the arrival of European settlers (ca. A. D. 1850), the predisturbance assemblage of oligotrophic Cyclotella diatoms was replaced by Synedra spp., which then succeeded to a eutrophic flora dominated by Stephanodiscus hantzschii. Meanwhile, synuracean algae were almost completely excluded. Over the last 30 years, the algal microfossils indicate that the lake underwent a marked return to oligotrophy. Evidence is presented which suggests that this shift was related to the cultural enhancement of meromixis by the seepage of road salt.

THE MORPHOLOGY AND DISTRIBUTION OF NEIDIUM DISTINCTE-PUNCTATUM HUSTEDT AND ITS SYSTEMATIC POSITION WITHIN THE GENUS

Neidium distincte-punctatum was described by Hustedt in 1922 from Lunz Untersee, Austria. Since then, this extant species has been identified from northern Germany, Finland, and Greenland. In North America, N. distincte-punctatum is recorded from the Arctic islands, Alaska, an isolated region in the central United States and Holocene sediments in southern Canada. In this study, data for N. distincte-punctatum is presented from 8 lakes and ponds on Ellesmere Island in the Canadian High Arctic. Valves range in size from 32 to 70 um in length, from 12 to 21 μm in width, and from 12 to 21 μm in depth, with 10-12 striae in 10 μm. There are 11-14 areolae in 10 μm. The morphology of the external valve surface (with large depressions around the areolae) is distinctive for this taxon. The structure of the internal valve surface is unique, with a complex network of braiding silica projections over the areolae. Only one other species, N. boyeri Reimer, is known to have such an internal valve wall structure. This int...

Community structure in epilinmetic and metalimnetic phytoplankton assemblages

Epilimnetic and metalimnetic phytoplankton communities were compared to assess the relative importance of autogenic and allogenic factors regulating community composition and structure. In the epilimnion, opportunistic species were predominant, and as a result there was considerabletemporal change in the community as measured by the community coefficient. In the metalimnion, however, temporal changes in the community coefficient were small compared with changes along the vertical gradient. It appears that non-equilibrium dynamics caused by short term changes in physical processes might be regulating the phytoplankton community in the epilimnion, and vertical niche partitioning along the vertical gradient was minor compared with temporal changes of community structure. The metalimnion supported communities with a more persistent structure over time, although vertical resource partitioning between density layers resulted in considerable change of community structure with depths.